Duct Sizing Calculator

Why Your Duct Size Matters More Than You Think

A contractor once showed me a system he'd inherited from another installer—brand new 3-ton AC, top-of-the-line Carrier, barely a year old. The homeowner couldn't keep the master bedroom below 78 degrees in August. I pulled the grilles, measured the ducts, and found the problem instantly: 6-inch round ducts trying to deliver 180 CFM. Those tiny pipes were choking airflow like breathing through a straw. The noise? Like a jet engine spooling up every time the system kicked on. We replaced them with proper 8-inch ducts, and suddenly the room cooled perfectly, dead silent, energy bills dropped $40 monthly. Wrong duct sizing turns expensive equipment into worthless junk, and most homeowners never realize that's the problem.

Here's what HVAC schools teach but contractors often ignore: ductwork is the circulatory system of your home. Your furnace is the heart, pumping air through arteries (supply ducts) and veins (return ducts). Size those ducts wrong, and it's like forcing your heart to pump blood through narrowed arteries—excessive strain, poor performance, early failure. The numbers are precise: 800 CFM through a 10-inch round duct flows at 880 feet per minute (FPM), whisper quiet with minimal friction loss. Squeeze that same 800 CFM through an 8-inch duct? Velocity jumps to 1,375 FPM—you'll hear a roaring whoosh, static pressure spikes 0.15 inches, and your blower motor works 40% harder. This calculator does the math contractors should do but rarely bother with.

Understanding CFM and Air Velocity

CFM (cubic feet per minute) is just counting air—how many cubic feet flow past a point every minute. Sounds simple until you realize your 2,000-square-foot house with 8-foot ceilings contains 16,000 cubic feet of air. A typical 3-ton AC moves about 1,200 CFM, meaning it completely changes your home's air every 13 minutes. But here's where it gets interesting: that 1,200 CFM splits across maybe 8 different supply runs. Your master bedroom gets 280 CFM, the living room 320 CFM, each kid's room 140 CFM. Now you're sizing individual ducts for each run—get one wrong, and that room either freezes or swelters.

Air velocity is how fast that air moves, measured in feet per minute. Think highway traffic: 100 cars per hour (CFM) can flow smoothly at 25 mph on a four-lane road (big duct, low velocity) or cause chaos at 60 mph on a two-lane road (small duct, high velocity). HVAC has a sweet spot: 600-900 FPM for branch ducts, 700-900 FPM for main trunks. Below 600 FPM wastes space with oversized ducts that cost extra and barely fit. Above 900 FPM creates noise—I measured a system running 1,400 FPM once, sounded like a wind tunnel. The homeowner thought their blower motor was dying. Nope, just undersized 6-inch flex duct where 8-inch should've been installed. Swapped the ducts, velocity dropped to 780 FPM, problem solved. Velocity is the difference between peaceful and unbearable.

Round vs. Rectangular Duct Sizing

Round ducts are the gold standard—air flows smoothest through circular cross-sections with zero corners disrupting flow. A 10-inch round duct moving 600 CFM at 830 FPM has friction loss around 0.08 inches of water per 100 feet. That's practically nothing—your blower barely notices. Plus, round ducts are cheap: rigid galvanized costs $12-15 per 10-foot section, flexible insulated runs about $2 per foot. Installation is quick, connections seal easily, and they last forever. The downside? They take up vertical space. That 10-inch round duct needs at least 12 inches of clearance—tough to fit in a 2x10 floor joist bay. That's where rectangular ducts save the day.

Rectangular ducts solve space problems but create airflow challenges. A 6x10 rectangular duct has the same cross-sectional area as an 8-inch round duct (48 square inches), so it handles the same CFM. But those sharp corners create turbulence, increasing friction loss by 15-25%. The equivalent diameter concept helps here—that 6x10 rectangular duct performs like a 7.2-inch round duct, not a true 8-inch. I learned this installing ducts in a 1920s Colonial with 2x8 floor joists. We needed 600 CFM through a joist bay—an 8-inch round wouldn't fit. Solution? 4x12 rectangular duct with a 6.7-inch equivalent diameter, running at 825 FPM. Friction loss was slightly higher (0.10 vs 0.08), but it fit perfectly and delivered the airflow we needed. The aspect ratio (ratio of width to height) matters too—keep it under 4:1 or airflow gets weird. A 3x12 duct (4:1 ratio) flows reasonably well. A 2x16 duct (8:1 ratio) is a disaster waiting to happen.

Friction Loss and Static Pressure

Friction loss is air scraping against duct walls, bleeding energy as heat. Every 100 feet of ductwork creates friction loss measured in inches of water column—seems weird until you realize HVAC pressures are tiny. Your blower might generate 0.5 inches of total static pressure. Lose 0.08 inches per 100 feet to friction, and a 50-foot duct run costs you 0.04 inches—no big deal. But undersized ducts? I measured a 6-inch duct moving 400 CFM (velocity screaming at 1,470 FPM), friction loss hit 0.38 inches per 100 feet. That 50-foot run ate 0.19 inches of static pressure—38% of the blower's capacity gone just fighting duct resistance. The remaining 0.31 inches barely pushed air through the grilles and filters. The homeowner complained of weak airflow despite a brand new blower. We upsized to 8-inch ducts, friction dropped to 0.10 per 100 feet, and airflow doubled.

Total static pressure is your blower's budget—how much push it has to overcome all resistance in the system. Filters eat 0.1-0.2 inches, grilles take 0.03-0.05 each, every elbow adds 0.02-0.05, and ductwork friction compounds with every foot. Add it all up, and a typical system runs 0.4-0.5 inches total static. Residential blowers handle 0.5 inches comfortably, 0.8 inches maximum before performance tanks. I audited a system running 0.92 inches of static—way over capacity. The 1/3 HP blower motor was drawing 9 amps instead of rated 6 amps, running screaming hot. It died six months later. The culprit? Combination of undersized ducts, crimped flex duct, and a clogged filter nobody changed for two years. Proper duct sizing keeps friction loss low—typically 0.06-0.10 inches per 100 feet—leaving plenty of static pressure budget for everything else.

Common Duct Sizing Mistakes

The "use whatever fits" approach kills system performance more than any other mistake. I watched a crew install an entire system last year—gorgeous new Trane unit, brand new ductwork throughout. Their method? Measure the joist bay, install the biggest round duct that fits. Sounds logical, except they ended up with 6-inch ducts for 200 CFM runs (perfect at 730 FPM), 10-inch ducts for 150 CFM runs (way oversized at 330 FPM), and zero consistency. The system worked but felt wrong—some rooms overcooled, others struggled. We ran the numbers, resized three critical runs, and suddenly it balanced perfectly. Proper sizing means matching duct diameter to CFM and velocity targets, not duct diameter to available space.

Flex duct compression is the silent killer nobody sees until they crawl into the attic. Flex duct—that silver fabric-covered insulated stuff—comes in long compressed bundles. Fully stretched, 25 feet of flex duct performs as designed. But most installers are lazy: they pull it partway, leave it saggy with curves and dips, crushing the inner liner. A fully compressed 8-inch flex duct performs like a 6-inch duct—30% airflow loss instantly. I measured a system where 40 feet of flex duct had 22 feet of compression and sag. Calculated airflow: 450 CFM. Actual measured airflow: 280 CFM. We pulled it tight, supported it every 4 feet with proper strapping, and airflow jumped to 425 CFM. Cost to fix? $30 in hangers and an hour of labor. The lesson? Flex duct must be pulled tight with less than 1% compression, and flex runs shouldn't exceed 10-15 feet where possible. Long runs need rigid duct—it doesn't sag, doesn't compress, and flows air exactly as designed.

How to Use This Duct Sizing Calculator

Start with your CFM requirement—if you don't know it, you need to calculate it first using a CFM calculator or your Manual J load calculation. Let's say your master bedroom needs 280 CFM for a 12,000 BTU cooling load (typical 400 BTU per CFM rule). Punch 280 into the calculator. Next, choose your target velocity. For quiet bedrooms, I recommend 600-700 FPM. Living areas can handle 700-800 FPM. Utility rooms and basements tolerate 800-900 FPM. Select 650 FPM for our bedroom example. Choose your duct shape (round or rectangular) and material type—galvanized steel is standard, flex duct has higher friction (30% increase), and PVC runs smoothest (20% lower friction). Hit calculate, and boom—you get your duct size with accurate friction loss based on the material you selected. For our example with galvanized steel: 8-inch round duct running at 646 FPM with 0.074 inches friction loss per 100 feet. Perfect sizing, whisper quiet, efficient airflow.

Rectangular duct sizing requires one extra step: selecting aspect ratio. If you've got a 2x10 floor joist (9.25 inches of clear space), you need a flat rectangular duct. Choose "Round" first to see you need 8 inches equivalent. Switch to "Rectangular," enter 280 CFM and 650 FPM velocity, keep galvanized steel material selected. Try aspect ratio 2:1—calculator shows you need 5x10 (50 square inches, equivalent diameter 7.8 inches, velocity 685 FPM). That won't fit in your 2x10 joist bay with insulation clearance. Try 3:1 aspect ratio—calculator suggests 4x12 (48 square inches, equivalent diameter 7.5 inches, velocity 715 FPM). Still tight. Go 4:1 aspect ratio—3x14 duct (42 square inches, equivalent diameter 7.0 inches, velocity 775 FPM). That's your winner: fits the space, velocity acceptable, gets the job done. The calculator automatically adjusts friction loss for rectangular ducts (about 10% higher than round due to corner turbulence). Real-world duct sizing involves balancing physics, space constraints, material selection, and cost.

Duct Sizing Quick Reference Chart

I keep a laminated version of this chart in my truck—saves me ten minutes every job site visit when I'm verifying installed ducts or quoting duct replacements. A homeowner called last week complaining about a noisy bedroom vent. I measured 240 CFM coming through a 6-inch duct—this chart showed instantly that's 982 FPM, way too fast. Swapped it for an 8-inch duct (437 FPM), problem solved in twenty minutes. The chart works both ways: if you know your CFM requirement, scan across to find the duct size at your target velocity. If you've got existing ducts and measured CFM, use it to verify the system's running at reasonable velocity. I caught an installer using 7-inch ducts for a 400 CFM run—the chart screamed "911 FPM, too loud!" We upsized to 9-inch (722 FPM) before the homeowner ever heard that wind tunnel sound.

Here's your professional quick-reference for round duct sizing across common CFM loads and velocity targets:

Pro Tip: The blue-highlighted rows (200, 300, 400, 600, 1,000, 1,200, 1,500 CFM) represent the most common residential duct sizing scenarios. If you're sizing branch runs for bedrooms (typically 150-250 CFM), living rooms (300-400 CFM), or main trunks (1,000-1,500 CFM), these are your go-to reference points. I've probably sized a thousand ducts using just these seven values—they cover 90% of residential HVAC applications.

Understanding When to Use Each Velocity Target

Velocity selection isn't random—it's driven by noise tolerance, space type, and duct location. Here's how I choose velocity targets on actual jobs:

How to Read and Use This Chart Like a Pro

Start with your known variable—usually CFM from your load calculation. Find that CFM in the left column (or the closest value—interpolate for in-between numbers). Scan across the row to your target velocity column. That intersection shows your round duct size. For example, 300 CFM at 700 FPM needs an 8-inch round duct. The rectangular column shows common equivalent sizes—300 CFM could use 5x12 or 6x10 rectangular if space demands it. Remember that rectangular ducts need slightly larger cross-sectional area than the equivalent round duct due to corner losses.

Working backwards from existing ducts? Measure the duct diameter, find it in the appropriate velocity column, then scan left to see maximum CFM capacity. I found a 7-inch duct on an inspection and needed to verify it wasn't overloaded. The 700 FPM column shows 7-inch handles about 200-250 CFM comfortably. I measured actual airflow at 185 CFM—perfect sizing, well within capacity. If I'd measured 350 CFM through that 7-inch duct, the chart would've screamed trouble: 350 CFM needs 9 inches at 700 FPM, meaning that 7-inch duct was running close to 1,000 FPM—way too fast, explaining the noise complaint. The chart works both directions: CFM to size, or size to CFM capacity.

Practical Tips for Using the Chart in Real Applications

Manual D Duct Design Basics

Manual D is the ACCA standard for residential duct design—the rule book for proper HVAC systems. Manual J calculates loads room by room (how many BTUs each space needs), Manual S selects equipment (which furnace and AC to install), and Manual D designs the duct system (sizing every duct run, grille, and fitting). The process starts with total system CFM—usually 400 CFM per ton of cooling (a 3-ton system moves 1,200 CFM). That total splits proportionally to room loads. If your whole house needs 36,000 BTUs and the master bedroom calculates at 10,800 BTUs, the bedroom gets 30% of total CFM (360 CFM of the 1,200 total). Now you size the duct: 360 CFM at 700 FPM needs a 9-inch round duct or 6x12 rectangular.

The full Manual D process accounts for duct material (smooth metal vs. flex), duct length (friction compounds), fittings (every elbow, tee, and transition adds loss), and available static pressure (your blower's capacity). I ran a complete Manual D on a 2,400-square-foot two-story last month. Total cooling: 38,000 BTUs (3.17 tons), rounded to a 3.5-ton system at 1,400 CFM. Eight supply runs ranged from 120 CFM (small bathroom) to 340 CFM (great room). The longest run—62 feet to the master bedroom—needed 280 CFM. Quick math said 8-inch duct, but Manual D dug deeper: 62 feet of flex duct at 0.074 per 100 feet equals 0.046 friction loss, plus three elbows (0.015 each, 0.045 total), plus grille and boot (0.08 combined). Total static pressure for that run: 0.171 inches. With 0.5 total available and 8 runs to feed, we had budget to spare. But if we'd used 7-inch duct to save $20? Friction jumps to 0.12 per 100 feet, that run alone eats 0.233 inches, and suddenly we're tight on static budget. Proper sizing isn't just about velocity—it's system-wide pressure management.

Practical Examples With Real Numbers

Let's size ducts for a real house—my neighbor's 1,800-square-foot ranch. Manual J showed 30,000 BTUs cooling (2.5 tons), so we're moving 1,000 CFM total. Four bedrooms, one bath, living room, kitchen, dining room—eight zones total. The master bedroom calculated at 9,000 BTUs (30% of load), earning 300 CFM. Target velocity: 650 FPM for quiet sleeping. Calculator says 8-inch round duct, velocity 689 FPM, friction 0.075 per 100 feet. The attic has room, 8-inch round fits perfectly, friction loss over the 35-foot run is just 0.026 inches. Easy decision: 8-inch it is. The kids' bedrooms? Each needs 6,000 BTUs (20% load), getting 200 CFM apiece. At 650 FPM, that's 7-inch round duct—but we used 8-inch anyway for two reasons: future-proofing (kids get older, add electronics and heat load) and bulk pricing (buying 250 feet of 8-inch was cheaper per foot than mixing sizes).

The great room presented challenges—combination living/dining/kitchen, 12,000 BTUs (40% of load), needing 400 CFM. Standard 10-inch round duct at 700 FPM would work great, but the room is 45 feet from the furnace across eight 2x8 floor joists. A 10-inch duct won't fit through 7.25-inch joist spaces. Options: drop ceiling (ugly), run ducts under the floor (expensive crawlspace work), or go rectangular. We chose 6x14 rectangular ducts (84 square inches, equivalent diameter 10.3 inches) at aspect ratio 2.33:1. Velocity calculated at 690 FPM, friction 0.084 per 100 feet. The 45-foot run costs 0.038 inches friction loss—totally acceptable. Those rectangular ducts fit perfectly between joists with 1-inch clearance on each side for insulation. Cost? $140 in sheet metal versus $800 to frame a soffit. The kitchen needed extra: appliances add heat, so we bumped the 400 CFM to 450 CFM using 7x14 rectangular (same 6x14 duct with one side extended). Velocity rose to 740 FPM—still quiet for a kitchen where noise isn't critical.

Return Air Duct Sizing

Return ducts are the forgotten stepchild of HVAC design—everyone obsesses over supply ducts while returns get a "whatever fits" approach. Big mistake. Your blower pulls air through the return, pushes it through supply ducts. Undersized returns strangle the blower like running a marathon breathing through your nose. I diagnosed a system pulling 1,200 CFM through a single 12x12 return grille. That's 900 square inches needed for 1,200 CFM at 80 FPM face velocity—we had 144 square inches. Return velocity screamed at 500 FPM (should be under 300), sucking the grille against the wall with a groaning sound. The blower worked so hard fighting return restriction that it overheated weekly. We added two more 14x20 returns, total area jumped to 704 square inches, velocity dropped to 102 FPM, and the groan disappeared.

Return duct sizing is simpler than supply—just size for low velocity and unrestricted flow. Rule of thumb: 2 square inches of free area per CFM for grilles (1,200 CFM needs 600 square inches of grille face), and return ducts oversized compared to supply. If your main supply trunk is 20-inch round moving 1,200 CFM at 530 FPM, your return should be 24-inch round moving that same 1,200 CFM at just 368 FPM. Slower return air means lower restriction, quieter operation, and easier breathing for your blower. I see new construction homes with massive 3-ton units and single 16x20 return grilles (320 square inches for 1,200 CFM). That's criminally undersized—velocity hits 395 FPM, static pressure at the return jumps 0.15 inches, and the blower loses 15% capacity fighting that restriction. Add two more returns (900 square inches total), and suddenly the system delivers rated airflow. Returns are cheap—there's zero excuse for undersizing them.

Ductwork Materials and Installation

Rigid sheet metal ductwork is the Cadillac of duct materials—smooth interior, zero compression, lasts 50+ years. Galvanized steel is standard: 26-gauge for residential, pre-fabricated sections with snap-lock seams. A 10-inch round duct section costs $12-15, 8-inch runs $9-11. Installation requires tin snips, drive cleats, and screws—basic tools but moderate skill. The payoff? Friction loss exactly matches calculated values, no performance degradation over time, and you can seal joints properly with mastic. I renovated a 1965 house last year with original sheet metal ducts still performing flawlessly—minor rust on exteriors, but interiors smooth and clean. Compare that to flex duct from the same era (doesn't exist—it would've disintegrated decades ago).

Flex duct—that insulated silver fabric tube—dominates new construction because it's fast and cheap. A 25-foot section of 8-inch insulated flex duct costs $45-50 versus $120+ in sheet metal fittings and sections to cover the same run. Installation is stupid simple: stretch it tight, secure with clamps, done. But the tradeoffs bite you: interior liner isn't smooth (friction loss runs 15-20% higher than sheet metal), any compression kills performance, and it sags over time if not properly supported. Maximum recommended length for flex duct runs is 10-15 feet—longer than that, and you're asking for problems. The smart approach I use: rigid metal for main trunks and long runs, flex duct for final 6-10 foot connections to boots. This hybrid system costs 20% more than all-flex but performs 40% better long-term. One client saved $180 on installation by using all flex duct. Five years later, he spent $1,400 replacing sagging, compressed runs that had lost 35% airflow. Sometimes cheap is expensive.

Duct Fittings and Transitions

Every elbow, tee, and transition adds friction loss—the hidden tax on airflow nobody calculates until pressure problems appear. A smooth 90-degree elbow in sheet metal adds about 10 feet of equivalent length (if your friction loss is 0.08 per 100 feet, that elbow costs 0.008 inches). Stack three elbows in a run, and you've added 30 equivalent feet—suddenly your 40-foot duct performs like 70 feet with proportional pressure loss. Flex duct elbows are worse: bend a flex duct 90 degrees, and you've crimped the liner, created turbulence, and added 25-30 equivalent feet. I measured airflow before and after replacing a U-shaped flex duct run (two 90-degree bends in 8 feet of duct) with a smooth swept elbow. Airflow jumped from 245 CFM to 315 CFM—same blower, same duct size, just better fittings.

Transitions need careful design—going from 10-inch to 8-inch duct can't happen in 6 inches or airflow separates, creating turbulence and pressure loss. The rule: taper angle under 15 degrees. Transitioning from 10-inch (10-inch diameter) to 8-inch (8-inch diameter) means reducing 2 inches in diameter. At 15 degrees, you need 7.6 inches of length for that transition—round up to 8 inches minimum. I've seen duct boots with 10-inch duct crimped directly into a 4x10 boot opening—a brutal transition that shreds airflow efficiency. Proper approach: use a gradual reducing fitting, transitioning smoothly over 12-18 inches. The $15 fitting costs peanuts compared to the performance gain. Wyes and tees are pressure killers: a straight tee (one supply trunk splitting into two branches) can add 35-50 equivalent feet depending on design. Angled wyes with 45-degree takeoffs perform better—maybe 20-25 equivalent feet. When you've got static pressure budget to spare, fittings don't matter much. When you're tight on pressure (complex system, long runs, multiple zones), every fitting needs optimization.

Duct Insulation and Sealing

Duct insulation serves two purposes: preventing condensation and reducing heat gain/loss. Ducts in unconditioned spaces (attics, crawlspaces, garages) absolutely need insulation—R-6 minimum, R-8 preferred. I measured a system with uninsulated ducts in a 135-degree Texas attic. Supply air left the plenum at 58 degrees, arrived at the register at 68 degrees—10-degree gain from attic heat soaking through metal ducts. That's 15-20% cooling capacity lost before air even enters the room. We wrapped those ducts in R-8 duct insulation (fiberglass with foil vapor barrier), and supply temps dropped to 60 degrees at the register. Energy savings? About $35 monthly, insulation paid for itself in 18 months. Flex duct comes pre-insulated (typically R-4.2 or R-6), which is adequate for most attic installations but marginal in extreme climates. For ducts in conditioned basements or between floors, insulation isn't critical for energy—but it prevents condensation when cold AC air meets humid air.

Duct sealing is non-negotiable—leaky ductwork throws money out of your house with every cycle. The Department of Energy found that typical duct systems leak 20-30% of conditioned air through gaps, holes, and poor connections. Your 1,200 CFM system? It's actually delivering 850 CFM—the other 350 CFM is cooling your attic or crawlspace. I pressure-tested a system last month showing 380 CFM of leakage at 25 pascals test pressure. That's $95 monthly in wasted energy, year after year. We sealed it with mastic (the gooey paste that actually works) on every joint and seam—not duct tape, which fails in 3-5 years despite its name. Post-sealing leakage: 68 CFM. Cost of materials? $85. Annual energy savings? $1,140. The sealing paid for itself in three weeks. Mastic is messy—looks like peanut butter, goes on with a gloved hand or brush—but it lasts decades and seals gaps up to 1/4 inch. For larger gaps, use fiberglass mesh tape embedded in mastic. Spray foam sealers work but cost 5x more than mastic. Bottom line: seal every duct joint like your wallet depends on it, because it does.

Special Considerations for Ductwork

High-velocity mini-duct systems flip conventional duct sizing upside down. Standard HVAC runs 600-900 FPM through large ducts. High-velocity systems scream at 1,800-2,500 FPM through 2-inch flexible tubes, using specialized equipment and entirely different physics. I installed an Unico system in a historic renovation where we couldn't fit conventional ducts. Those tiny 2-inch supplies deliver 40-60 CFM each at face-melting velocity, but specialized diffusers slow the air and spread it quietly into rooms. The trunk lines run 6-8 inch insulated flex at 1,200-1,500 FPM. Total system CFM is lower (800 CFM for a 2-ton system vs. conventional 800 CFM), but the equipment is engineered specifically for high static pressure. Don't try sizing these systems with a standard ductulator—the manufacturers provide detailed engineering guides, and deviation means disaster.

Zoned systems need zone dampers, bypass dampers, and careful balancing to avoid disaster. When you close zone dampers (blocking airflow to unoccupied areas), static pressure spikes—your blower suddenly fights closed dampers instead of open ducts. Three solutions exist: variable-speed blower that ramps down when pressure rises (best option, built into modern ECM motors), bypass damper that dumps excess air back to the return (simple but wastes energy), or zone-specific equipment (multiple small systems instead of one large zoned system). I installed a four-zone system last year with a basic single-speed blower and no bypass damper—total amateur move. With all zones open, static pressure sat at 0.48 inches, perfect. Close three zones, and pressure spiked to 0.92 inches. The blower screamed, airflow dropped 40%, and the one open zone froze. We added a 12-inch bypass damper from supply plenum to return plenum, solving the pressure problem but wasting 200 CFM of conditioned air when zones closed. Lesson: zone dampers require compatible equipment and proper design, or you've built an expensive disaster.

Common Duct Sizing Questions Answered

"Can I just use 6-inch ducts for everything?" No, unless you enjoy noise, poor airflow, and sky-high energy bills. Six-inch ducts max out around 100-120 CFM at acceptable velocity. Anything more, and you're in jet-engine territory with friction loss through the roof. "What about those flexible duct kits at Home Depot?" They work for small rooms—bathrooms, closets, bonus spaces—if sized correctly. But the 25-foot run of 6-inch flex rated at "200 CFM" only delivers that if pulled perfectly straight with zero compression. Reality? You'll see 140-160 CFM after typical installation mistakes. For critical rooms or long runs, hire a pro or at minimum use rigid duct for mains and only short flex connections to boots.

"My contractor says bigger ducts are always better." Your contractor is wrong—and probably lazy. Oversized ducts cost more, take more space, and can cause velocity issues in heating mode where you want 600-900 FPM to throw heat across rooms. A 14-inch duct moving 600 CFM only flows at 450 FPM—not enough velocity for proper air mixing, leading to stratification where heat pools at the ceiling. Proper sizing means matching duct diameter to CFM and velocity targets, period. "Do I really need to calculate friction loss?" For simple systems (single-story ranch, short duct runs, straightforward layout), you can probably skip detailed friction calculations and rely on velocity targets. For complex systems (two-story, long runs, multiple zones, tight static budgets), friction loss calculations prevent expensive failures. This calculator shows friction loss automatically—use it to verify you're not in the danger zone above 0.15 per 100 feet, where pressure problems lurk.

Next Steps After Duct Sizing

Got your duct sizes calculated? Print or screenshot the results—you'll reference them during material ordering and installation. Next, sketch your duct layout showing every supply and return run, measuring actual distances in your attic or basement. Count fittings: every elbow, tee, transition, and boot. This gives you a shopping list. For a typical 1,500-square-foot home with 8 supply runs, expect to buy: 200-250 feet of trunk duct (large diameter, 14-18 inch round or equivalent rectangular), 150-200 feet of branch ducts (6-10 inch round based on your calculations), 30-50 feet of flex duct for final connections, 8 supply boots, 2-3 return grilles, 1 filter base, and about $120 in fittings (elbows, wyes, end caps, starting collars). Don't forget mastic (two gallons minimum) and foil tape for sealing.

Installation sequence matters: main trunks first, then branch takeoffs, finally flex connections to boots. Support everything—rigid duct needs hangers every 8-10 feet, flex duct every 4 feet maximum with straps that don't compress the duct. Pull flex tight with less than 1% sag. Seal every joint with mastic, then cover with foil tape for UV protection and durability. After installation, commission the system: measure airflow at each register (use a flow hood if you're fancy, a cardboard box and stopwatch if you're budget), verify velocities fall in the 600-900 FPM range, and check total static pressure at the blower (should be under 0.5 inches for residential). If numbers look wrong, diagnose before drywall goes up. I've torn out and redone entire systems after drywall when someone skipped commissioning—costs 4x more than fixing it during rough-in. For detailed system design beyond individual duct sizing, check our residential load calculator for complete Manual J/D calculations. Your perfectly sized ducts mean nothing if the system design is wrong.