IECC climate zones tell you two things at a glance: how cold your winters get and how hot your summers get. The number captures temperature severity, and the letter captures humidity. A house in zone 6A (Minneapolis) needs a very different HVAC setup than a house in zone 3C (Los Angeles), even if both buildings are the same square footage.
Zone boundaries follow county lines (not ZIP codes), and the IECC uses heating degree days (HDD) and cooling degree days (CDD) to assign each county to a zone. Higher HDD means longer, colder winters. Higher CDD means longer, hotter summers. For HVAC design, you also need the local 99 percent winter and 1 percent summer design temperatures, which are the outdoor temperatures your system must handle without falling behind.
Find Your Climate Zone by City
These are the most-searched US cities and their IECC climate zones. Click any zone to see the design temperatures, degree days, and HVAC equipment guidance for that zone.
| City | State | IECC zone | What it means |
|---|---|---|---|
| Miami | Florida | 1A | Very Hot, moist |
| Honolulu | Hawaii | 1A | Very Hot, moist |
| Phoenix | Arizona | 1B | Very Hot, dry |
| Las Vegas | Nevada | 1B | Very Hot, dry |
| Houston | Texas | 2A | Hot, moist |
| New Orleans | Louisiana | 2A | Hot, moist |
| Jacksonville | Florida | 2A | Hot, moist |
| El Paso | Texas | 2B | Hot, dry |
| Atlanta | Georgia | 3A | Warm, moist |
| Charlotte | North Carolina | 3A | Warm, moist |
| Memphis | Tennessee | 3A | Warm, moist |
| Dallas | Texas | 3A | Warm, moist |
| Austin | Texas | 3B | Warm, dry |
| San Antonio | Texas | 3B | Warm, dry |
| Los Angeles | California | 3C | Warm, marine |
| San Francisco | California | 3C | Warm, marine |
| San Diego | California | 3C | Warm, marine |
| Richmond | Virginia | 4A | Mixed, moist |
| Kansas City | Missouri | 4A | Mixed, moist |
| Baltimore | Maryland | 4A | Mixed, moist |
| St. Louis | Missouri | 4A | Mixed, moist |
| Albuquerque | New Mexico | 4B | Mixed, dry |
| Seattle | Washington | 4C | Mixed, marine |
| Portland | Oregon | 4C | Mixed, marine |
| Chicago | Illinois | 5A | Cool, moist |
| Indianapolis | Indiana | 5A | Cool, moist |
| Philadelphia | Pennsylvania | 5A | Cool, moist |
| Boston | Massachusetts | 5A | Cool, moist |
| Denver | Colorado | 5B | Cool, dry |
| Salt Lake City | Utah | 5B | Cool, dry |
| Minneapolis | Minnesota | 6A | Cold, moist |
| Detroit | Michigan | 6A | Cold, moist |
| Milwaukee | Wisconsin | 6A | Cold, moist |
| Burlington | Vermont | 6A | Cold, moist |
| Helena | Montana | 6B | Cold, dry |
| Duluth | Minnesota | 7 | Very Cold, variable |
| Fargo | North Dakota | 7 | Very Cold, variable |
| Anchorage | Alaska | 7 | Very Cold, variable |
| Fairbanks | Alaska | 8 | Subarctic, variable |
What the Zone Numbers Mean
The number is temperature severity, measured by heating and cooling degree days. Lower numbers are hotter, higher numbers are colder.
| Zone | Climate | Summer design temp | Winter design temp | HVAC focus |
|---|---|---|---|---|
| 1 | Very hot | 95 to 115°F | 25 to 50°F | Cooling and dehumidification |
| 2 | Hot | 90 to 105°F | 15 to 35°F | Cooling dominates, some heat |
| 3 | Warm | 75 to 95°F | 10 to 40°F | Balanced, heat pump territory |
| 4 | Mixed | 70 to 95°F | 0 to 35°F | Real heating and cooling load |
| 5 | Cool | 80 to 90°F | -10 to 15°F | Heating leads, AC supports |
| 6 | Cold | 75 to 85°F | -20 to 5°F | Heating dominates, cold-climate equipment |
| 7 | Very cold | 65 to 75°F | -30 to -10°F | Heating only matters |
| 8 | Subarctic | 60 to 70°F | -50 to -20°F | Extreme heat, no cooling |
What the Letters Mean
The letter after the number tells you the humidity pattern. This matters because cooling load is not just about temperature. A house in a humid climate spends a lot of cooling capacity removing moisture, not just lowering air temperature.
| Letter | Type | What it means | Example cities |
|---|---|---|---|
| A | Moist | High humidity, regular rainfall. Dehumidification matters year-round. | Miami (1A), Houston (2A), Atlanta (3A) |
| B | Dry | Low humidity, arid. Evaporative cooling is an option. Sensible cooling dominates. | Phoenix (1B), El Paso (2B), Denver (5B) |
| C | Marine | Coastal, moderate humidity, mild temperatures. Cooling is minimal, heating is moderate. | San Francisco (3C), Seattle (4C), Portland (4C) |
All US Climate Zones
Sixteen IECC zones cover the US. Each link below opens that zone's design temperatures, degree days, code-required insulation, and HVAC equipment recommendations.
| Zone | Classification | Example states | Example cities |
|---|---|---|---|
| 1A | Very Hot, moist | Florida, Hawaii, Louisiana | Miami, Key West, Honolulu |
| 1B | Very Hot, dry | Arizona, California, Nevada | Phoenix, Tucson, Las Vegas |
| 2A | Hot, moist | Louisiana, Mississippi, Alabama | Houston, New Orleans, Mobile |
| 2B | Hot, dry | Arizona, California, Nevada | Albuquerque, El Paso, Bakersfield |
| 3A | Warm, moist | North Carolina, South Carolina, Georgia | Atlanta, Charlotte, Memphis |
| 3B | Warm, dry | California, Texas, Oklahoma | Fresno, Austin, San Antonio |
| 3C | Warm, marine | California, Oregon | San Francisco, Los Angeles, San Diego |
| 4A | Mixed, moist | Virginia, Kentucky, Missouri | Richmond, Louisville, Kansas City |
| 4B | Mixed, dry | Colorado, Kansas, Oklahoma | Denver, Colorado Springs, Albuquerque |
| 4C | Mixed, marine | Washington, Oregon | Seattle, Portland, Olympia |
| 5A | Cool, moist | Illinois, Indiana, Ohio | Chicago, Indianapolis, Columbus |
| 5B | Cool, dry | Colorado, Wyoming, Utah | Denver, Salt Lake City, Cheyenne |
| 6A | Cold, moist | Minnesota, Wisconsin, Michigan | Minneapolis, Milwaukee, Detroit |
| 6B | Cold, dry | Montana, Wyoming, Idaho | Helena, Casper, Bozeman |
| 7 | Very Cold, variable | Alaska, Minnesota, North Dakota | Duluth, International Falls, Fargo |
| 8 | Subarctic, variable | Alaska | Fairbanks, Barrow, Bettles |
Why Climate Zone Affects HVAC Sizing
Two things change with your climate zone: the design temperatures your system has to handle, and the insulation levels your code requires. Both feed directly into your Manual J load calculation.
A 2,000 square foot house in Phoenix might need 3.5 tons of cooling because the outdoor design temperature is 110°F. The same house in Boston might only need 2.5 tons because the cooling design temperature is 87°F. The Phoenix house also needs very little heating capacity. The Boston house needs a furnace or heat pump that can hold setpoint when it's 5°F outside.
IECC code also sets minimum insulation R-values by zone. Zone 1 might require R-30 in the ceiling. Zone 7 requires R-49 to R-60. Better-insulated houses have lower loads, which means smaller HVAC equipment. Skip the load calculation and you end up with oversized equipment that short-cycles, wastes energy, and never dehumidifies properly. Our free residential load calculator uses your climate zone design temperatures to give you accurate BTU/hr numbers.
How the IECC Climate Map Was Built (And the 2021 Update)
The IECC zone map is not just a guess by code writers. It comes from ASHRAE Standard 169, which is the underlying technical standard that assigns every US county to a zone based on long-term weather data. The IECC pulls the zones directly from ASHRAE 169 and publishes the result in Table C301.1 of the code.
The map sat untouched from 2003 until the 2021 IECC, when ASHRAE rebuilt it using measured temperature data from over 4,000 weather stations across North America covering the previous 25 years. About 10 percent of US counties moved to a different zone in the 2021 update. Almost every county that moved shifted to a warmer (lower) zone, not a colder one. Parts of Kentucky, Tennessee, and Virginia that used to be 4A are now 3A. Counties in the Texas panhandle moved from 3 to 2. The pattern matches the broader warming trend in US climate data.
The 2024 IECC kept the same zone boundaries but tightened airtightness rules for zones 0 through 2 and zones 6 through 8 (lower allowed air changes per hour at 50 pascals during the blower door test). Ceiling insulation requirements were loosened back to 2018 IECC levels after the 2021 code pushed them higher. State adoption of the 2024 IECC is rolling out slowly, so check your state's adopted edition before you assume a number is current.
How Climate Zone Is Calculated (HDD and CDD Thresholds)
Heating degree days and cooling degree days are the math behind the zone assignment. For HDD, the base temperature is 65°F. If the outdoor temperature averages 50°F for one day, that's 15 heating degree days. Add up every day in a year and you get the annual HDD for that location. CDD works the same way using a 50°F base.
ASHRAE 169 uses CDD to assign zones 1 through 4 (where cooling matters more) and HDD to assign zones 3 through 8 (where heating dominates). Zones 3 and 4 use both. Here are the rough cutoffs the standard uses:
| Zone | Heating degree days (HDD65) | Cooling degree days (CDD50) |
|---|---|---|
| 1 | Under 2,000 | Over 5,000 |
| 2 | Under 3,000 | 3,500 to 5,000 |
| 3 | Under 3,500 | 2,500 to 3,500 |
| 4 | Under 5,400 | Under 2,500 |
| 5 | 5,400 to 7,200 | Under 1,500 |
| 6 | 7,200 to 9,000 | Under 1,000 |
| 7 | 9,000 to 12,600 | Under 500 |
| 8 | Over 12,600 | Near 0 |
These thresholds explain why two cities in the same state can end up in different zones. Colorado is the textbook example: Pueblo (lower elevation, fewer HDD) is zone 4B, Denver is zone 5B, Steamboat Springs sits in zone 7. Elevation shifts HDD enough to bump a county up one or even two full zones.
Climate Zone vs ASHRAE 169 vs California Title 24
Three different climate zone systems get mentioned in HVAC and code work, and they don't all mean the same thing. Here's how they differ:
| System | Used for | Zone count | Relationship to IECC |
|---|---|---|---|
| IECC (Table C301.1) | Residential energy code in most US states | 8 zones, A/B/C suffixes | The reference for this page |
| ASHRAE 169 | Technical standard defining the zones | 8 zones, same boundaries | Identical to IECC. IECC adopts ASHRAE 169 directly |
| ASHRAE 90.1 | Commercial energy code | 8 zones, same numbering | Same zones, different building requirements per zone |
| California Title 24 | California-only residential energy code | 16 zones | Does not line up with IECC. California uses both. |
| Building America | DOE research and design guidance | 5 regions (groups IECC zones) | Cold/Very Cold = IECC 5-8. Hot-Humid = IECC 1A-3A |
If you're a homeowner sizing residential HVAC anywhere in the US, IECC is the reference you want. If you're working on commercial construction, your state may use IECC commercial chapter, ASHRAE 90.1, or both. California always needs the Title 24 zone in addition to the IECC zone because the state energy code is stricter than IECC for most measures.
How to Look Up Your Exact Climate Zone
Three reliable lookup paths, in order of how often they work for a homeowner:
- City table above: Most US cities of any size show up in the city table on this page. If your city is listed, click the zone link. Done.
- By county on the IECC map: Climate zones follow county boundaries. Find your county on the Building America Solution Center map (free, maintained by the DOE and Pacific Northwest National Lab) and you'll get the exact zone for any US address.
- By ZIP code on a lookup tool: ZIP codes don't actually map to climate zones (a ZIP can span multiple counties), but most online lookup tools cross-reference your ZIP to its primary county and return the zone. Accurate for the vast majority of US addresses.
Edge cases that trip people up: mountainous counties where elevation changes the zone (parts of one county can be zone 5, the high-elevation parts can be zone 6 or 7), counties that span the boundary between two zones, and US territories that have their own zone definitions (Hawaii uses zone 1A, Puerto Rico uses zone 1A, Guam uses zone 0A under the newer IECC editions).
What Climate Zone Alone Doesn't Tell You
Climate zone is the starting point for HVAC sizing but not the finish line. Knowing you're in 4A doesn't tell you whether your house needs 2.5 tons of cooling or 4 tons. Several other factors layer on top of the zone:
- Building envelope: Wall R-value, attic R-value, window U-factor, and air leakage rate. A leaky uninsulated house in zone 4A can have double the load of a tight well-insulated house in the same zone.
- Orientation and windows: A house with a wall of west-facing glass picks up solar heat gain that adds tons of cooling load. Same square footage with north-facing windows runs cooler.
- Occupancy and internal gains: People, appliances, and lighting all add heat. A 2,000 square foot home with 6 people, two ovens, and a server rack has a different load than the same shell with 2 people.
- Microclimate: Coastal vs inland, urban heat island, valley vs ridgetop. Local conditions can shift design temperatures 5 to 10°F from the regional average.
- Ductwork condition: Leaky ducts in unconditioned attic space add 15 to 25 percent to your load on top of everything else.
This is why HVAC sizing requires a real Manual J calculation, not just a square-footage-per-ton rule of thumb. Climate zone sets the design temperatures you plug in. The Manual J does the math that turns those temperatures plus your house's specifics into a BTU/hr number. See our Manual J load calculation guide for what a complete calculation requires.
Frequently Asked Questions
What climate zone is Miami in?
Miami sits in IECC climate zone 1A (very hot, humid). That means cooling and dehumidification dominate. Heating loads are minimal, so most homes use a heat pump or air conditioner with no separate furnace.
What climate zone is Phoenix in?
Phoenix is in IECC climate zone 1B (very hot, dry). Summer design temperatures hit 110 to 115 degrees, so cooling capacity is the main concern. Humidity is low year-round, which makes evaporative cooling an option in some buildings.
What climate zone is Houston in?
Houston is in IECC climate zone 2A (hot, humid). Cooling dominates but winters still pull a meaningful heating load. Most houses run a heat pump with strip backup or a furnace plus AC combo.
What climate zone is Atlanta in?
Atlanta is in IECC climate zone 3A (warm, humid). Heating and cooling loads are both meaningful, which makes Atlanta classic heat pump territory.
What climate zone is Chicago in?
Chicago is in IECC climate zone 5A (cool, humid). Winters are long and cold, so heating drives the system selection. Most Chicago homes run a gas furnace with central AC, though cold-climate heat pumps are growing fast.
What climate zone is Minneapolis in?
Minneapolis is in IECC climate zone 6A (cold, humid). Heating degree days run 6,000 to 8,500. High-efficiency 95 percent AFUE furnaces, cold-climate heat pumps, and strong envelope insulation are standard.
How do I find my climate zone by ZIP code?
IECC climate zones follow county lines, not ZIP codes. Look up your county on the IECC climate zone map or the Building America Solution Center tool, then cross-reference with the city table on this page. Every major US city we list is mapped to its climate zone here.
What does the letter after the zone number mean?
The letter is the moisture suffix. A means moist (humid summers, regular rain). B means dry (arid, low humidity). C means marine (coastal, mild, often foggy). So 3A is warm and humid (Atlanta), 3B is warm and dry (San Antonio), and 3C is warm and marine (Los Angeles).
Which climate zones are heating dominated vs cooling dominated?
Zones 1 and 2 are cooling dominated. Zones 3 and 4 are balanced. Zones 5 and up are heating dominated. Zones 7 and 8 have almost no cooling load and equipment selection is driven entirely by winter performance.
Why does my climate zone matter for HVAC sizing?
Your climate zone sets the design temperatures (the hottest summer day and coldest winter day your system needs to handle), the insulation R-values your code requires, and the equipment efficiencies that make sense. A 3-ton AC sized for Phoenix is not the same size as a 3-ton AC sized for Boston. Use our free residential load calculator to plug in your climate zone and get accurate BTU/hr numbers for your specific home.