Montana climate zones and HVAC design temperatures
All of Montana is in IECC climate zone 6B (cold, dry). In Billings, heating and cooling equipment is sized for 92°F in summer and -4°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Billings | 6B | 92°F | -4°F | 51°F |
| Missoula | 6B | 92°F | 4°F | 48°F |
| Great Falls | 6B | 91°F | -10°F | 48°F |
| Bozeman | 6B | 89°F | -6°F | 46°F |
| Helena | 6B | 92°F | -5°F | 49°F |
Summer: exceeded in 1% of hours. Winter: colder in only 1% of hours. Water in: estimated groundwater temperature for tankless water heater sizing. Computed for each county from NOAA hourly weather data, 2015-2024; see where our numbers come from. Hottest summer design in the state: 94°F (Custer County); coldest winter design: -18°F (Sheridan County).
Typical equipment sizes in Billings
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Billings's design temperatures. Your house can need more or less; use the calculators for yours.
| House | Central AC | Gas furnace (95% AFUE) | Heat pump |
|---|---|---|---|
| 1,500 sq ft | 2 tons | 40,000 BTU | 2 tons + 10 kW backup |
| 2,000 sq ft | 2.5 tons | 60,000 BTU | 3 tons + 10 kW backup |
| 2,500 sq ft | 3 tons | 60,000 BTU | 3.5 tons + 15 kW backup |
AC size calculator Furnace size calculator Heat pump size calculator
Climate zone by county
From the 2021 International Energy Conservation Code (IECC), as published by the U.S. Department of Energy. The zone sets the insulation and window requirements in the energy code; the number runs from 1 (hottest) to 8 (coldest), and the letter is A for moist, B for dry and C for marine.
Zone 6B: cold, dry (56 counties, 100% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Beaverhead County | 86°F | -4°F |
| Big Horn County | 92°F | -4°F |
| Blaine County | 93°F | -15°F |
| Broadwater County | 92°F | -5°F |
| Carbon County | 88°F | -4°F |
| Carter County | 93°F | -6°F |
| Cascade County | 91°F | -10°F |
| Chouteau County | 90°F | -11°F |
| Custer County | 94°F | -9°F |
| Daniels County | 94°F | -16°F |
| Dawson County | 92°F | -12°F |
| Deer Lodge County | 86°F | -9°F |
| Fallon County | 92°F | -11°F |
| Fergus County | 88°F | -9°F |
| Flathead County | 89°F | 2°F |
| Gallatin County | 89°F | -6°F |
| Garfield County | 94°F | -14°F |
| Glacier County | 88°F | -14°F |
| Golden Valley County | 92°F | -4°F |
| Granite County | 92°F | 4°F |
| Hill County | 93°F | -15°F |
| Jefferson County | 92°F | -5°F |
| Judith Basin County | 86°F | -8°F |
| Lake County | 92°F | 4°F |
| Lewis and Clark County | 92°F | -5°F |
| Liberty County | 93°F | -15°F |
| Lincoln County | 90°F | 9°F |
| Madison County | 86°F | -4°F |
| McCone County | 94°F | -16°F |
| Meagher County | 86°F | -8°F |
| Mineral County | 92°F | 4°F |
| Missoula County | 92°F | 4°F |
| Musselshell County | 92°F | -4°F |
| Park County | 91°F | -5°F |
| Petroleum County | 88°F | -9°F |
| Phillips County | 94°F | -14°F |
| Pondera County | 88°F | -14°F |
| Powder River County | 94°F | -9°F |
| Powell County | 86°F | -9°F |
| Prairie County | 92°F | -12°F |
| Ravalli County | 92°F | 4°F |
| Richland County | 91°F | -15°F |
| Roosevelt County | 94°F | -16°F |
| Rosebud County | 94°F | -9°F |
| Sanders County | 92°F | 4°F |
| Sheridan County | 86°F | -18°F |
| Silver Bow County | 86°F | -9°F |
| Stillwater County | 92°F | -4°F |
| Sweet Grass County | 91°F | -5°F |
| Teton County | 91°F | -10°F |
| Toole County | 88°F | -14°F |
| Treasure County | 94°F | -9°F |
| Valley County | 94°F | -14°F |
| Wheatland County | 88°F | -9°F |
| Wibaux County | 90°F | -13°F |
| Yellowstone County | 92°F | -4°F |
Climate data as of 2026-10. Design temperatures may differ by a few degrees from the ACCA Manual J tables your contractor uses; winter values run a little warmer because recent winters have been milder.