BTU Calculator by Room Size & Climate Zone
Enter the room's size, ceiling height and sun exposure, pick your state, and get the cooling BTU/h and tons it needs for a window AC, portable AC or a single mini split head.
BTU calculator
C01Scenario A
Sets the electricity rate. Pick US average if you're not sure.
From EIA for US average. Use the rate on your bill if you know it.
Rate used: 18.4¢/kWh · US average · EIA reference 2025–26 · climate zone 4 (data as of 2026-09-26) ·
Recommended cooling capacity
Pick the size at or just above the load: 12,000 BTU. For a whole house, use a Manual J load calculation.
- Base BTU (20 × sq ft × climate-zone factor)
- 8,000
- Adjustments (sun, kitchen, people, ceiling)
- 0
- Recommended BTU/h
- 8,000
- Tons
- 0.7
- Nearest standard size
- 6,000 BTU
- Next size up
- 12,000 BTU
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
Formula
How it works
The calculator starts from the ENERGY STAR room-AC rule of 20 BTU per square foot and scales it by your state's climate zone, then adds or subtracts for the room itself:
- Base BTU/h = square feet × 20 × zone factor. Zone factors: zone 1 = 1.3, zone 2 = 1.2, zone 3 = 1.1, zone 4 = 1.0, zone 5 = 0.9, zone 6 = 0.85, zones 7 and 8 = 0.8.
- Sun: +10% of base for a sunny room, −10% for a shaded one.
- Kitchen: +4,000 BTU/h.
- People: +600 BTU/h for each person above 2.
- Ceiling: +10% of base for every 8 ft above an 8 ft ceiling, prorated (a 10 ft ceiling adds 2.5% of base).
- Recommended BTU/h = base + adjustments. Tons = BTU/h ÷ 12,000.
This sizes one room. For a whole house, the heat-loss and heat-gain math is ACCA Manual J, which an installer runs with your windows, insulation and orientation.
Sources: 20 BTU/sq ft base and the sun, kitchen and occupancy adjustments: ENERGY STAR room air conditioner sizing guidance. Climate zone per state: IECC 2021 (DOE/PNNL), zone factor derived from cooling degree-days (NOAA CPC normals 1991–2020).
Worked example
A worked example
Arizona is climate zone 2, so the zone factor is 1.2: 600 sq ft × 20 × 1.2 = 14,400 BTU/h of base load. The sun adds 10%, the third person adds 600 BTU/h and the 10 ft ceiling adds a little more, for 2,400 BTU/h of adjustments. The room needs about 16,800 BTU/h, or 1.4 tons. The nearest standard size is 18,000 BTU.
Reference
By state, at the default inputs
Recommended cooling capacity, with this page's default inputs and each state's reference prices (EIA reference 2025–26, data as of 2026-09-26).
| US average | 8,000 BTU/h | 18.4¢/kWh |
|---|---|---|
| Alabama | 8,800 BTU/h | 15.5¢/kWh |
| Alaska | 6,400 BTU/h | 25.0¢/kWh |
| Arizona | 9,600 BTU/h | 15.0¢/kWh |
| Arkansas | 8,800 BTU/h | 13.0¢/kWh |
| California | 8,800 BTU/h | 32.0¢/kWh |
| Colorado | 7,200 BTU/h | 16.0¢/kWh |
| Connecticut | 7,200 BTU/h | 30.0¢/kWh |
| Delaware | 8,000 BTU/h | 17.0¢/kWh |
| District of Columbia | 8,000 BTU/h | 19.0¢/kWh |
| Florida | 9,600 BTU/h | 15.0¢/kWh |
| Georgia | 8,800 BTU/h | 14.5¢/kWh |
| Hawaii | 10,400 BTU/h | 42.0¢/kWh |
| Idaho | 7,200 BTU/h | 12.0¢/kWh |
| Illinois | 7,200 BTU/h | 17.0¢/kWh |
| Indiana | 7,200 BTU/h | 15.5¢/kWh |
| Iowa | 7,200 BTU/h | 13.5¢/kWh |
| Kansas | 8,000 BTU/h | 14.5¢/kWh |
| Kentucky | 8,000 BTU/h | 13.0¢/kWh |
| Louisiana | 9,600 BTU/h | 12.5¢/kWh |
| Maine | 6,800 BTU/h | 26.0¢/kWh |
| Maryland | 8,000 BTU/h | 19.0¢/kWh |
| Massachusetts | 7,200 BTU/h | 29.0¢/kWh |
| Michigan | 7,200 BTU/h | 19.5¢/kWh |
| Minnesota | 6,800 BTU/h | 15.5¢/kWh |
| Mississippi | 8,800 BTU/h | 13.5¢/kWh |
| Missouri | 8,000 BTU/h | 13.0¢/kWh |
| Montana | 6,800 BTU/h | 13.0¢/kWh |
| Nebraska | 7,200 BTU/h | 12.0¢/kWh |
| Nevada | 8,800 BTU/h | 15.0¢/kWh |
| New Hampshire | 6,800 BTU/h | 27.0¢/kWh |
| New Jersey | 8,000 BTU/h | 22.0¢/kWh |
| New Mexico | 8,000 BTU/h | 15.0¢/kWh |
| New York | 7,200 BTU/h | 25.0¢/kWh |
| North Carolina | 8,800 BTU/h | 14.5¢/kWh |
| North Dakota | 6,400 BTU/h | 11.5¢/kWh |
| Ohio | 7,200 BTU/h | 16.5¢/kWh |
| Oklahoma | 8,800 BTU/h | 13.0¢/kWh |
| Oregon | 8,000 BTU/h | 14.5¢/kWh |
| Pennsylvania | 7,200 BTU/h | 18.5¢/kWh |
| Rhode Island | 7,200 BTU/h | 29.0¢/kWh |
| South Carolina | 8,800 BTU/h | 15.0¢/kWh |
| South Dakota | 6,800 BTU/h | 13.0¢/kWh |
| Tennessee | 8,000 BTU/h | 13.5¢/kWh |
| Texas | 9,600 BTU/h | 15.5¢/kWh |
| Utah | 7,200 BTU/h | 12.0¢/kWh |
| Vermont | 6,800 BTU/h | 22.0¢/kWh |
| Virginia | 8,000 BTU/h | 15.5¢/kWh |
| Washington | 8,000 BTU/h | 12.0¢/kWh |
| West Virginia | 7,200 BTU/h | 15.5¢/kWh |
| Wisconsin | 6,800 BTU/h | 17.5¢/kWh |
| Wyoming | 6,800 BTU/h | 12.5¢/kWh |
Questions
FAQ
How many BTU do I need for a 12 × 12 room?
A 12 × 12 room is 144 sq ft. At the US average zone factor of 1.0 that's 144 × 20 = 2,880 BTU/h before adjustments. In Arizona (zone 2) it's 3,456 BTU/h; in Minnesota (zone 6) 2,448 BTU/h. That is below the smallest window units, so the smallest unit you can buy will cover it; add a kitchen or a sunny wall and run the numbers again.
Is a bigger air conditioner better?
No. An oversized unit cools the air fast and shuts off before it has pulled moisture out, so the room ends up cold and clammy, and the compressor wears from starting and stopping. Size to the recommended BTU/h and pick the nearest size at or just above it.
How many BTU per square foot by climate?
20 BTU per square foot is the zone 4 baseline. With the zone factors it ranges from 26 BTU/sq ft in zone 1 (Hawaii) and 24 in zone 2 (Texas, Florida, Arizona) down to 16 in zones 7 and 8 (Alaska, North Dakota).
How much does a kitchen add?
4,000 BTU/h, for the heat from the stove, oven and refrigerator. In the Minnesota test case, a shaded 300 sq ft kitchen needs 8,590 BTU/h instead of the 4,590 BTU/h the same room would need as a bedroom.
How many tons is my room?
Divide BTU/h by 12,000. The Arizona example above needs 16,800 BTU/h, or 1.4 tons. Central systems are sized for the whole house; use the Heat Pump Size calculator for that.
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