Home Kilowatt

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.

A tape measure across the floor of an empty, bright room.

BTU calculator

C01

Scenario A

Sets the electricity rate. Pick US average if you're not sure.

$/kWh

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) ·

sq ft
ft

Recommended cooling capacity

8,000BTU/h

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.

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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.

A tape measure across the floor of an empty, bright room.

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.

Open this example in the calculator

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 average8,000 BTU/h18.4¢/kWh
Alabama8,800 BTU/h15.5¢/kWh
Alaska6,400 BTU/h25.0¢/kWh
Arizona9,600 BTU/h15.0¢/kWh
Arkansas8,800 BTU/h13.0¢/kWh
California8,800 BTU/h32.0¢/kWh
Colorado7,200 BTU/h16.0¢/kWh
Connecticut7,200 BTU/h30.0¢/kWh
Delaware8,000 BTU/h17.0¢/kWh
District of Columbia8,000 BTU/h19.0¢/kWh
Florida9,600 BTU/h15.0¢/kWh
Georgia8,800 BTU/h14.5¢/kWh
Hawaii10,400 BTU/h42.0¢/kWh
Idaho7,200 BTU/h12.0¢/kWh
Illinois7,200 BTU/h17.0¢/kWh
Indiana7,200 BTU/h15.5¢/kWh
Iowa7,200 BTU/h13.5¢/kWh
Kansas8,000 BTU/h14.5¢/kWh
Kentucky8,000 BTU/h13.0¢/kWh
Louisiana9,600 BTU/h12.5¢/kWh
Maine6,800 BTU/h26.0¢/kWh
Maryland8,000 BTU/h19.0¢/kWh
Massachusetts7,200 BTU/h29.0¢/kWh
Michigan7,200 BTU/h19.5¢/kWh
Minnesota6,800 BTU/h15.5¢/kWh
Mississippi8,800 BTU/h13.5¢/kWh
Missouri8,000 BTU/h13.0¢/kWh
Montana6,800 BTU/h13.0¢/kWh
Nebraska7,200 BTU/h12.0¢/kWh
Nevada8,800 BTU/h15.0¢/kWh
New Hampshire6,800 BTU/h27.0¢/kWh
New Jersey8,000 BTU/h22.0¢/kWh
New Mexico8,000 BTU/h15.0¢/kWh
New York7,200 BTU/h25.0¢/kWh
North Carolina8,800 BTU/h14.5¢/kWh
North Dakota6,400 BTU/h11.5¢/kWh
Ohio7,200 BTU/h16.5¢/kWh
Oklahoma8,800 BTU/h13.0¢/kWh
Oregon8,000 BTU/h14.5¢/kWh
Pennsylvania7,200 BTU/h18.5¢/kWh
Rhode Island7,200 BTU/h29.0¢/kWh
South Carolina8,800 BTU/h15.0¢/kWh
South Dakota6,800 BTU/h13.0¢/kWh
Tennessee8,000 BTU/h13.5¢/kWh
Texas9,600 BTU/h15.5¢/kWh
Utah7,200 BTU/h12.0¢/kWh
Vermont6,800 BTU/h22.0¢/kWh
Virginia8,000 BTU/h15.5¢/kWh
Washington8,000 BTU/h12.0¢/kWh
West Virginia7,200 BTU/h15.5¢/kWh
Wisconsin6,800 BTU/h17.5¢/kWh
Wyoming6,800 BTU/h12.5¢/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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