BTU per Square Foot: Chart by Room Size and Climate
The fastest way to size an air conditioner is to multiply the floor area by a BTU per square foot figure. It is a rough method, but it gets you to the right size range in seconds. This guide gives the usual BTU per square foot values, a sizing chart by room size, and the adjustments that move the number up or down.
The rule of thumb
For cooling a room with an 8 ft ceiling in a moderate climate, use about 20 BTU per square foot:
BTU per hour = Area (sq ft) × 20
A 15 by 20 ft room of 300 sq ft needs about 6,000 BTU per hour. Hot, sunny climates push the figure to 25 to 30 BTU per square foot, and cool climates bring it down to 15 to 20.
Air conditioner BTU chart by room size
| Area (sq ft) | Cooling capacity (BTU/h) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
These are the room air conditioner sizes widely published for energy-efficient sizing. The BTU calculator applies the same idea with your room size, sun, and occupancy.
Adjustments to the BTU per square foot figure
- Sunny room: add 10%.
- Heavily shaded room: subtract 10%.
- More than two people regularly: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU.
- Ceilings above 8 ft: scale up by the ceiling height ÷ 8.
Worked example
A 400 sq ft sunny living room used by four people: 400 × 20 = 8,000; plus 10% = 8,800; plus 2 × 600 = 10,000 BTU per hour. A 10,000 BTU unit fits.
From BTU to tons
Central systems are rated in tons, where one ton is 12,000 BTU per hour. A whole house of 1,800 sq ft at 20 BTU per square foot needs 36,000 BTU, or 3 tons. The AC tonnage calculator does that conversion with climate factors.
Heating BTU per square foot
Heating needs depend far more on climate and insulation. A rough range runs from about 30 BTU per square foot in mild areas to 50 or 60 in cold ones. For a real number, total the losses through walls, windows, and air leaks with the heat load calculator.
Common mistakes
- Oversizing. A unit far above the load short-cycles and leaves the air damp.
- Ignoring ceiling height. The rule assumes 8 ft ceilings.
- Using cooling figures for heating. Heating loads are higher in most climates.
Quick reference
Cooling: about 20 BTU per square foot, 25 to 30 in hot climates; 12,000 BTU = 1 ton. Size any room with a free online calculator from CalculateGuide.