How the BTU Calculator Works
A BTU (British thermal unit) is a small amount of heat, and BTU per hour (BTU/h) is a measure of power. It tells how much heat an air conditioner can take out of a room in one hour. If the air conditioner is too small, the room does not get cool. If it is too big, it turns on and off too often, cools the air too fast to remove the moisture, and the room feels cold and damp. The BTU Calculator estimates the size that a room needs.
The basic formula
capacity (BTU per hour) = area (square feet) x BTU for each square foot
How many BTU each square foot needs depends on the climate:
| Climate | BTU per sq ft | 500 sq ft |
|---|---|---|
| Cold (north) | 20 | 10,000 |
| Moderate | 25 | 12,500 |
| Hot (south or desert) | 35 | 17,500 |
The chart compares the three climates for a room of 500 square feet. Each block is about 440 BTU/h.
Cold (20) ███████████████████████ 10,000 BTU/h Moderate (25) █████████████████████████████ 12,500 BTU/h Hot (35) ████████████████████████████████████████ 17,500 BTU/h
The corrections
A room is not only its floor area. The calculator adds four corrections that are used by many sizing guides. With the usual choices (a ceiling of 8 feet, an average amount of sun, 2 people and no kitchen), the result is only the basic formula.
| Condition | Correction |
|---|---|
| Ceiling higher or lower than 8 feet | Multiply by the height / 8. A ceiling of 10 feet gives x 1.25. |
| Very sunny room | Add 10% |
| Shady room | Take off 10% |
| More than 2 people | Add 600 BTU/h for each person above 2 |
| Kitchen | Add 4,000 BTU/h, because of the heat of the stove and the oven |
Worked example: 500 sq ft, moderate climate, with all the corrections
| Step | Calculation | Result |
|---|---|---|
| Base | 500 x 25 | 12,500 |
| Ceiling of 10 feet | 12,500 x 10 / 8 | 15,625 |
| Very sunny room | 15,625 x 1.10 | 17,188 |
| 5 people | + 3 x 600 | 18,988 |
| Kitchen | + 4,000 | 22,988 BTU/h |
The bars show how each correction adds to the total. Each block is about 575 BTU/h.
Base ██████████████████████ 12,500 Ceiling 10 ft ███████████████████████████ 15,625 Very sunny ██████████████████████████████ 17,188 5 people █████████████████████████████████ 18,988 Kitchen ████████████████████████████████████████ 22,988
Table: the capacity for different areas
| Area (sq ft) | Cold (20) | Moderate (25) | Hot (35) |
|---|---|---|---|
| 200 | 4,000 | 5,000 | 7,000 |
| 400 | 8,000 | 10,000 | 14,000 |
| 600 | 12,000 | 15,000 | 21,000 |
| 800 | 16,000 | 20,000 | 28,000 |
| 1000 | 20,000 | 25,000 | 35,000 |
| 1500 | 30,000 | 37,500 | 52,500 |
Each block is about 940 BTU/h, for a moderate climate.
200 sq ft █████ 5,000 BTU/h 400 sq ft ███████████ 10,000 BTU/h 600 sq ft ████████████████ 15,000 BTU/h 800 sq ft █████████████████████ 20,000 BTU/h 1000 sq ft ███████████████████████████ 25,000 BTU/h 1500 sq ft ████████████████████████████████████████ 37,500 BTU/h
Other units
- Kilowatts: 1 BTU/h = 0.29307 watts, so 12,000 BTU/h is 3.52 kW.
- Tons of cooling: 1 ton = 12,000 BTU/h. This has nothing to do with weight. It comes from the heat needed to melt a ton of ice in one day.
- Square meters: 1 m² is 10.764 sq ft. The calculator changes it for you.
Standard sizes
Air conditioners are made in fixed sizes, for example 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000, 30,000 and 36,000 BTU/h. The calculator names the smallest standard size that is equal to or above your result. Each block is 1,500 BTU/h; the area is for the moderate climate:
6,000 BTU ████ 0.5 tons, about 240 sq ft 12,000 BTU ████████ 1 ton, about 480 sq ft 18,000 BTU ████████████ 1.5 tons, about 720 sq ft 24,000 BTU ████████████████ 2 tons, about 960 sq ft 36,000 BTU ████████████████████████ 3 tons, about 1,440 sq ft 60,000 BTU ████████████████████████████████████████ 5 tons, about 2,400 sq ft
Tips
- Do not choose a much bigger unit. An oversized unit turns on and off all the time, wears out sooner and does not take the moisture out of the air.
- Insulation and windows matter. A room with big windows facing the sun or a poorly insulated attic needs more. A well-insulated room needs less.
- Open rooms. If a room is open to the next one, add the area of both.
- Ask a professional for a whole house. A proper sizing (called a Manual J calculation) looks at every wall, window and duct.
What this calculator does not include
- Heating. It gives the cooling capacity.
- The size of the windows, the insulation, the air leaks and the heat of machines.
- The efficiency of the unit (its SEER or EER rating), and the electricity that it uses.
This tool gives a rule of thumb for planning. Ask a qualified installer to size a system for a house or a large space.