Sizing an air conditioner by the room
Buying a window or portable air conditioner starts with one number: its cooling capacity in BTU per hour. Too small, and it runs nonstop without ever cooling the room. Too large, and it cools the air in short bursts without removing enough humidity. This calculator uses the sizing chart published by ENERGY STAR and adjusts it for the factors that change a room’s heat load.
The method
- Measure the room: length × width gives the floor area in square feet.
- Read the base capacity from the chart below.
- Adjust for sun: add 10 percent for a very sunny room, subtract 10 percent for a heavily shaded one.
- Add 600 BTU for each person beyond two who regularly uses the room.
- Add 4,000 BTU for a kitchen, where cooking appliances add heat.
| Floor area | Cooling capacity |
|---|---|
| 100 to 150 sq ft | 5,000 BTU |
| 150 to 250 sq ft | 6,000 BTU |
| 250 to 300 sq ft | 7,000 BTU |
| 300 to 350 sq ft | 8,000 BTU |
| 350 to 400 sq ft | 9,000 BTU |
| 400 to 450 sq ft | 10,000 BTU |
| 450 to 550 sq ft | 12,000 BTU |
| 550 to 700 sq ft | 14,000 BTU |
| 700 to 1,000 sq ft | 18,000 BTU |
Worked example: a 15 by 12 foot bedroom
- Area: 15 × 12 = 180 sq ft, in the 150 to 250 band.
- Base capacity: 6,000 BTU per hour, which is half a ton or 1.76 kW of cooling.
- Normal sun, two occupants and no kitchen: no adjustment.
A 6,000 BTU window unit fits this room. If the bedroom faces west and bakes in the afternoon sun, the result rises to 6,600 BTU, and the next size up, often 8,000 BTU, is the better buy.
Heating the same room
For electric heat, a common rule of thumb is 10 watts per square foot with standard 8-foot ceilings and average insulation. The 180 sq ft bedroom needs about 1,800 W, more than the 1,500 W a standard plug-in heater delivers. In that case, either accept a slower warm-up, add a second heater on a different circuit, or improve the room’s insulation and drafts first. Older, drafty rooms may need 12 to 15 watts per square foot, while a well-insulated room may need only 7 or 8.
Units at a glance
- 1 ton of cooling = 12,000 BTU per hour.
- 1 kW = 3,412 BTU per hour.
- A heat pump’s heating capacity is also stated in BTU, so the same chart gives a first estimate for a ductless mini split serving one room.
Things the chart does not cover
Ceilings higher than 8 feet, large windows, poor insulation, attic rooms under a hot roof and very hot climates all raise the load. For a whole house or a central system, a contractor should run a Manual J load calculation, which accounts for every wall, window and duct rather than floor area alone.