Air conditioners are sized in BTU per hour — how much heat they can remove from a room. Too small and the room never gets cool; too large and the unit short-cycles without drying the air. This calculator uses the sizing bands and adjustments from ENERGY STAR's room air conditioner guidance, which also serve as a quick check for single-zone mini splits and portable units.
How to use the BTU calculator
- Enter the room's length and width. If the room opens into another space without a door, include that space too.
- Choose the sun exposure. Heavily shaded rooms need less; rooms with large sunny windows need more.
- Enter how many people regularly use the room. Each person beyond two adds about 600 BTU of heat.
- Tell it if the room is a kitchen, which adds 4,000 BTU for cooking heat.
BTU formula and adjustments
Adjusted = base × 0.9 (shaded) or × 1.1 (very sunny)
+ 600 BTU for each person beyond 2
+ 4,000 BTU if the room is a kitchen
One ton of cooling equals 12,000 BTU per hour, so the tons figure is BTU ÷ 12,000.
Worked example: a sunny living room
- Room: 15 × 12 ft = 180 sq ft → base 6,000 BTU
- Very sunny: 6,000 × 1.10 = 6,600 BTU
- Three regular occupants: + 600 = 7,200 BTU
Choose the nearest available size at or slightly above 7,200 BTU — commonly an 8,000 BTU unit.
Recommended BTU by room size
ENERGY STAR room air conditioner sizing, before adjustments.
| Area to be cooled | Capacity needed |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/h |
| 150 to 250 sq ft | 6,000 BTU/h |
| 250 to 300 sq ft | 7,000 BTU/h |
| 300 to 350 sq ft | 8,000 BTU/h |
| 350 to 400 sq ft | 9,000 BTU/h |
| 400 to 450 sq ft | 10,000 BTU/h |
| 450 to 550 sq ft | 12,000 BTU/h |
| 550 to 700 sq ft | 14,000 BTU/h |
| 700 to 1,000 sq ft | 18,000 BTU/h |
| 1,000 to 1,200 sq ft | 21,000 BTU/h |
| 1,200 to 1,400 sq ft | 23,000 BTU/h |
| 1,400 to 1,500 sq ft | 24,000 BTU/h |
| 1,500 to 2,000 sq ft | 30,000 BTU/h |
| 2,000 to 2,500 sq ft | 34,000 BTU/h |
Source: ENERGY STAR, room air conditioner sizing guidance. Rooms under 100 sq ft use the smallest band.
When to go beyond this calculator
- Whole houses and ducted systems should be sized with a proper load calculation (ACCA Manual J) that accounts for insulation, windows, climate and air leakage.
- High ceilings above about 8 ft add volume; consider the next size up.
- Poor insulation or many windows push the real load higher; good insulation can bring it down.
Wondering what it will cost to run? The Electricity Cost Calculator converts watts and hours into dollars.
Frequently asked questions
How many BTU do I need per square foot?
Room air conditioner guides work out to roughly 20 BTU per square foot for typical rooms, but the recommendation steps up in bands rather than scaling exactly. A 150–250 sq ft room calls for about 6,000 BTU; 450–550 sq ft about 12,000 BTU.
What size mini split do I need for a 500 sq ft room?
The ENERGY STAR sizing guide recommends about 12,000 BTU (1 ton) for 450–550 sq ft under normal conditions. Add 10% for a very sunny room and 600 BTU per extra person beyond two. For a whole-home or multi-zone system, a professional load calculation (Manual J) is the better tool.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and shuts off before it removes much humidity, leaving the room cold and clammy, and it cycles on and off more often. Choose the size that matches the room.
How many BTU is one ton of cooling?
12,000 BTU per hour. A 24,000 BTU unit is a 2-ton system.
How do I size an AC for a garage?
Use the floor area here, then select very sunny if the garage has a west-facing door or a dark roof. Garages are often poorly insulated, so an uninsulated garage may need more than the table suggests — insulating the door and walls usually helps more than buying a bigger unit.