What to measure
How to use the BTU calculator
Enter the room’s length and width, its sun exposure, how many people usually use it and whether it’s a kitchen. The result is the cooling capacity to look for on a window, portable or mini-split unit.
How it’s calculated
- Look up the room’s area in the ENERGY STAR chart.
- Heavily shaded: −10%. Very sunny: +10%.
- Add 600 BTU for each person beyond two, and 4,000 BTU for a kitchen.
Worked example: 20 × 25 ft living room
- 500 sq ft → 12,000 BTU from the chart
- Very sunny: 13,200; four people: +1,200 → 14,400 BTU
- If it were an open kitchen: +4,000 → 18,400 BTU
ENERGY STAR sizing chart
| Room area (sq ft) | BTU per hour |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,400–1,500 | 24,000 |
| 1,500–2,000 | 30,000 |
| 2,000–2,500 | 34,000 |
For a whole house, central air should be sized with a Manual J load calculation by an HVAC contractor.
AC size for common rooms
Two people, not a kitchen.
| Room | Area | Average sun | Very sunny |
|---|---|---|---|
| 10 × 12 ft | 120 sq ft | 5,000 BTU | 5,500 BTU |
| 12 × 15 ft | 180 sq ft | 6,000 BTU | 6,600 BTU |
| 14 × 16 ft | 224 sq ft | 6,000 BTU | 6,600 BTU |
| 15 × 20 ft | 300 sq ft | 8,000 BTU | 8,800 BTU |
| 20 × 20 ft | 400 sq ft | 10,000 BTU | 11,000 BTU |
| 20 × 25 ft | 500 sq ft | 12,000 BTU | 13,200 BTU |
| 25 × 30 ft | 750 sq ft | 18,000 BTU | 19,800 BTU |
Common mistakes
- Buying the biggest unit that fits the budget. An oversized unit short-cycles and leaves the room clammy.
- Measuring only the room in an open plan. If the room opens onto a hall or dining area without a door, the unit cools that space too, so include it.
- Forgetting the kitchen allowance when the unit will cool a kitchen or an open kitchen area.
- Using a room chart for the whole house. Central air needs a Manual J load calculation.
Frequently asked questions
How many BTUs do I need per square foot?
Roughly 20–30 BTU per sq ft, and small rooms need more per square foot than large ones. The ENERGY STAR chart is more accurate: 12,000 BTU covers 450–550 sq ft, 6,000 BTU covers 150–250 sq ft.
What size AC for a 500 square foot room?
12,000 BTU per hour (1 ton). Make it 13,200 if the room is very sunny, and add 600 BTU for each person beyond two.
Is a bigger air conditioner better?
No. An oversized unit cools quickly and shuts off before removing humidity, leaving the room cold and clammy. Size it to the room.
Does ceiling height matter?
Yes. The chart assumes 8 ft ceilings; for higher ceilings, choose a slightly larger unit.
What size AC for a 12x12 bedroom?
5,000 BTU per hour, since 144 sq ft falls in the chart's smallest band. Make it 4,500 if the room is heavily shaded or 5,500 if it's very sunny.
How many square feet does a 10,000 BTU air conditioner cool?
400–450 sq ft, according to the ENERGY STAR chart, before adjusting for sun, people or a kitchen.
What does 1 ton of air conditioning mean?
12,000 BTU per hour. In the ENERGY STAR chart, that's the size for a 450–550 sq ft room.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.