AC BTU Calculator
Measure the room and answer three quick questions. You'll get the air conditioner size in BTUs per hour from ENERGY STAR's sizing chart, adjusted for sun, people and kitchens.
- ENERGY STAR sizing chart
- Sun, people and kitchen adjustments
- Feet or metric
2 What size to buy
Updates as you type| ENERGY STAR chart size | 8,000 BTU per hour |
|---|---|
| Size to look for | 8,000 BTU per hour or the nearest size above it |
The chart assumes an 8 ft ceiling; for taller ceilings ask an HVAC contractor. A much bigger unit isn't better: it cools the room before it has time to pull out humidity, so the air stays clammy.
Quick answer
A 15 × 20 ft room (300 sq ft) needs an 8,000 BTU air conditioner on ENERGY STAR's sizing chart, before adjusting for sun, extra people or a kitchen.
AC size for common rooms
| Room | Area | AC size | Very sunny |
|---|---|---|---|
| 10 × 12 ft | 120 sq ft | 5,000 BTU | 5,500 BTU |
| 12 × 12 ft | 144 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 |
BTU per hour from the ENERGY STAR chart, for a room with normal sun and up to two people. Use the calculator above for your exact size.
How to size a room air conditioner
ENERGY STAR publishes a sizing chart for room air conditioners: find the room's floor area, and the chart gives the cooling capacity in BTUs per hour. Then adjust for the room:
- Very sunny room: add 10%.
- Heavily shaded room: take off 10%.
- More than two people: add 600 BTU for each extra person who regularly uses the room.
- Kitchen: add 4,000 BTU.
The 10% is worked out on the chart size; add people and the kitchen after that. The chart assumes an 8 ft ceiling and ENERGY STAR gives no figure for taller ceilings, so for those rooms ask an HVAC contractor.
Worked example: a 15 × 20 ft living room
The room is 300 sq ft, which falls in the chart's "300 up to 350" row: 8,000 BTU. If it faces west and gets afternoon sun, add 10% to make 8,800 BTU. With three people using it most evenings, add another 600: 9,400 BTU. If nobody sells that exact size, take the next one up, not two.
ENERGY STAR sizing chart
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU per hour |
| 150 up to 250 sq ft | 6,000 BTU per hour |
| 250 up to 300 sq ft | 7,000 BTU per hour |
| 300 up to 350 sq ft | 8,000 BTU per hour |
| 350 up to 400 sq ft | 9,000 BTU per hour |
| 400 up to 450 sq ft | 10,000 BTU per hour |
| 450 up to 550 sq ft | 12,000 BTU per hour |
| 550 up to 700 sq ft | 14,000 BTU per hour |
| 700 up to 1,000 sq ft | 18,000 BTU per hour |
| 1,000 up to 1,200 sq ft | 21,000 BTU per hour |
| 1,200 up to 1,400 sq ft | 23,000 BTU per hour |
| 1,400 up to 1,500 sq ft | 24,000 BTU per hour |
| 1,500 up to 2,000 sq ft | 30,000 BTU per hour |
| 2,000 up to 2,500 sq ft | 34,000 BTU per hour |
A room exactly on a boundary, like 300 sq ft, goes in the higher row.
Why bigger isn't better
An oversized air conditioner costs more, uses more energy and doesn't cool any better, according to ENERGY STAR. It cools the room before it has time to remove the humidity, so the room ends up cool but damp and clammy. Size it to the room, not a notch up "to be safe".
Your attic insulation is worth checking too: the climate zone lookup shows what your area's code expects.
Frequently asked questions
What size air conditioner do I need for 500 sq ft?
12,000 BTU per hour, from the ENERGY STAR chart row for 450 up to 550 sq ft. Add 10% for a very sunny room and 600 BTU for each regular occupant over two.
What size AC for a 12 × 12 bedroom?
A 12 × 12 room is 144 sq ft, which ENERGY STAR sizes at 5,000 BTU, the smallest size on its chart.
How many BTUs is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour. A 24,000 BTU unit is 2 tons.
Is a bigger air conditioner better?
No. ENERGY STAR says an oversized unit costs more, wastes energy and leaves the room damp and clammy, because it cools the air before it can remove the humidity.
Last reviewed 2026-10-25. See how we calculate for our sources and assumptions.