How to use the BTU calculator
- Measure the floor of the room (or the open area) you want to cool.
- Pick the sun exposure. Rooms with big south or west windows run hotter.
- Enter how many people regularly use the room, and choose kitchen if it's where you cook.
- Enter the ceiling height. Rooms taller than 8 ft have more air to cool.
How AC size is worked out
The calculator starts from the ENERGY STAR chart below, then adjusts it the way ENERGY STAR suggests:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
For ceilings above 8 ft, the calculator also scales the result by ceiling height ÷ 8. That's a common rule of thumb, not part of the ENERGY STAR chart.
AC size chart by room size
| Room area (sq ft) | Capacity (BTU per hour) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Source: ENERGY STAR room air conditioner sizing guide. 12,000 BTU per hour equals 1 ton of cooling.
Why size matters
A unit that's too small runs constantly and never gets the room cool. One that's too big cools the air quickly and shuts off before it removes the humidity, which leaves the room cold and clammy. Pick the size closest to the result, and round up only if the room is hot or sunny.
For a whole house or central air, a contractor should do a Manual J load calculation, which accounts for insulation, windows, ducts and your local climate.
Frequently asked questions
What size air conditioner do I need for a 12x12 room?
A 144 sq ft room needs about 5,000 BTU per hour, the smallest common window unit. Go up to 6,000 BTU if the room gets a lot of sun.
How many square feet does a 12,000 BTU air conditioner cool?
About 450–550 sq ft in normal conditions, according to the ENERGY STAR chart.
How many BTU is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour, so a 24,000 BTU unit is a 2 ton unit.
Is a bigger air conditioner better?
No. An oversized unit short-cycles, wastes energy and leaves the room humid. Choose the size closest to what the room needs.