BTU Calculator
Multiply the room's square footage by 20 to get the BTU per hour an air conditioner needs. A 300 sq ft room needs about 6,000 BTU. Add 10% for a sunny room, 600 BTU per person beyond two and 4,000 BTU for a kitchen.
How to use this BTU calculator
- Measure the length and width of the room you want to cool. If the room is open to another space with no door, measure both areas together.
- Enter the ceiling height. Rooms taller than 8 feet hold more air and need more capacity.
- Choose the sunlight level, the number of people who usually use the room and whether it is a kitchen.
- Read the BTU result and the nearest standard unit size. You can also switch to the second mode to see what room size a given air conditioner suits.
How do I calculate what air conditioner size I need?
This calculator starts from the widely used guideline of about 20 BTU per hour for each square foot of living space, then applies common adjustments.
- Base: area in square feet × 20.
- Ceiling height: if the ceiling is above 8 feet, multiply by height ÷ 8.
- Sunlight: multiply by 1.10 for a very sunny room or 0.90 for a mostly shaded one.
- People: add 600 BTU for each person beyond two.
- Kitchen: add 4,000 BTU because of heat from cooking appliances.
Example: a 450 sq ft sunny living room with 4 people. 450 × 20 = 9,000 BTU, × 1.10 for sun = 9,900, plus 2 extra people × 600 = 1,200, for about 11,100 BTU. A 12,000 BTU unit fits.
Room size to BTU guide
The table shows the base formula result and the nearest standard unit size at or above it.
| Room (sq ft) | Formula (BTU/h) | Nearest standard size |
|---|---|---|
| 150 | 3,000 | 5,000 |
| 250 | 5,000 | 5,000 |
| 350 | 7,000 | 7,000 |
| 450 | 9,000 | 10,000 |
| 550 | 11,000 | 12,000 |
| 700 | 14,000 | 14,000 |
Air conditioner sizing chart by room size
This chart and the adjustments used above follow the ENERGY STAR room air conditioner sizing guidance (checked November 12, 2026). It assumes a room used by one or two people.
| Room area (sq ft) | Capacity needed (BTU/h) |
|---|---|
| 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 |
Why not just buy a bigger air conditioner?
An oversized unit cools the room fast and then shuts off before it has pulled much moisture out of the air. The result is a room that is cool but clammy, with uneven temperatures and more wear from frequent starting. An undersized unit runs constantly and never catches up on hot days. Aim for the nearest size above your result rather than the biggest one you can fit.
How many tons and kilowatts is my BTU number?
One ton of cooling equals 12,000 BTU per hour. So 6,000 BTU is half a ton, 18,000 BTU is 1.5 tons and 24,000 BTU is 2 tons. In metric units, 1 BTU per hour is about 0.293 watts, so 12,000 BTU per hour is about 3.5 kilowatts of cooling. The calculator shows both.
When a simple rule is not enough
The formula suits an ordinary room. It will come up short for a top-floor room under a poorly insulated roof, a room with large west-facing windows, or a space packed with computers. Open-plan areas need to be measured as one space, because the unit cools all of it. Portable units are often rated more generously than window units, so many buyers size up. For central air or a whole house, a professional load calculation is the right tool, not a room formula.
This tool gives a general estimate. It does not replace a professional heat-load calculation.
BTU Calculator FAQ
What size air conditioner do I need for my room?
Multiply the room area in square feet by 20 to get BTU per hour, then adjust for ceiling height, sun, the number of people and kitchens. A 300 square foot room needs about 6,000 BTU in normal conditions.
How big of a room will an 8,000 BTU air conditioner cool?
About 350 to 400 square feet under typical conditions, or 400 square feet at the 20 BTU per square foot rule. Hot, sunny or crowded rooms are smaller.
How many square feet does a 12,000 BTU air conditioner cool?
About 450 to 550 square feet in common sizing charts. At 20 BTU per square foot it is 600 square feet, but real rooms need adjustments, so size to your own conditions.
How do I calculate what air conditioner size I need?
Area × 20 gives the base BTU. Multiply by ceiling height ÷ 8 if the ceiling is above 8 feet, add 10% for a sunny room or subtract 10% for a shady one, add 600 BTU for each person beyond two and add 4,000 BTU for a kitchen.
What does BTU mean on an air conditioner?
BTU stands for British thermal unit, a measure of heat. Air conditioners are rated in BTU per hour, the amount of heat they can remove from a room in an hour.
Is a bigger air conditioner better?
No. An oversized unit cools quickly, shuts off early and removes less moisture, which can leave the room cool but damp. Match the size to the room.
How many tons is a 12,000 BTU air conditioner?
One ton. A ton of cooling is 12,000 BTU per hour.
Does this calculator work for heating?
No. It sizes cooling for a single room. Heating needs depend on climate and insulation, so use a heat-load calculation or ask a heating professional.
Related calculators
Last updated: November 11, 2026 · How we check this calculator