Not sure what size air conditioner you need? This free AC BTU calculator works out the exact cooling capacity for any room in seconds. Enter your room size, ceiling height, sun exposure and climate, and get the right BTU rating, tonnage and running cost, so you do not overspend on a unit that is too big or sweat with one that is too small.
AC BTU Calculator
Find the right air conditioner size for any room in BTU and tons.
Estimates follow ENERGY STAR room-size guidance with standard adjustments. For central AC or whole-home systems, ask an HVAC pro for a Manual J load calculation.
How to Use the AC BTU Calculator
- Enter the room length and width in feet or meters. For L-shaped rooms, split them into two rectangles and add the results.
- Pick the ceiling height. Taller ceilings mean more air to cool, so the BTU goes up.
- Choose the room type. Kitchens add 4,000 BTU for cooking heat, and offices add a little for computers.
- Set sun exposure, insulation and climate to match your home.
- Add the number of people who normally use the room. Each person beyond two adds about 600 BTU of body heat.
The calculator then rounds up to the nearest standard air conditioner size you can actually buy.
AC BTU Chart by Room Size
These are the base cooling capacities recommended by ENERGY STAR before any adjustments for sun, climate or occupants.
| Room area (sq ft) | Room area (m²) | BTU per hour |
|---|---|---|
| 100 – 150 | 9 – 14 | 5,000 |
| 150 – 250 | 14 – 23 | 6,000 |
| 250 – 300 | 23 – 28 | 7,000 |
| 300 – 350 | 28 – 33 | 8,000 |
| 350 – 400 | 33 – 37 | 9,000 |
| 400 – 450 | 37 – 42 | 10,000 |
| 450 – 550 | 42 – 51 | 12,000 |
| 550 – 700 | 51 – 65 | 14,000 |
| 700 – 1,000 | 65 – 93 | 18,000 |
| 1,000 – 1,200 | 93 – 111 | 21,000 |
| 1,200 – 1,400 | 111 – 130 | 23,000 |
| 1,400 – 1,500 | 130 – 139 | 24,000 |
| 1,500 – 2,000 | 139 – 186 | 30,000 |
| 2,000 – 2,500 | 186 – 232 | 34,000 |
How the BTU Is Calculated
The tool starts with the base BTU from the chart above, then applies these adjustments:
- Ceiling height: BTU × (ceiling height ÷ 8 ft).
- Sun: −10% for a shady room, +10% for a very sunny room.
- Insulation: −10% for good insulation, +15% for poor insulation.
- Climate: −10% for mild areas, +15% for hot regions.
- People: +600 BTU for each person above two.
- Kitchen: +4,000 BTU.
Example: a 16 × 14 ft bedroom (224 sq ft) with 8 ft ceilings, normal sun and two people needs 6,000 BTU. Move the same room to a hot climate with poor insulation and it jumps to about 7,900 BTU, so an 8,000 BTU unit is the right buy.
What Happens If Your AC Is the Wrong Size?
Too small: the unit runs nonstop, never reaches the set temperature and wears out faster. Too big: it cools the room quickly and shuts off before removing humidity, leaving the room cold and clammy. It also short-cycles, which wastes energy. Getting the BTU right is the single most important step when buying an air conditioner.
Tips to Cut Cooling Costs
- Use a smart thermostat or a smart AC controller to schedule cooling around your day.
- Look for an ENERGY STAR unit with a high CEER (window AC) or SEER2 (mini-split and central) rating.
- Close blinds on south and west windows during the afternoon.
- Set the thermostat to 78°F (26°C) when you are home and higher when you are out.
- Clean or replace the filter every month during heavy use.
Frequently Asked Questions
How many BTU do I need for a 12×12 room?
A 12×12 room is 144 sq ft, which needs about 5,000 BTU in normal conditions. Choose 6,000 BTU if the room is very sunny, in a hot climate or used as a kitchen.
How many square feet does 12,000 BTU cool?
A 12,000 BTU (1 ton) air conditioner cools roughly 450 to 550 sq ft in average conditions. In a hot, sunny climate it may only handle about 400 sq ft.
Is it better to oversize an air conditioner?
No. An oversized AC short-cycles, wastes energy and leaves the room humid. Choose the closest standard size at or just above your calculated BTU.
How do I convert BTU to tons?
Divide BTU by 12,000. For example, 24,000 BTU equals a 2-ton air conditioner and 36,000 BTU equals 3 tons.
Does ceiling height affect AC size?
Yes. BTU charts assume 8 ft ceilings. A 10 ft ceiling has 25% more air volume, so you need about 25% more cooling capacity.
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