What is a BTU?
A BTU, or British thermal unit, is a measure of heat. One BTU is the heat it takes to raise one pound of water by 1°F.
Air conditioners and heaters are rated by how much heat they move each hour. When a store sells a “5,000 BTU air conditioner,” it means 5,000 BTUs per hour: the unit can pull 5,000 BTUs of heat out of your room for every hour it runs. The “per hour” part is usually left off the box.
To picture that, think of a small electric space heater. A 1,500 watt heater gives off about 5,100 BTUs of heat per hour, so a 5,000 BTU air conditioner removes about as much heat as one of those heaters adds. A heater or furnace works the other way around: its BTU rating is how much heat it puts into the room each hour.
A room that soaks up more heat, from sun, people, or cooking, needs an air conditioner with a bigger BTU number.
How many BTUs do you need to cool a room?
ENERGY STAR sizes room air conditioners by floor area. Its chart assumes an 8 ft ceiling.
| Room size | Cooling needed |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/hr |
| 150 to 250 sq ft | 6,000 BTU/hr |
| 250 to 300 sq ft | 7,000 BTU/hr |
| 300 to 350 sq ft | 8,000 BTU/hr |
| 350 to 400 sq ft | 9,000 BTU/hr |
| 400 to 450 sq ft | 10,000 BTU/hr |
| 450 to 550 sq ft | 12,000 BTU/hr |
| 550 to 700 sq ft | 14,000 BTU/hr |
| 700 to 1,000 sq ft | 18,000 BTU/hr |
| 1,000 to 1,200 sq ft | 21,000 BTU/hr |
| 1,200 to 1,400 sq ft | 23,000 BTU/hr |
| 1,400 to 1,500 sq ft | 24,000 BTU/hr |
| 1,500 to 2,000 sq ft | 30,000 BTU/hr |
| 2,000 to 2,500 sq ft | 34,000 BTU/hr |
Then ENERGY STAR adjusts that number for a few things that add or take away heat in your room:
- The room gets heavy shade: take off 10%. Trees or buildings that block the sun mean less heat comes in.
- The room is very sunny: add 10%. Big windows or strong afternoon sun let in extra heat.
- More than two people use the room: add 600 BTU for each person after the first two. Every person gives off body heat.
- The room is a kitchen: add 4,000 BTU for the heat from the stove and oven.
For ceilings other than 8 ft, this calculator scales the chart number by your ceiling height divided by 8. A 10 ft ceiling holds 25% more air than an 8 ft one, so the room gets 25% more cooling.
Example: a three-room mini split
Say you are cooling three rooms with 10 ft ceilings and lots of afternoon sun. Four people live there, and the living room is open to the kitchen.
First, each room gets its ENERGY STAR chart number. The 10 ft ceiling adds 25% and the sun adds 10%, so each number is multiplied by 1.25 x 1.10 = 1.375 and rounded to the nearest 100.
| Room | Size | Chart | Adjusted |
|---|---|---|---|
| Bedroom | 15 x 12 ft, 180 sq ft | 6,000 | 8,300 |
| Living room | 20 x 15 ft, 300 sq ft | 8,000 | 11,000 |
| Office | 10 x 10 ft, 100 sq ft | 5,000 | 6,900 |
The three rooms add up to 26,200 BTU per hour. Next come the people and the kitchen. The ENERGY STAR chart already allows for two people, and four people live here, so the two extra people add 600 BTU each, or 1,200 BTU. The kitchen adds another 4,000 BTU.
26,200 + 1,200 + 4,000 = 31,400 BTU/hr
31,400 / 12,000 = 2.62 tons
A multi-zone mini split puts an indoor unit in each room, so the room lines help you pick each one. A single central system would be sized to the total.
What to do with your number
Shop for the size closest to your result. ENERGY STAR warns that an oversized air conditioner cools the room before it can remove the humidity, which leaves the air damp and clammy, and it uses more energy doing it.
Poor insulation and drafty windows let extra heat in, so sealing and insulating your home can mean a smaller air conditioner does the job. The spray foam insulation cost calculator prices an upgrade from the area and foam depth. You can find our other tools for the home in Home Improvement.
Limitations
This is a rule-of-thumb estimate. It does not know your window sizes, wall construction, duct leaks, or local design temperatures. Before you buy a whole-house system, ask a heating and cooling contractor for a Manual J load calculation, which works out how much heat each room gains and loses.
The ENERGY STAR chart is built for room air conditioners and stops at 2,500 sq ft per room. The ceiling height scaling is an approximation, since the chart itself is set for 8 ft ceilings.
Sources
- ENERGY STAR: Room Air Conditioners
- U.S. Energy Information Administration: British thermal units (Btu)
- NIST Guide to the SI, Appendix B.9: Conversion factors
- U.S. Department of Energy: Electric Resistance Heating
Frequently asked questions
How many BTUs do I need for a 500 square foot room?
ENERGY STAR lists 12,000 BTU per hour for 450 to 550 sq ft with an 8 ft ceiling. Take off 10% for heavy shade, add 10% for a very sunny room, and add 600 BTU for each extra person if more than two people regularly use the room.
Is a bigger air conditioner better?
No. ENERGY STAR says an oversized air conditioner costs more, wastes energy, and does not cool better. It cools the room before it has time to pull out the humidity, so the air feels damp and clammy. Pick the size that matches the room instead of rounding way up.
How many BTUs is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour. Central air conditioners are usually sold in tons, so divide your BTU total by 12,000 to compare. A 24,000 BTU per hour load is 2 tons, and a 36,000 BTU per hour load is 3 tons.
How do I convert BTU per hour to watts?
Multiply BTU per hour by 0.293 to get watts. A 5,000 BTU per hour load is about 1,465 watts. Going the other way, a 1,500 watt electric space heater puts out about 5,100 BTU per hour, since electric resistance heat turns all the power it draws into heat.
