BTU Calculator by Square Footage

A BTU is a British thermal unit, and an air conditioner's BTU rating is how much heat it can pull out of a room in an hour. Size it off the room's square footage, adjust for how the room is used, and you land on the right unit. Enter your area below and the calculator works from the official ENERGY STAR chart rather than a rough multiplier.

The ENERGY STAR sizing chart

This is the table the whole industry quotes for room air conditioners, published by ENERGY STAR. Find your square footage, read across.

Area to cool (sq ft)	Capacity needed (BTU/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

If you don't know your square footage yet, measure length times width and use the room calculator, or break an odd-shaped space into rectangles with the irregular shape calculator.

The four adjustments

The chart assumes an ordinary room with eight foot ceilings and a couple of people in it. ENERGY STAR gives four corrections on top:

Very sunny room: increase capacity by 10 percent.
Heavily shaded room: reduce capacity by 10 percent.
More than two people regularly in the room: add 600 BTU per additional person.
Kitchen: add 4,000 BTU.

Higher ceilings matter too, because you are cooling volume rather than floor. Ten foot ceilings mean roughly 25 percent more air in the same footprint, and the calculator scales for that.

Worked example. A 400 square foot living room, south facing with big windows, four people in it most evenings.

Chart size for 400 square feet: 9,000 BTU
Very sunny, add 10 percent: 9,900 BTU
Two extra people at 600 each: 1,200 BTU
Total: 11,100 BTU, so a 12,000 BTU unit

Kitchen example. A 250 square foot kitchen starts at 6,000 BTU from the chart, plus 4,000 for the cooking load, which is 10,000 BTU.

Why "20 BTU per square foot" drifts

The rule of thumb you'll see everywhere is 20 BTU per square foot. It is a reasonable memory aid in the middle of the range and it falls apart at both ends.

Room size	ENERGY STAR chart	20 BTU per sq ft	Difference
150 sq ft	5,000	3,000	Rule undersizes by 40%
300 sq ft	7,000	6,000	Rule undersizes
500 sq ft	12,000	10,000	Rule undersizes
700 sq ft	14,000	14,000	Match
1,000 sq ft	18,000	20,000	Rule oversizes
1,500 sq ft	24,000	30,000	Rule oversizes by 25%
2,000 sq ft	30,000	40,000	Rule oversizes by a third

Small rooms carry a fixed overhead of walls, windows, and door openings that doesn't shrink with the floor area, which is why the chart never drops below 5,000. Large rooms benefit from the opposite effect. Use the chart, and treat 20 BTU per square foot as a sanity check rather than the answer.

BTU, tons, and whole-house sizing

Central systems are sold in tons. One ton of cooling is 12,000 BTU per hour, a figure that goes back to the amount of heat needed to melt a ton of ice in 24 hours. So a 24,000 BTU system is 2 tons, and 36,000 BTU is 3 tons.

Whole-house sizing is a different exercise from cooling one room, because it has to account for insulation, window area and orientation, air leakage, duct losses, and local climate. Rules of thumb do badly here. The Department of Energy's building science guidance is blunt about it: "Rules of thumb" are too often used to size comfort systems, which results in excessively oversized systems, and it points contractors to ACCA's Manual J, Eighth Edition for a real load calculation.

An oversized system costs more up front and then runs badly. DOE lists the consequences as increased cost, wasted energy, and too-frequent on-and-off cycling. That short cycling is the comfort problem people actually notice: the unit hits the thermostat setpoint fast, shuts off before it has run long enough to pull moisture out of the air, and leaves the house cold and clammy. For tonnage by house size and what a Manual J involves, see what size AC unit do I need.

Heating BTU is a separate calculation

Furnaces, boilers, and heat pumps are also rated in BTU, but the heating load depends on your climate far more than the cooling load does. The same 2,000 square foot house needs a very different furnace in Minnesota than in Georgia, so a single BTU-per-square-foot figure can't cover both. Heat pumps complicate it further, since their output falls as the outdoor temperature drops.

The same answer applies: a Manual J load calculation, not a multiplier. If a contractor sizes your system off floor area alone and a quick look at the windows, get a second quote.

Common mistakes

Buying bigger to be safe. Oversizing is the single most common HVAC error and it makes comfort worse, not better, because of short cycling and poor dehumidification.

Using the whole floor plan for one unit. A window or portable unit cools the room it's in. Open-plan spaces that flow into each other should be added together, but a closed bedroom down the hall should not.

Ignoring the kitchen. A cooktop and an oven throw off a lot of heat. That 4,000 BTU adjustment exists for a reason.

Forgetting ceiling height. The chart assumes eight feet. A vaulted ceiling or a converted loft needs more.

Sizing off gross square footage. Use the area you are actually cooling. Garages, unfinished basements, and unconditioned porches are not part of it, which what counts as square footage covers.

Frequently asked questions

How many BTU do I need per square foot?

About 20 BTU per square foot is the common rule, but the ENERGY STAR chart is more accurate. It effectively runs above 30 BTU per square foot for a small 150 square foot room and closer to 15 for a 2,000 square foot space, because small rooms carry fixed heat loads that don't scale with floor area.

How many BTU do I need for a 500 square foot room?

12,000 BTU, since 500 square feet falls in the 450 to 550 band. Add 10 percent if the room is very sunny, 600 BTU for each person beyond two who is regularly in it, and 4,000 if it's a kitchen.

What size air conditioner do I need for 1,000 square feet?

18,000 BTU covers 700 to 1,000 square feet, which is 1.5 tons. For a whole home of that size rather than one open room, get a Manual J load calculation instead of sizing off the chart, since insulation and window area change the answer.

How many square feet does a 12,000 BTU air conditioner cool?

450 to 550 square feet under the ENERGY STAR chart. A 12,000 BTU unit is one ton of cooling. Adjust downward if the room is sunny, has a high ceiling, or doubles as a kitchen.

How do I convert BTU to tons?

Divide by 12,000. A 24,000 BTU system is 2 tons and 36,000 BTU is 3 tons. The ton comes from the heat absorbed by a ton of melting ice over a day.

Is it bad to buy an air conditioner that's too big?

Yes. An oversized unit cools the air quickly, shuts off, and never runs long enough to remove humidity, so the room feels cold and damp. It also cycles on and off constantly, which wastes energy and wears the compressor. DOE names oversizing as the main consequence of sizing by rule of thumb.

Do I need more BTU for a room with high ceilings?

Yes. The sizing chart assumes eight foot ceilings, and you are really cooling the volume of air. A room with ten foot ceilings holds about 25 percent more air, so scale the capacity up accordingly.

BTU Calculator by Square Footage