BTU Calculator by Square Footage
This BTU calculator sizes a room air conditioner from the floor area you need to cool. Enter the square footage, ceiling height and how many people use the room, then pick the sun exposure and whether it's a kitchen. You get the BTU per hour and the same figure in tons, next to the plain ENERGY STAR chart size and the 20 BTU per square foot rule of thumb.
BTU to square feet chart
A 300 square foot room needs 7,000 BTU, a 500 square foot room needs 12,000 and 1,000 square feet needs 18,000. These are the capacities in the ENERGY STAR room air conditioner sizing chart, which the calculator starts from.
| Area to cool (sq ft) | 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 |
Don't have the area yet? Multiply length by width with the room calculator, or split an odd-shaped space into pieces with the irregular shape calculator.
How many BTU per square foot?
The rule of thumb is 20 BTU per square foot, but the ENERGY STAR chart asks for more in small rooms and less in big ones: 33 BTU per square foot at 150 square feet, 15 at 2,000.
So the rule undersizes a 150 square foot bedroom by 2,000 BTU (3,000 against 5,000) and oversizes a 2,000 square foot space by 10,000 (40,000 against 30,000).
Adjust for sun, people, kitchens and ceilings
ENERGY STAR adds four corrections to the chart size:
of the chart size
of the chart size
BTU, for people who use the room regularly.
BTU, if the unit cools a kitchen.
A very sunny 400 square foot room used by four people needs 11,100 BTU, so you buy a 12,000 BTU unit:
Kitchen example: a 250 square foot kitchen starts at 6,000 BTU on the chart. Add 4,000 for the cooking heat and you need 10,000 BTU.
ENERGY STAR's capacities assume an 8 foot ceiling. Above 8 feet, the calculator scales the chart size by the ceiling height divided by 8, so a 10 foot ceiling adds 25 percent before the other adjustments.
BTU to tons
One ton of cooling is 12,000 BTU per hour, the conversion NIST lists for a ton of refrigeration. Divide BTU by 12,000 to get tons.
12,000 BTU
18,000 BTU
24,000 BTU
36,000 BTU
Whole house and heating BTU
Don't size central air, a heat pump or a furnace from this chart. The Department of Energy says "rules of thumb" too often lead to "excessively oversized systems" and points to a load calculation with ACCA's Manual J, which works out both the heating and the cooling load of the house.
There's no single heating BTU per square foot figure for the same reason: the heating load comes out of that same Manual J calculation. Tonnage by house size is in what size AC unit do I need, and the house square footage calculator gives you the floor area to start from.
Common sizing mistakes
ENERGY STAR warns an oversized unit cools the room before it removes the humidity, leaving it damp and clammy, and costs more to buy and run.
Size for the room or open area being cooled. Garages and unfinished basements aren't part of it; see what counts as square footage.
Frequently asked questions
How many BTU do I need for 600 square feet?
14,000 BTU, because 600 square feet falls in the ENERGY STAR band for 550 to 700 square feet. Add 10 percent for a very sunny room and 600 BTU for each regular occupant over two.
How many BTU do I need for 1,000 square feet?
18,000 BTU, or 1.5 tons, for a single open space of 700 to 1,000 square feet. For a whole home of that size, get a Manual J load calculation instead.
How many square feet does a 12,000 BTU air conditioner cool?
450 to 550 square feet on the ENERGY STAR chart. That's one ton of cooling.
How many BTU per square foot do I need for heating?
There isn't a reliable per square foot figure. The Department of Energy recommends sizing heating and cooling equipment from a Manual J load calculation for the house.