BTU Calculator
BTU Calculator
Work out what size air conditioner a room needs in BTU from its floor area, using the Energy Star sizing table and its adjustments for sun, shade, people and kitchens. The page gives the cooling load, the nearest common unit size to buy, tons and kilowatts, and compares the table with the quick 20 BTU per square foot rule.
Where your room sits on the Energy Star table
How the adjustments add up
Sizing follows the Energy Star room air conditioner table: 5,000 BTU an hour up to 150 sq ft, rising to 34,000 at 2,500 sq ft, with each band including its upper figure. Adjustments are Energy Star’s: 10 percent less for a heavily shaded room, 10 percent more for a very sunny one, 600 BTU for each person beyond two, and 4,000 BTU for a kitchen. Ceilings above 8 ft are scaled in proportion to the extra room volume, a common allowance rather than part of the table. One ton of cooling is 12,000 BTU an hour and 1 BTU an hour is 0.293 W. Whole house systems should be sized with an ACCA Manual J load calculation. The CSV, image and printed report contain the values you entered, nothing preloaded.
How to use this tool
Enter the room’s length and width and the floor area fills in, or type the area directly in square feet or square meters. Then set how sunny the room is, how many people usually use it, and whether it is a kitchen. If the ceiling is higher than the usual 8 ft, enter its height as well. The cooling load in BTU an hour appears at the top, with the next common unit size to buy, and the same load in tons and kilowatts.
The first chart shows where the room falls on the Energy Star table, with the quick 20 BTU per square foot rule drawn over it for comparison. The second builds the total from the table size and each adjustment in turn, so you can see which one is doing the work.
How the size is worked out
The starting point is the Energy Star sizing table for room air conditioners, which matches bands of floor area to a cooling capacity: 5,000 BTU an hour up to 150 sq ft, 7,000 BTU up to 300 sq ft, 12,000 BTU up to 550 sq ft and so on to 34,000 BTU at 2,500 sq ft. Each band here includes its upper figure, so a 300 sq ft room reads 7,000 BTU.
Energy Star then adjusts that figure for the room: 10 percent less if it is heavily shaded, 10 percent more if it is very sunny, 600 BTU for each person beyond two who regularly uses the room, and 4,000 BTU if the unit cools a kitchen. For ceilings above 8 ft the page also scales the table size in proportion to the extra volume, a common allowance that sits outside the Energy Star guidance. Because units are sold in fixed sizes, the result is rounded up to the next common size.
Energy Star sizing table
| Floor area | Example room | BTU an hour | Tons | kW |
|---|---|---|---|---|
| up to 150 sq ft | 10 by 15 ft | 5,000 | 0.42 | 1.47 |
| over 150 to 250 sq ft | 10 by 25 ft | 6,000 | 0.50 | 1.76 |
| over 250 to 300 sq ft | 15 by 20 ft | 7,000 | 0.58 | 2.05 |
| over 300 to 350 sq ft | 14 by 25 ft | 8,000 | 0.67 | 2.34 |
| over 350 to 400 sq ft | 20 by 20 ft | 9,000 | 0.75 | 2.64 |
| over 400 to 450 sq ft | 18 by 25 ft | 10,000 | 0.83 | 2.93 |
| over 450 to 550 sq ft | 22 by 25 ft | 12,000 | 1.00 | 3.52 |
| over 550 to 700 sq ft | 25 by 28 ft | 14,000 | 1.17 | 4.10 |
| over 700 to 1,000 sq ft | 25 by 40 ft | 18,000 | 1.50 | 5.28 |
| over 1,000 to 1,200 sq ft | 30 by 40 ft | 21,000 | 1.75 | 6.15 |
| over 1,200 to 1,400 sq ft | 35 by 40 ft | 23,000 | 1.92 | 6.74 |
| over 1,400 to 1,500 sq ft | 30 by 50 ft | 24,000 | 2.00 | 7.03 |
| over 1,500 to 2,000 sq ft | 40 by 50 ft | 30,000 | 2.50 | 8.79 |
| over 2,000 to 2,500 sq ft | 50 by 50 ft | 34,000 | 2.83 | 9.96 |
Energy Star room air conditioner sizing, before adjustments. Each band includes its upper figure; the example room is one that fills the band.
BTU, tons and kilowatts
A BTU, or British thermal unit, is the heat needed to warm a pound of water by one degree Fahrenheit. Air conditioner ratings are really BTU per hour, a rate of heat removal, though the label usually drops the per hour. One ton of cooling is 12,000 BTU an hour, a unit that comes from the heat needed to melt a ton of ice in a day. In metric terms 1 BTU an hour is 0.293 W, so 12,000 BTU an hour is about 3.5 kW of cooling, which is how split systems are usually rated outside the US.
Portable units often carry two ratings: an older ASHRAE figure and a lower Department of Energy figure called SACC, which allows for the heat the unit leaks back into the room. Compare the SACC figure with the size on this page.
A worked example you can check on paper
A bedroom measures 14 by 20 ft, which is 280 sq ft. That falls in the band over 250 and up to 300 sq ft, so the table gives 7,000 BTU an hour. The room faces west and gets strong afternoon sun, so add 10 percent: 7,700 BTU. Two people use it, so there is no occupancy adjustment. Room units come in 6,000 and 8,000 BTU, so the right choice is 8,000 BTU, or 0.67 tons.
Now take a 12 by 15 ft kitchen, 180 sq ft. The table gives 6,000 BTU, and the kitchen adjustment adds 4,000, for 10,000 BTU. The 20 BTU per square foot rule would have said 3,600 BTU, far too little for a room with a stove and oven running.
Room units and whole house systems
The Energy Star table is written for room air conditioners: window, through the wall and portable units that cool one space. It works for a single open area up to 2,500 sq ft, but a whole house is different. Heat gain depends on insulation, window area and direction, air leakage, ducts and the local climate, and square footage alone can miss by a ton or more.
For central air or a heat pump, ask for a Manual J load calculation, the method published by the Air Conditioning Contractors of America. It works room by room and is what a good installer uses to choose the equipment size.
Mistakes that catch people out
Buying the biggest unit that fits the window. Oversized units short cycle and leave the air damp. Match the table.
Measuring only part of an open plan space. If the living room opens onto the dining room and kitchen, the unit is cooling all of it.
Forgetting the sun. A west facing room with large windows can need 10 percent more, and a room under a hot roof more again.
Leaving out the kitchen allowance. Cooking adds a lot of heat; Energy Star adds 4,000 BTU for it.
Comparing portable unit ratings with window units. Use the lower SACC rating for portables.
Popular calculations
Each opens the calculator with its own address, and every value can be changed once it is open.
By room size in square feet
By room size in square meters
Sunny and shaded rooms
Kitchens
More people and high ceilings
How this was checked
The engine was tested in Node against the Energy Star table band by band, including the edges: 150 sq ft reads 5,000 BTU and 151 sq ft reads 6,000, 300 sq ft reads 7,000 and 301 reads 8,000. The adjustments were checked by hand: a 300 sq ft sunny kitchen for four people gives 7,000 times 1.1, plus 1,200, plus 4,000, exactly 12,900 BTU and 1.075 tons; a 30 m² room converts to 322.9 sq ft and 8,000 BTU; and a 10 ft ceiling scales 12,000 BTU to 15,000.
Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.
Common questions
What size air conditioner do I need for 300 square feet?
By the Energy Star table a 300 sq ft room needs 7,000 BTU an hour. Room units are rarely sold at that size, so the usual choice is 8,000 BTU. A very sunny room needs 7,700 BTU, which still fits an 8,000 BTU unit.
How many BTU per square foot do I need?
About 20 BTU per square foot is the common rule, but the Energy Star table works out at anything from about 14 BTU per square foot for large open spaces to 30 or more for small rooms, because small rooms need more per square foot. The table is the better guide.
How many square feet will a 12,000 BTU air conditioner cool?
Energy Star matches 12,000 BTU an hour to rooms over 450 and up to 550 sq ft. In a heavily shaded room it stretches a little further; in a sunny room or a kitchen it covers less.
What is a ton of air conditioning?
One ton of cooling is 12,000 BTU an hour, the rate of heat needed to melt a ton of ice in a day. A 24,000 BTU unit is a 2 ton unit, which is about 7 kW of cooling.
Is it bad to buy an air conditioner that is too big?
Yes. An oversized unit cools the air quickly and shuts off before it has removed much moisture, so the room feels cold and damp, and the frequent starts waste energy and wear the compressor. Buy the size the table gives, rounded up to the next size sold.
How many BTU do I need for 1,000 square feet?
The Energy Star table gives 18,000 BTU an hour for over 700 and up to 1,000 sq ft, before adjustments. For a whole floor or house rather than one open room, a Manual J load calculation is the proper way to size a central system.
Related Calculators and Reference
- Square Footage Calculator: for rooms that are not simple rectangles.
- Power Unit Converter: BTU an hour, kilowatts, tons and horsepower.
- Energy Converter: BTU, kWh and joules.
- Electricity Cost Calculator: what the unit will cost to run.
- Temperature Converter: for thermostat settings in either scale.
- Furnace Size Calculator: furnace BTU from a heat loss estimate.
- Insulation Calculator: depth and bags of insulation to reach the recommended R value.
- Generator Size Calculator: running and starting watts to generator size.
- Bathroom Fan CFM Calculator: exhaust fan size for any bathroom.
- Ceiling Fan Size Calculator: a ceiling fan eases a room cooling load but does not replace it, and the fan is sized by room area.
- Energy Star room air conditioners: the sizing table and adjustments used here.