BTU Calculator
Size a room air conditioner or heater in BTU per hour, tons and kW from floor area, ceiling, sun, occupants and kitchen use, based on the ENERGY STAR chart.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the BTU Calculator?
Air conditioners and heaters are sized in BTU per hour, the amount of heat they can move. Too small and the room never cools on a hot afternoon; too large and the unit short-cycles, switching off before it has pulled moisture from the air, which leaves the room cold and clammy. This BTU calculator starts from the room air conditioner sizing chart that ENERGY STAR publishes and applies its own adjustments for sun, people and kitchens.
Enter the room as length by width or as a floor area, in feet or metres, and the ceiling height. Choose how sunny the room is, how many people usually use it, whether it is a kitchen, and how well it is insulated. The result is a cooling figure in BTU per hour, the same in tons (12,000 BTU/h each) and kilowatts, and a heating estimate based on your climate.
The chart is shown in full with your row highlighted, and the formula panel lists every step from the chart value to the final number, so you can see which adjustment moved the answer.
- ENERGY STAR room AC sizing chart with sun, occupant and kitchen adjustments
- Ceiling height and insulation adjustments, labelled as rules of thumb
- Heating estimate by climate band
- Results in BTU/h, tons and kW; feet or metres
- Full chart with your row highlighted and a step-by-step formula
How to use the BTU Calculator
- Choose feet or metres and enter the room length and width, or the floor area.
- Enter the ceiling height. Rooms above 8 feet (2.4 m) get a proportional increase.
- Pick sun exposure, the number of people who regularly use the room, and whether it is a kitchen.
- Choose the insulation level and your climate for the heating estimate.
- Read the cooling and heating figures in BTU per hour, tons and kW, and buy the nearest unit at or slightly above the cooling number.
Worked examples
20 x 20 ft living room
Input
400 ft², 8 ft ceiling, average sun, 2 people, mixed climate
Output
Cooling 9,000 BTU/h (0.75 tons, 2.64 kW); heating 16,000 BTU/h
This is the default example: the 350 to 400 ft² chart row with no adjustments.
Sunny bedroom used by four
Input
300 ft², very sunny, 4 people
Output
7,000 x 1.1 + 1,200 = 8,900 BTU/h
Sun adds 10% to the chart value and two extra people add 600 BTU/h each. A 9,000 or 10,000 BTU unit fits.
Open kitchen
Input
500 ft², kitchen, 2 people
Output
12,000 + 4,000 = 16,000 BTU/h (1.33 tons)
Cooking heat adds a flat 4,000 BTU/h, which is a big share of the load in a small room.
Shaded 20 m² room in metres
Input
20 m² (215 ft²), 2.4 m ceiling, heavily shaded
Output
6,000 x 0.9 = 5,400 BTU/h (1.58 kW)
Metric inputs are converted to the chart square footage, so a 5,000 BTU unit would be slightly small; pick 6,000.
Background and insight
ENERGY STAR sizes room air conditioners by floor area for an 8 foot ceiling: 5,000 BTU/h for 100 to 150 ft², 6,000 for 150 to 250, rising to 12,000 for 450 to 550 and 34,000 for 2,000 to 2,500 ft². It then reduces capacity by 10% for a heavily shaded room, increases it by 10% for a very sunny one, adds 600 BTU/h for each regular occupant beyond two, and adds 4,000 BTU/h if the unit cools a kitchen. Those are the rules this tool follows.
Two factors are rules of thumb added here rather than part of the chart: capacity scales in proportion for ceilings above 8 feet, and insulation shifts the figure by -10% (good) or +20% (poor). The heating estimate uses typical BTU per square foot by climate, from about 30 in hot regions to 60 in very cold ones. For a whole house, a Manual J load calculation from an HVAC contractor replaces all of these shortcuts.
Common use cases
Buying a window or portable AC
Pick the right BTU rating for a bedroom, office or living room before you shop.
Choosing a mini-split head
Get a per-room figure in BTU/h and kW to compare indoor units across brands that use different units.
Sizing a space heater or radiator
Turn the heating estimate in kW into the wattage of an electric heater.
Garage and workshop conversions
Account for high ceilings, poor insulation and strong sun in rooms the chart does not describe well.
Tips and limitations
- If the result falls between two unit sizes, choose the larger one only when the room is very sunny or heavily used; otherwise the smaller one dehumidifies better.
- Portable air conditioners are often rated by an older standard that overstates capacity. Compare the DOE SACC figure, not the larger ASHRAE number.
- Open-plan spaces cool as one room. Add the areas together rather than sizing each zone separately.
- Sealing gaps and adding window film can lower the load more cheaply than buying a bigger unit.
- kW here is cooling or heating output, not electricity use. A heat pump moves two to four times more heat than the electricity it draws.
Frequently asked questions
How many BTU do I need per square foot?
The ENERGY STAR chart works out to roughly 20 to 35 BTU/h per square foot for small and mid-size rooms, falling to about 14 to 17 for the largest. A 400 ft² room needs about 9,000 BTU/h before adjustments for sun, people and kitchens.
What size air conditioner do I need for a 12 x 12 room?
A 144 ft² room falls in the 100 to 150 ft² row of the ENERGY STAR chart, so 5,000 BTU/h is enough unless it is very sunny, used by more than two people, or has a high ceiling.
How many BTU is a ton of air conditioning?
One ton is 12,000 BTU per hour, the heat needed to melt a short ton of ice in 24 hours. A 24,000 BTU unit is a 2-ton unit.
How do I convert BTU to kW?
Multiply BTU per hour by 0.000293. So 12,000 BTU/h is 3.52 kW and 1 kW is about 3,412 BTU/h. This is thermal capacity, not the power the unit draws.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools the air quickly and shuts off before it removes much humidity, so rooms feel damp and the compressor cycles more, which wears it faster and uses more energy.
How much do people and kitchens add to the BTU requirement?
ENERGY STAR adds 600 BTU/h for each person beyond two who regularly use the room, and 4,000 BTU/h if the air conditioner cools a kitchen.
Is there an in-depth guide to the BTU Calculator?
Yes. “Roof Pitch, Stair Layout, AC Sizing and Running Costs Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
Related Tools You May Like
Free, in-browser tools that pair well with this one.