BTU Calculator guide
Find the right size air conditioner or heater for a room. Enter the room size, sun exposure, number of people, and whether it is a kitchen, and get the BTU rating to buy, based on ENERGY STAR sizing guidance, plus a heating estimate in BTU and watts.
What a BTU rating means
A BTU (British thermal unit) is the heat needed to warm one pound of water by 1°F. Air conditioners are rated in BTU per hour of heat they can pull out of a room, and heaters in BTU per hour they can add. A 12,000 BTU air conditioner is also called a one-ton unit, from the old days of cooling with blocks of ice.
Bigger is not better. An oversized AC cools the room so fast it shuts off before it has removed much humidity, and you end up cold and clammy. ENERGY STAR says this plainly: size to the room. An undersized unit runs nonstop and never catches up on hot afternoons.
How the calculator sizes an air conditioner
Cooling follows the ENERGY STAR room air conditioner sizing chart, which maps floor area to capacity: 100 to 150 sq ft needs 5,000 BTU, 150 to 250 needs 6,000, 350 to 400 needs 9,000, 450 to 550 needs 12,000, and so on up to 34,000 BTU for 2,500 sq ft. Then it applies ENERGY STAR's adjustments. A heavily shaded room gets 10% less. A very sunny room gets 10% more. Add 600 BTU for each person beyond two who regularly uses the room. And add 4,000 BTU if the room is a kitchen, because stoves and ovens dump a lot of heat.
The chart assumes a standard 8-foot ceiling. For taller rooms the calculator scales the base number by ceiling height ÷ 8. That is our assumption, not ENERGY STAR's, and it is deliberately simple. It then rounds up to a unit size you can actually buy.
Worked example: three rooms
A 12 × 15 ft bedroom is 180 sq ft. The chart says 6,000 BTU; average sun and two people add nothing. Buy a 6,000 BTU window unit.
A 20 × 20 ft living room is 400 sq ft, which is 9,000 BTU on the chart. It faces west and gets hammered by afternoon sun, so add 10%: 9,900. Four people watch TV there every evening, which is two more than the chart assumes, so add 1,200. Total 11,100 BTU. Buy a 12,000 BTU unit.
A 12 × 14 ft kitchen is 168 sq ft, so 6,000 BTU plus 4,000 for the kitchen is 10,000 BTU. A 6,000 BTU unit that would be perfect in a bedroom would struggle there every time the oven is on.
Sizing a heater
Heating has no official chart like cooling, so the calculator uses a common rule of thumb: about 30 BTU per sq ft in hot climates, 40 in mixed climates like Virginia or Missouri, 50 in cold ones like Chicago or Denver, and 60 in Minnesota and northern New England. It then adjusts for insulation, since a drafty room with old single-pane windows can need 25% more heat than the same room in a new, tight house.
To convert to watts for an electric heater, divide BTU per hour by 3.412. A plug-in space heater tops out at 1,500 watts (5,118 BTU) on a standard 120-volt outlet. If the room needs more than that, you are looking at a 240-volt hardwired heater, two heaters on separate circuits, or a mini-split heat pump, which heats for roughly a third of the electricity cost of resistance heat.
Common mistakes
Comparing portable ACs by the big number. Portable units list an ASHRAE rating and a lower SACC rating. The SACC number reflects real performance with the exhaust hose. Compare portables on SACC, and expect it to be well below the ASHRAE figure.
Buying for the whole floor. One window unit will not cool three rooms through doorways. Size for the room it sits in.
Ignoring the circuit. Units over about 12,000 BTU and heaters over 1,500 W often need a dedicated circuit or 240 volts. Check before you buy.
When to get a load calculation
Room units are forgiving. Central air, heat pumps, and furnaces are not. For any whole-house system, ask the contractor for a Manual J load calculation, the ACCA standard that accounts for windows, insulation, orientation, duct losses, and your local design temperatures. A contractor who sizes a central system by square footage alone, or by matching whatever was there before, is guessing, and an oversized system costs more to buy and more to run.
How we calculate: sources
Frequently asked questions
How many BTU do I need for my room?
ENERGY STAR's chart starts at 5,000 BTU for 100 to 150 sq ft, 6,000 for 150 to 250, 8,000 for 300 to 350, 10,000 for 400 to 450, and 12,000 for 450 to 550 sq ft. Then adjust for sun, people, and kitchens.
What size AC do I need for a 12x12 room?
A 12 × 12 ft room is 144 sq ft, so ENERGY STAR recommends 5,000 BTU. Go up 10% for a very sunny room and add 600 BTU for each regular occupant beyond two.
How much should I add for a sunny room or a kitchen?
ENERGY STAR says increase capacity by 10% for a sunny room, reduce it 10% for a heavily shaded one, add 600 BTU per person beyond two, and add 4,000 BTU if the unit is in a kitchen.
Is a bigger air conditioner better?
No. An oversized unit cools the room quickly and shuts off before it removes much humidity, leaving the room cold and clammy. It also cycles more, which wastes energy. Size to the room.
How many BTU does it take to heat a room?
A rule of thumb is 30 BTU per sq ft in hot climates, 40 in mixed climates, 50 in cold climates, and up to 60 in very cold ones, more for poorly insulated rooms. A 200 sq ft room in a mixed climate needs about 8,000 BTU, or 2.3 kW.
How do I convert BTU to watts?
Divide BTU per hour by 3.412. A 1,500 W space heater produces about 5,118 BTU per hour. One ton of cooling is 12,000 BTU per hour.
Are my room details stored?
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