BTU Calculator
A rule-of-thumb estimate for one room or open space. For a whole building or a new system, ask an HVAC contractor for a Manual J load calculation.
How the BTU calculation works
Start with area × BTU per square foot
The Energy Star room air conditioner chart lists 12,000 BTU for a 450–550 sq ft room with an 8 ft ceiling, which is 24 BTU per sq ft at 500 sq ft. Bigger rooms need fewer BTU per sq ft: 1,500–2,000 sq ft is listed at 30,000 BTU. For heating, enter your own figure, because it depends on your climate.
Scale for ceiling height and insulation
The Energy Star chart assumes an 8 ft ceiling, so this calculator scales the load by height: a 10 ft ceiling adds 25%. It also adds 20% for poor insulation and takes 10% off for a well-insulated space. These are rough allowances, not a standard.
Adjust cooling for sun, people and cooking
Energy Star suggests +10% for a very sunny room, −10% for heavy shade, 600 BTU for each regular occupant over two and 4,000 BTU for a kitchen.
Divide by 12,000 for tons
One ton of cooling is 12,000 BTU per hour. Round up to the next unit size rather than down.
Worked example. A 500 sq ft sunny office with an 8 ft ceiling, average insulation and three people: 500 × 24 = 12,000 BTU, plus 10% for sun = 13,200 BTU, plus 600 for the third person = 13,800 BTU/h. That is 1.15 tons. A 14,000 BTU room unit (the next size on the Energy Star chart) covers it; in half-ton steps, round up to 1.5 tons.
BTU FAQ
How many BTU do I need per square foot?
For cooling with an 8 ft ceiling, the Energy Star chart works out to about 22–27 BTU per sq ft for rooms of 300–550 sq ft, falling to 15–20 for 1,500–2,000 sq ft. There is no official per-square-foot figure for heating; it depends on climate and insulation, so use a contractor's figure or a Manual J calculation.
What size air conditioner do I need for 500 square feet?
Energy Star lists 12,000 BTU (1 ton) for 450–550 sq ft. Add 10% for a very sunny room and 600 BTU for each regular occupant over two, so a sunny room with three people needs about 13,800 BTU.
What size air conditioner do I need for 1,000 square feet?
Energy Star lists 18,000 BTU (1.5 tons) for 700–1,000 sq ft and 21,000 BTU for 1,000–1,200 sq ft, before adjustments for sun, people and a kitchen.
How many BTU is a ton?
One ton of cooling equals 12,000 BTU per hour, the heat needed to melt a ton of ice in 24 hours. To convert BTU to tons, divide by 12,000: 18,000 BTU/h is 1.5 tons and 36,000 BTU/h is 3 tons.
How many watts is a BTU?
One watt equals 3.412 BTU per hour. Divide BTU/h by 3,412 to get kilowatts, so 12,000 BTU/h is about 3.52 kW of cooling or heating output.
How many BTU heater do I need?
Multiply the area by the BTU per sq ft figure for your climate. At 40 BTU per sq ft, a 1,000 sq ft space needs 40,000 BTU/h; with a 10 ft ceiling and poor insulation this calculator raises that to 60,000 BTU/h.
Is a bigger AC unit better?
No. Energy Star notes that an oversized unit costs more, wastes energy and cools the room before it removes the humidity, so the room feels damp and clammy.
Can I use this to size central air for a house?
Only as a first guess. Whole-house systems should be sized with a Manual J load calculation, which accounts for windows, walls, duct losses and your local climate.
Sources: Energy Star: room air conditioner sizing chart and adjustments