BTU Calculator
Estimate the heating and cooling load of a room: enter the size, ceiling height, insulation, climate and sun exposure, and get the BTU per hour the room needs, the air conditioner size in tons and the heater size in kilowatts.
This is the rule-of-thumb method that HVAC catalogs use for a first pass, not a Manual J load calculation: it cannot see your window glass, duct losses or air leaks. Size a window unit to the nearest 1,000 BTU, and round central equipment to the nearest half ton rather than oversizing it.
Formula and calculation
Worked example
A 300 sq ft room with an 8 ft ceiling in a moderate climate, average insulation and average sun needs 9,500 BTU per hour: the room itself accounts for 7,500, two windows add 2,000 and the people add nothing since two are already in the rate. That is 0.7917 tons, or a 1 ton unit. Heating the same room takes 13,500 BTU per hour, about 3.9565 kW.
Why not buy the biggest unit?
An oversized air conditioner cools the air fast and shuts off before it has pulled the humidity out of it, so the room feels cold and clammy at the same time, and the compressor starts and stops all day until it wears out. Undersizing means the unit runs continuously on the hottest afternoon and never quite catches up. The rule of thumb here gets you into the right half-ton band; if you are replacing a system, the size of the old one and the ductwork are worth as much as the calculation, which is why a Manual J by an HVAC contractor is the number that matters for anything bigger than a window unit.
Window units, portable units and central systems
Window air conditioners are sold in increments of about 1,000 BTU, so round your figure up to the nearest size: a 9,500 BTU load means a 10,000 BTU window unit. Portable units are rated the same way, but their real output runs lower than the label, so go up one size. Central systems and mini-splits come in half-ton steps, and the installer has to match the indoor coil, the refrigerant line and the airflow to the outdoor unit. For heating, electric space heaters and heat pumps are rated in kilowatts — multiply the kilowatts by 3,412 to get BTU per hour, or divide the BTU figure by 3,412.
What the results mean
- Cooling needed
- The total cooling load of the room in BTU per hour — the number to match against an air conditioner's rated capacity.
- Air conditioner size
- The air conditioner size in tons, rounded to the nearest standard half-ton step. Central units and mini-split systems are sold in half-ton sizes.
- Exact load
- The exact cooling load in tons of refrigeration. One ton is 12,000 BTU per hour, so 24,000 BTU is a 2 ton unit.
- Heating needed
- The heating load of the same room in BTU per hour, from the climate and insulation you picked. Heating needs are usually larger than cooling needs in a cold climate.
- Heater size
- The heating load in kilowatts, the unit electric space heaters, baseboard heaters and heat pumps are rated in. Four times the kilowatts is roughly the BTU per hour.
- Room and sun
- The part of the cooling load that comes from the room itself: area times the climate rate, adjusted for insulation, sun exposure and ceiling height. It covers the walls, roof, floor and the sun beating on them.
- Windows
- The load added by the windows you entered, at about 1,000 BTU per hour each. Glass lets heat in far faster than an insulated wall does.
- People
- The load added by the people in the room, at 600 BTU per hour for each person beyond the first two.
Frequently asked questions
How many BTU do I need for a 300 square foot room?
Around 9,000 to 10,000 BTU for a typical room: 300 sq ft at 25 BTU per sq ft is 7,500, plus 1,000 for each window. A sunny west-facing room with four windows needs closer to 12,000. Enter your own numbers above and the total updates with the climate, insulation and windows.
How many BTU is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour, so a 2 ton unit is 24,000 BTU and a 3 ton unit is 36,000. The ton comes from the old measure of melting one ton of ice in a day, and it is still how central equipment is sold today.
What size air conditioner for a 1,000 square foot house?
Roughly 24,000 to 30,000 BTU, or 2 to 2.5 tons, in a moderate climate: 1,000 sq ft at 25 BTU per sq ft is 25,000 BTU before windows and sun. In a hot climate, or with poor insulation and a lot of west glass, the same house can need a half ton more.
Is 12,000 BTU enough for a 500 square foot room?
Usually not quite. At 25 BTU per sq ft a 500 sq ft room needs 12,500 BTU before windows, so a 12,000 BTU unit will struggle on a hot day unless the room is shaded and well insulated. Step up to 14,000 or 15,000 BTU.
How many BTU of heating do I need per square foot?
Thirty to 60 BTU per square foot is the usual range, from about 35 in a warm climate to 55 in a cold one, and a poorly insulated house pushes it higher. That is why a 300 sq ft room needs 13,500 BTU to heat in a moderate climate but only about 9,500 to cool.