BTU Calculator
Your details
Result
BTUs required
- Nearest unit size
- 10,000 BTU unit
- In tons
- 0.83 tons
- Floor area
- 500 sq ft
- Base load before adjustments
- 10,000 BTU at 20/sq ft
- Adjustments applied
- none
- Do not round up "to be safe". An oversized air conditioner cools the air to the thermostat setpoint before it has run long enough to remove humidity, then shuts off — so the room ends up cold and clammy, the compressor short-cycles, and both efficiency and equipment life suffer. Undersizing is uncomfortable; oversizing is uncomfortable and wasteful.
- A ton of cooling is 12,000 BTU per hour — a legacy of how much heat melting a ton of ice absorbs in a day. 10,000 BTU is 0.83 tons.
Room size to unit size
At 20 BTU per square foot with no adjustments, which is the starting point before conditions.
| Floor area | BTU needed | Typical unit |
|---|---|---|
| 150 sq ft | 3,000 | 5,000 BTU |
| 250 sq ft | 5,000 | 5,000 BTU |
| 350 sq ft | 7,000 | 8,000 BTU |
| 450 sq ft | 9,000 | 10,000 BTU |
| 550 sq ft | 11,000 | 12,000 BTU |
| 700 sq ft | 14,000 | 14,000 BTU |
| 1,000 sq ft | 20,000 | 24,000 BTU |
| 1,400 sq ft | 28,000 | 30,000 BTU |
How many BTUs do I need per square foot?
About 20 BTU per square foot of floor area for cooling. A 500 square foot room needs roughly 10,000 BTU, adjusted up 10% for a very sunny room, 4,000 for a kitchen, and 600 for each person beyond two.
BTU = area × 20 × (ceiling ÷ 8) × sun factor + 600 × (occupants − 2) + kitchen- area
- Floor area in square feet
- 20
- BTU per square foot, the ENERGY STAR starting point
- sun factor
- 1.1 very sunny, 0.9 heavily shaded, otherwise 1
- kitchen
- A flat 4,000 BTU for cooking appliances
Twenty BTU per square foot is the published rule of thumb, and the adjustments around it account for the loads that floor area alone cannot see. Ceiling height matters because a cooling unit conditions volume, not floor — a 10 ft ceiling is 25% more air than the 8 ft the baseline assumes. Solar gain through large south or west-facing glass is worth about 10% either way. People are heat sources at roughly 600 BTU each at rest, and a kitchen's appliances add a concentrated 4,000. The important thing this calculation tells you is not to round up. An oversized air conditioner reaches the thermostat setpoint quickly and shuts off, which sounds efficient and is the opposite: dehumidification happens only while the coil runs, so a unit that satisfies the thermostat in short bursts leaves the room cold and damp while short-cycling the compressor. For whole-house sizing, a Manual J calculation accounts for insulation, orientation, infiltration and local design temperatures, and a rule of thumb does not substitute for it.
- Multiply room length by width for the floor area.
- Multiply the area by 20 BTU per square foot.
- Scale by ceiling height divided by eight feet.
- Add or subtract 10% for a very sunny or heavily shaded room.
- Add 600 BTU for each person beyond two, and 4,000 for a kitchen.
Worked example: A 25 × 20 ft sunny living room with an 8 ft ceiling and three regular occupants
A 25 × 20 ft sunny living room with an 8 ft ceiling and three regular occupants
The floor area is 25 × 20 = 500 square feet, so the base load is 500 × 20 = 10,000 BTU. The ceiling is the standard 8 ft, so that factor is 1 and changes nothing. Being a very sunny room multiplies by 1.1, adding 1,000 BTU to reach 11,000. The third occupant adds 600 beyond the two the baseline covers, giving 11,600 BTU an hour — which is 0.97 tons, since a ton of cooling is 12,000 BTU. The nearest unit you can actually buy is 12,000 BTU, and that is the right choice: it is the smallest size that meets the load. Jumping to the 14,000 or 18,000 unit would be the common mistake. The larger unit would hit the thermostat setpoint faster, stop running sooner, and therefore remove less moisture from the air — leaving the room cool but clammy while short-cycling its compressor.
What each btu input means
- Room length
- Interior length of the space being conditioned.
- Room width
- Interior width. Length × width gives the floor area the sizing is based on.
- Ceiling height
- The baseline assumes 8 ft. Anything higher scales the load proportionally, because you are conditioning volume rather than floor.
- Sun exposure
- A room with large south or west-facing windows gains significant solar heat. ENERGY STAR adjusts the load by 10% either way.
- People regularly in the room
- The baseline covers two. Each additional person adds 600 BTU, because a body at rest emits roughly that in heat.
- This is a kitchen
- Cooking appliances add a large, concentrated heat load — ENERGY STAR adds a flat 4,000 BTU.
What this btu does not account for
- Do not round up to the next size for safety margin. Oversized cooling short-cycles, removes less humidity and wears the compressor, leaving a room that is cold and damp rather than comfortable.
- This is a rule of thumb for a single room. Whole-house sizing needs a Manual J load calculation accounting for insulation, orientation, air infiltration and local design temperatures.
- Heating BTUs are not the same as cooling BTUs for the same space. Heating load depends on your climate's design temperature and the building envelope, and in cold climates it is substantially higher.
- Twenty BTU per square foot assumes reasonably insulated modern construction. A poorly insulated room, a top floor under an uninsulated roof, or a space with single-glazed windows needs considerably more.
- A ton of cooling is 12,000 BTU per hour. Residential central systems are sold in half-ton steps, so the usable sizes are coarser than this calculation suggests.
- Rooms that open into each other are effectively one space for cooling purposes. Size for the combined area rather than per room.
- Portable units deliver less effective cooling than their rating because the exhaust hose and single-duct design pull conditioned air out of the room.
Frequently asked questions
How many BTU do I need for a 500 square foot room?
About 10,000 BTU as a starting point, at 20 BTU per square foot. A very sunny room needs around 11,000, and each person beyond two adds 600. The nearest common unit size is 12,000 BTU.
Is it better to oversize or undersize an air conditioner?
Neither, but oversizing is the worse error. An oversized unit cools the air before it has dehumidified it, then shuts off — so the room feels cold and clammy and the compressor short-cycles.
How many BTU is a ton of cooling?
12,000 BTU per hour. The term comes from the amount of heat absorbed by a ton of ice melting over 24 hours. A 2-ton system is 24,000 BTU.
How many square feet does 12,000 BTU cool?
Roughly 550 square feet at the 20 BTU per square foot rule, in average conditions. A sunny room, high ceiling or kitchen reduces that figure.
Do BTU requirements differ for heating and cooling?
Yes, substantially. Cooling load depends mostly on area and solar gain; heating load depends on your climate's design temperature and the building envelope, and is usually higher in cold climates.
Does ceiling height affect BTU requirements?
Yes, proportionally — a unit conditions air volume, not floor area. A 10 ft ceiling is 25% more air than the 8 ft baseline, so scale the requirement by the same ratio.
Sources and review
- Room air conditioners — sizing guidance — ENERGY STAR, U.S. Environmental Protection Agency
Last reviewed .