BTU Calculator
The right AC size for any room — Energy Star base, sun, ceiling height, and occupancy included.
Updated
You need
6,000BTU
cooling capacity this space needs per hour
- Buy this unit size
- 6,000 BTU
- Floor area
- 180 sq ft
- Energy Star base
- 6,000 BTU
In short
What size air conditioner do I need?
Energy Star sizes a room air conditioner by floor area: 6,000 BTU covers 150 to 250 square feet, 10,000 covers 400 to 450, and 18,000 covers 700 to 1,000. Then adjust — add 10 percent for a very sunny room, subtract 10 percent for a shaded one, add 600 BTU per occupant beyond two, and add 4,000 for a kitchen.
This is a rule-of-thumb estimate, not a load calculation: proper HVAC sizing uses the ACCA Manual J procedure, which accounts for insulation, window area, orientation, air leakage and your specific climate.
How to use the BTU calculator
Measure the space the unit will actually serve rather than the room it will sit in. For an open layout, that means the whole connected volume — a living room that flows into a dining area through a wide opening is one cooling zone, and sizing for half of it guarantees a unit that runs continuously and never quite gets there.
Enter length and width in feet and the calculator finds the floor area, then looks it up in the Energy Star room air conditioner sizing chart, which is the same chart printed on the back of most window-unit boxes. That chart is a floor-area lookup and nothing more, which is exactly why the next four inputs exist: floor area alone is a proxy for heat load, not a measurement of it.
6,000 BTU
Energy Star base
a 15 by 12 ft bedroom, 180 sq ft
±10%
Sun exposure
add for very sunny, subtract for shaded
+600 BTU
Per occupant past two
plus 4,000 BTU for a kitchen
Then tell the calculator what makes your room different from the average one the chart assumes. Energy Star publishes four adjustments and this tool applies all of them: reduce capacity by 10 percent for a shaded room, increase it by 10 percent for a very sunny one, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU if the unit is cooling a kitchen, where a range and a refrigerator both dump heat into the space.
Ceiling height is the one adjustment Energy Star does not publish, and this tool adds it on its own reasoning: an air conditioner cools a volume of air, the chart implicitly assumes a conventional 8 ft ceiling, and a 10 ft ceiling holds 25 percent more air, so the base figure is scaled by height divided by eight. Treat that scaling as a sensible correction rather than an official figure.
Do
- Measure the whole connected area the unit will serve, not one room of it.
- Measure floor to ceiling rather than assuming a standard eight feet.
- Add 600 BTU for every regular occupant beyond the first two.
- Switch the kitchen toggle on when a range and refrigerator share the space.
Don't
- Round up to the next unit size for headroom without a real extra heat source.
- Read CEER as capacity, because it describes watts rather than cooling.
- Compare a portable on a headline number instead of its SACC figure.
- Size a furnace, heat pump or central system off a floor-area chart.
The Energy Star floor-area chart this tool looks up, next to what two common adjustments do to it. The ceiling-height columns are the ones people skip, and they move the answer by a whole unit size well before the sun exposure does.
| Floor area | Energy Star base BTU | With a 9 ft ceiling | With a 10 ft ceiling | Very sunny, 8 ft ceiling | Unit size to buy at the base figure |
|---|---|---|---|---|---|
| Under 150 sq ft | 5,000 | 5,600 | 6,300 | 5,500 | 5,000 |
| 150 to 250 sq ft | 6,000 | 6,800 | 7,500 | 6,600 | 6,000 |
| 250 to 300 sq ft | 7,000 | 7,900 | 8,800 | 7,700 | 8,000 |
| 300 to 350 sq ft | 8,000 | 9,000 | 10,000 | 8,800 | 8,000 |
| 350 to 400 sq ft | 9,000 | 10,100 | 11,300 | 9,900 | 10,000 |
| 400 to 450 sq ft | 10,000 | 11,300 | 12,500 | 11,000 | 10,000 |
| 450 to 550 sq ft | 12,000 | 13,500 | 15,000 | 13,200 | 12,000 |
| 550 to 700 sq ft | 14,000 | 15,800 | 17,500 | 15,400 | 14,000 |
| 700 to 1,000 sq ft | 18,000 | 20,300 | 22,500 | 19,800 | 18,000 |
| 1,000 to 1,200 sq ft | 21,000 | 23,600 | 26,300 | 23,100 | 21,000 |
| 1,200 to 1,400 sq ft | 23,000 | 25,900 | 28,800 | 25,300 | 24,000 |
| 1,400 to 1,500 sq ft | 24,000 | 27,000 | 30,000 | 26,400 | 24,000 |
| 1,500 to 2,000 sq ft | 30,000 | 33,800 | 37,500 | 33,000 | 34,000 |
| 2,000 sq ft and up | 34,000 | 38,300 | 42,500 | 37,400 | 34,000 |
What do SACC and CEER on the label mean?
Two label numbers are worth knowing before you compare units. Portable air conditioners lose cooling through the exhaust hose and the air it pulls into the room, so the US Department of Energy requires them to be rated by seasonally adjusted cooling capacity.
That figure, SACC, is a weighted average of the capacity measured at the 95 and 83 degree Fahrenheit test conditions, each adjusted for duct heat transfer and infiltration losses, and it is the honest figure to compare against this calculator’s result.
Efficiency is separate: CEER, the combined energy efficiency ratio on the yellow EnergyGuide label, says how much electricity a given amount of cooling costs, not how much cooling you get. Two units at the same BTU cool identically; the higher-CEER one does it on fewer watts.
Moving between BTU and kilowatt-hours?
Appliance labels and utility bills do not speak the same unit. The converter moves a figure from BTU to kWh directly, with no sizing assumptions layered on top of it.
Open the BTU to kWh converter →Full guide
What Size Air Conditioner Do I Need? BTU by Room Size
Floor area gives you the base number. Ceiling height, sun, bodies and a kitchen decide whether you buy that unit or the next one up.
Read the full guide →The formula, worked line by line
The starting point is a lookup rather than a calculation. Energy Star publishes a chart that maps the floor area you are cooling onto a cooling capacity in BTU per hour, and this tool encodes that chart directly: multiply length by width, find the band your area falls into, and read the base figure. It is a coarse instrument by design, because it has to work printed on the side of a carton with no information about your house.
Everything after the lookup is a correction for what the chart cannot see. Some of those corrections scale the base figure — sun exposure and, in this tool, ceiling height — because they change how much heat enters or how much air there is to cool.
Others are flat additions, because they are heat sources that exist regardless of how big the room is: each occupant past the first two, and a kitchen’s range and refrigerator. The order matters: scale first, then add, then round.
base BTU = Energy Star chart value for length × width
BTU = base × (ceiling height ÷ 8) × sun factor [0.9 shaded · 1.0 average · 1.1 very sunny]
+ (occupants − 2) × 600 when above 2, + 4,000 when a kitchen is in the space
rounded to the nearest 100 · unit = next common size ≥ BTU- Floor area
- 15 × 12 ft = 180 sq ft
- Energy Star band
- 150 to 250 sq ft, base 6,000 BTU
- Ceiling and sun
- 8 ft × 1.0, average sun × 1.0
- Occupants and kitchen
- two people +0, no kitchen +0
- Unit to buy
- 6,000 BTU
Change one thing at a time and watch the leverage: strong afternoon sun takes it to 6,600, which climbs the unit ladder to an 8,000; a 10 ft ceiling takes it to 7,500, also an 8,000; and a shaded north-facing version drops to 5,400, which still buys a 6,000 unit because that is the next size up.
A busier example shows how quickly a room leaves window-unit territory. A 20 by 20 ft family room is 400 sq ft and returns 10,000 BTU. Put it under a lofted 10 ft ceiling and the requirement becomes 12,500, so you are buying a 14,000 unit rather than a 10,000 — two sizes up from the chart, on a room whose floor area never changed.
Add strong sun on top of that and it reaches 13,800. Once a room needs more than 34,000 BTU there is no common window unit left, and the honest answer is central air or a multi-zone mini-split rather than a bigger box in the window.
Questions people ask
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The room air conditioner floor-area sizing chart and its adjustments — minus 10 percent for a shaded room, plus 600 BTU for each occupant beyond two, plus 4,000 BTU for a kitchen. The ceiling-height scaling is this tool own extension, not part of the chart.
How To Choose the Right Sized Window AC — ENERGY STAR (US EPA)
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