BTU calculator
The ENERGY STAR cooling chart, applied to your room: enter the size, say how sunny it is and how many people use it, and read the BTU per hour to look for on the box.
BTU calculator for a room air conditioner
180 square feet: the chart gives 6,000 BTU/h for an 8-foot ceiling. That is about 0.5 tons of cooling.
Pick the unit whose rating is at or just above this figure. ENERGY STAR notes that an oversized unit cools less well: it cycles off before it has removed the humidity.
Lookup, not a formula: the ENERGY STAR room air conditioner chart gives a capacity for each range of floor area at an 8-foot ceiling, then minus 10% for heavy shade, plus 10% for strong sun, plus 600 BTU/h per person beyond two, plus 4,000 BTU/h in a kitchen. Taller ceilings and poor insulation push the need up; a contractor's load calculation settles it.
Where the numbers come from
The calculator is a lookup in the cooling capacity chart that ENERGY STAR, the US Environmental Protection Agency's efficiency program, publishes for room air conditioners. The chart gives one capacity in BTU per hour for each band of floor area, assuming an 8-foot ceiling, and four adjustments: minus 10 percent for a heavily shaded room, plus 10 percent for a very sunny one, plus 600 BTU per hour for each person beyond two who regularly occupies the room, and plus 4,000 BTU per hour when the unit serves a kitchen. The tool applies the band and the adjustments; nothing else is added.
The ENERGY STAR chart
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU/h |
| 150 up to 250 sq ft | 6,000 BTU/h |
| 250 up to 300 sq ft | 7,000 BTU/h |
| 300 up to 350 sq ft | 8,000 BTU/h |
| 350 up to 400 sq ft | 9,000 BTU/h |
| 400 up to 450 sq ft | 10,000 BTU/h |
| 450 up to 550 sq ft | 12,000 BTU/h |
| 550 up to 700 sq ft | 14,000 BTU/h |
| 700 up to 1,000 sq ft | 18,000 BTU/h |
| 1,000 up to 1,200 sq ft | 21,000 BTU/h |
| 1,200 up to 1,400 sq ft | 23,000 BTU/h |
| 1,400 up to 1,500 sq ft | 24,000 BTU/h |
| 1,500 up to 2,000 sq ft | 30,000 BTU/h |
| 2,000 up to 2,500 sq ft | 34,000 BTU/h |
8-foot ceiling. Heavily shaded: minus 10 percent. Very sunny: plus 10 percent. More than two people: plus 600 BTU/h each. Kitchen: plus 4,000 BTU/h. Source: ENERGY STAR, read 24 October 2026.
Why a bigger unit is not safer
An air conditioner does two jobs: it lowers the temperature and it pulls water out of the air. The second takes time. A unit that is too large for the room reaches the thermostat setting in a few minutes and shuts off before the coil has condensed much moisture, so the room is cool but damp, and the compressor starts and stops far more often than designed. ENERGY STAR's guidance is explicit that bigger is not better. If your figure lands near the top of a band, the next size up is reasonable; two sizes up is not.
What the chart cannot see
The chart knows the floor area and four adjustments. It does not know your insulation, the size and orientation of your windows, the climate, the heat from electronics, or a ceiling above 8 feet. Any of these can move the real need by a few thousand BTU per hour. For a whole house, a ductless multi-zone system, or a room that the chart does not fit (above 2,500 square feet, cathedral ceilings, a sunroom), ask a contractor for a load calculation under ACCA Manual J, which is what the chart approximates.
Reading the box in the store
Room units list their cooling capacity in BTU per hour, sometimes as "BTU" alone. Portable units often show two figures, a higher ASHRAE rating and a lower SACC or DOE rating; the lower one is the capacity you can compare with the chart, because it accounts for the heat the portable unit's own exhaust hose puts back into the room. Window units show one figure. Check the electrical requirement at the same time: units above about 12,000 BTU per hour often need a 20 amp or 240 volt circuit rather than a standard outlet.
Questions people ask
How many BTU do I need for a 12 by 12 room?
A 12 by 12 room is 144 square feet, which falls in the 100 to 150 square foot band of the ENERGY STAR chart: 5,000 BTU per hour at an 8-foot ceiling. Add 10 percent if the room is very sunny, and 600 BTU per hour for each person beyond two who is usually in it.
How many BTU per square foot for air conditioning?
The chart does not use one figure per square foot: it runs from about 33 BTU per square foot for a small room (5,000 BTU for 150 square feet) down to about 14 for a large open space (34,000 BTU for 2,500 square feet), because big rooms lose proportionally less through their walls. Read the band for your area rather than multiplying.
Is a bigger air conditioner better?
No. ENERGY STAR warns that an oversized unit cools the air quickly and shuts off before it has removed the humidity, so the room feels clammy and the unit cycles more. Choose the capacity at or just above the chart figure.
How many BTU is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour, the heat needed to melt a ton of ice in a day. A 24,000 BTU unit is a 2-ton unit. Central systems are sold in tons, room units in BTU.
What if my ceiling is higher than 8 feet?
The chart assumes 8-foot ceilings. With higher ceilings there is more air to cool, and ENERGY STAR suggests choosing a unit with a higher airflow rating (CADR). For a vaulted room or a converted loft, a contractor's load calculation (Manual J) is the reliable way to size it.