AC BTU Calculator

Find the right air conditioner size in BTU for a room using the ENERGY STAR sizing chart, adjusted for ceiling height, sun exposure, occupants and kitchens.

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Results will appear here once you enter your values.

How the ENERGY STAR sizing chart works

Cooling capacity is rated in BTU per hour.

ENERGY STAR's room air conditioner guidance starts from floor area: 100-150 sq ft needs 5,000 BTU/h, 150-250 sq ft 6,000, 250-300 sq ft 7,000, 300-350 sq ft 8,000, 350-400 sq ft 9,000, 400-450 sq ft 10,000, 450-550 sq ft 12,000, 550-700 sq ft 14,000, 700-1,000 sq ft 18,000, 1,000-1,200 sq ft 21,000, 1,200-1,400 sq ft 23,000, 1,400-1,500 sq ft 24,000, 1,500-2,000 sq ft 30,000 and 2,000-2,500 sq ft 34,000.

The chart works out to roughly 20-25 BTU/h per square foot for small rooms, falling to about 14 for large open areas.

The four adjustments

ENERGY STAR then adjusts the base figure: reduce capacity 10% for a heavily shaded room, increase it 10% for a very sunny room, add 600 BTU/h for each person beyond two who regularly occupies the space, and add 4,000 BTU/h if the unit serves a kitchen.

This calculator also scales the base figure up when ceilings are taller than 8 feet, since the chart assumes standard height and a 10-foot ceiling holds 25% more air.

Example: a 250 sq ft sunny room used by three people with a kitchenette needs 7,000 × 1.1 + 600 + 4,000 = 12,300 BTU/h.

Why bigger is not better

An oversized unit reaches the thermostat set point in a few minutes and shuts off before its coil has run long enough to condense moisture from the air.

The room ends up cold but clammy, the compressor short-cycles, and efficiency and lifespan both fall.

The U.S.

Department of Energy notes that a properly sized unit dehumidifies better and runs more efficiently than a larger one.

Pick the standard size closest to the result.

BTU, tons and kilowatts

One ton of cooling is 12,000 BTU/h, the heat needed to melt a ton of ice in 24 hours, so a 3-ton central system delivers 36,000 BTU/h.

One BTU/h equals 0.293 watts of cooling output, so an 8,000 BTU/h unit moves about 2.3 kW of heat.

That is not its electrical draw: divide BTU/h by the unit's EER (or CEER) to get input watts.

An 8,000 BTU/h window unit with a CEER of 12 draws roughly 670 W.

When to use a load calculation instead

The room chart is meant for single rooms up to 2,500 sq ft served by a window, portable or through-the-wall unit.

For central air or multi-zone ductless systems, rooms with large west-facing glass, poor insulation or unusual heat sources, contractors should run an ACCA Manual J load calculation, which accounts for your climate's design temperature, window area and orientation, insulation levels and air leakage.

Frequently Asked Questions

Common questions about the AC BTU Calculator

Roughly 20 BTU/h per square foot for typical rooms, per the ENERGY STAR chart. Small rooms need more per square foot (a 150 sq ft room gets 5,000-6,000 BTU/h) and large open areas need less (about 14 BTU/h per sq ft at 2,000-2,500 sq ft).

Sources & References

Room Air Conditioners

Room AC sizing chart by floor area and adjustments for shade, sun, occupants and kitchens.
→Source: ENERGY STAR (U.S. EPA)

Manual J: Residential Load Calculation

The ANSI-recognized method for whole-house heating and cooling load calculations.
→Source: Air Conditioning Contractors of America (ACCA)

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