Window Air Conditioner Size Calculator

The 25 BTU Per Square Foot Baseline Cooling capacity is measured in British Thermal Units per hour (BTU). The Energy Star program’s simplified guideline starts with 20 to 25 BTU per square foot of cooled space, and this calculator uses the upper figure of 25. The rule works well for standard rooms with 8-foot ceilings, normal sun exposure, and typical insulation. For most bedrooms and living rooms that single calculation is enough to pick the right unit.

Bigger Is Definitely NOT Better This is the most common shopping mistake. An oversized air conditioner drops the air temperature quickly, but the room’s surfaces (walls, furniture, carpet) stay warm. The unit hits the thermostat setpoint, shuts off, and restarts minutes later, a cycle called short-cycling. It never runs long enough to pull moisture out of the air, so the room ends up cold and clammy at once, which is worse than either problem alone. An undersized unit runs constantly and still falls short on the hottest days.

Ceiling Height Matters Cooling load scales with the volume of air, not just the floor area. A room with a 12-foot vaulted ceiling holds 50% more air than an 8-foot room on the same footprint, and needs proportionally more capacity.

CEER and Energy Efficiency Window and portable units are rated by the Combined Energy Efficiency Ratio (CEER), which is cooling output in BTU per hour divided by watts drawn. Central and split systems use SEER instead, so do not compare the two numbers directly. A CEER 15 unit draws about two thirds the power of a CEER 10 unit for identical cooling. Energy Star certified window units sit at least 10% above the federal minimum.

Whether the efficient unit pays back depends entirely on your electricity rate and how many hours you actually run it, which is why this calculator asks for both rather than assuming.

Occupants and Kitchens Each person in a room adds roughly 600 BTU/hr of body heat. The 25 BTU per square foot baseline already assumes about two occupants, so add 600 for each additional person rather than for everybody, which is what this calculator does. Kitchens are the harder case: a stove, oven, and refrigerator compressor together add 4,000 BTU/hr or more during cooking, so a kitchen needs a noticeably bigger unit than a bedroom of the same size.

Standard Window Unit Sizes Manufacturers build to standard capacities: 5,000 (small bedroom), 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 (large rooms), and 24,000 BTU. This calculator rounds up to the nearest standard size. Round up by one step, not two, since the short-cycling problem above is real and gets worse the further you overshoot.