Window Air Conditioner Size Calculator

The 25 BTU Per Square Foot Baseline The Energy Star program’s simplified guideline for air conditioning starts with 20–25 BTU per square foot of cooled space. This rule of thumb works well for standard rooms with 8-foot ceilings, normal sun exposure, and typical insulation. For many bedrooms and living rooms, this single calculation is sufficient for choosing the right window AC unit.

Bigger Is Definitely NOT Better This is the most common AC shopping mistake. An oversized air conditioner cools the room’s air temperature quickly, but the room’s surfaces (walls, furniture, carpet) remain warm. The unit reaches the thermostat setpoint, shuts off, and restarts minutes later — a cycle called “short-cycling.” Short-cycling means the unit never runs long enough to dehumidify the air effectively. The result: the room feels cold and clammy at the same time — a very uncomfortable combination. An undersized unit runs constantly but falls short on the hottest days.

Ceiling Height Matters BTU requirements scale with the volume of air being cooled, not just the floor area. A room with a 12-foot vaulted ceiling has 50% more air volume than an 8-foot standard ceiling room of the same footprint — and needs proportionally more BTU.

SEER and Energy Efficiency SEER (Seasonal Energy Efficiency Ratio) measures how efficiently an AC converts electricity to cooling. A 16 SEER unit uses exactly half the electricity of an 8 SEER unit for the same amount of cooling. Energy Star certified window ACs are at least 10% more efficient than federal minimum standards. For a unit running 8 hours a day for 4 months, the energy cost difference between a 10 SEER and 14 SEER unit can be $30–$80 per season — the more efficient unit often pays for itself in 2–3 years.

Occupants and Kitchens Every person in a room generates approximately 600 BTU/hr of heat from their body. A bedroom for two people needs 1,200 BTU more than an empty room of the same size. Kitchens present a special challenge: cooking appliances (stove, oven, refrigerator compressor) add 4,000+ BTU/hr of heat load during meal preparation, requiring a significantly larger unit than a comparably-sized bedroom.

Standard Window AC Sizes Manufacturers produce window AC units in standard BTU ranges: 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 — it’s always better to have a unit that’s 10% too large than 10% too small.