Window AC Size by Room Calculator
Size a window air conditioner in BTUs from your room’s area, sun exposure and occupancy using the EPA’s 20-BTU-per-square-foot rule.
How it works
Base = area × 20 BTU · × sun factor (0.9 / 1.0 / 1.1) · + 600 per person over 2 · + 4,000 if a kitchen
The U.S. EPA’s Energy Star guidance sizes a room air conditioner at roughly 20 BTU per hour for each square foot of floor area, then nudges it for real-world load. A heavily shaded room can drop 10%, a sun-drenched one adds 10%, each occupant beyond two adds about 600 BTU, and a kitchen adds 4,000 BTU for the heat from cooking. Getting close matters: an oversized unit cools fast but cycles off before it removes humidity, leaving the room cold and clammy.
Worked example
A 250 ft² sunny bedroom for two: base is 250 × 20 = 5,000 BTU, plus 10% for sun = 5,500 BTU, with no occupancy or kitchen additions. Rounded to the nearest 500, look for a 5,500 BTU window unit — the common 6,000 BTU size would be the practical off-the-shelf pick.
Frequently asked questions
What happens if my window AC is too big?
It cools the air quickly and shuts off before it has run long enough to pull out moisture. The result is a cold but humid, clammy room, plus more wear from frequent on-off cycling. Right-sizing keeps the air both cool and dry.
Does ceiling height change the BTU I need?
The 20-BTU rule assumes a standard 8-foot ceiling. For higher ceilings the room holds more air, so add roughly 10% for each extra couple of feet, or size up one step.
Is BTU the same as tons of cooling?
They measure the same thing at different scales: one ton of cooling equals 12,000 BTU per hour. Window units are almost always quoted in BTU/hr, while central systems are quoted in tons.
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This calculator is for educational purposes. Double-check important results before acting on them.