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How to Choose the Right AC Size for Any Room
Air conditioner sizing is the rare purchase where bigger is genuinely worse. This guide explains how BTU sizing works and how to pick the right capacity for your room.
The 20 BTU rule — and its limits
The starting point is simple: 20 BTU/h per square foot (about 215 BTU/h per m²) of floor area. A 20 m² bedroom needs roughly 7,000 BTU/h; a 40 m² living room about 12,000.
But the rule assumes average insulation, average sun and 2.6 m ceilings. Real rooms deviate — which is why the BTU Calculator adjusts for insulation, sun exposure, kitchens and occupants instead of giving one flat number.
Why oversizing backfires
An air conditioner does two jobs: cooling and dehumidifying. Dehumidification needs sustained runtime. An oversized unit blasts cold air, hits the setpoint in minutes and shuts off — before it has wrung the moisture out. The result: a cold, clammy room and a compressor worn out by constant restarts.
One size step above your calculated load is the sensible maximum. Inverter (variable-speed) units tolerate oversizing better because they throttle down instead of switching off.
BTU, tons and kW — translating the labels
The same capacity is sold under three units: 12,000 BTU/h equals 1 ton of cooling equals about 3.5 kW of cooling power. The AC Size Calculator converts between them and rounds your load to the next standard size.
Do not confuse cooling kW with electrical kW: a 3.5 kW (cooling) unit draws only about 1.1 kW of electricity. The Electricity Cost Calculator shows what that means for your bill.
Special cases that change the answer
Kitchens add about 4,000 BTU/h from cooking. Rooms with floor-to-ceiling west-facing glass can need 25% more. Top-floor rooms under a flat roof, and rooms over uninsulated garages, run hot — add 10–20%.
Portable units deserve special caution: their single exhaust hose pulls conditioned air out of the room, cutting effective capacity by about 30%. A “12,000 BTU” portable performs like an 8,000–9,000 BTU split.