ac_unit AC BTU Calculator star

How Air Conditioner Size Is Calculated

The starting point is a simple rule of thumb — roughly 20 BTU per square foot (about 600 BTU per m²) of floor area — but that number alone is only a rough baseline. Real cooling capacity depends on several correcting factors on top of raw room size: how much direct sun the room gets, how well insulated the walls, roof and windows are, ceiling height, how many people typically occupy the room, and how much heat-generating equipment (kitchen appliances, computers, multiple TVs) is running inside it. This calculator applies the base rule, then layers on adjustments — commonly +10% for a sunny exposure, −10% for a shaded room, roughly +600 BTU per occupant beyond two, and +4,000 BTU if the space includes a kitchen.

Worked example: a 20 m² bedroom starts at 20 × 600 = 12,000 BTU (a "1-ton" unit) under the base rule. If that same room faces south with strong sun exposure, add 10% to reach about 13,200 BTU; if it instead faces north and is shaded, subtract 10% to land around 10,800 BTU. A 20 m² open-plan room that includes a kitchenette would instead add roughly 4,000 BTU on top of the base figure, landing near 16,000 BTU regardless of sun exposure.

What You Should Know

• Undersized capacity means the unit runs constantly at full power and often still never reaches the target temperature on hot days, because it simply cannot remove heat faster than the room and its occupants generate it. This also shortens compressor lifespan from continuous strain.
• Oversized capacity causes "short cycling" — the unit cools the air quickly, shuts off before it has run long enough to also remove much humidity, then restarts soon after. The room ends up cold on a thermometer but still damp and clammy to the skin, which is exactly the comfort problem air conditioning is supposed to solve.
• The correct size sits between those two failure modes — matched closely enough to the room's actual heat load that the unit runs in reasonably long, complete cycles that cool the air and dehumidify it together.
• The simple "BTU per square meter" rule is only a starting point, not a final answer — the same size room can need meaningfully different capacity depending on sun exposure, insulation quality, ceiling height and occupancy.
• Manufacturer sizing charts and professional load calculations (Manual J in the US, for example) go further still, accounting for window count and orientation, local climate data, and construction materials — worth commissioning for a whole-house system, even though a rule-of-thumb estimate is generally sufficient for a single room.