BTU Calculator
Air conditioner sizing is the classic 'bigger must be better' trap - and it's wrong in both directions. An undersized unit runs flat-out on hot afternoons and never pulls humidity down; an oversized one cools the air in five minutes, shuts off before it dehumidified anything, and leaves the room cold-but-clammy while short-cycling the compressor to death. The right number comes from area, ceiling height, insulation, sun, people and what appliances share the room.
This calculator starts from the 20 BTU per square foot rule of thumb, scales it for ceiling volume, then applies honest adjustments - poor insulation +15%, heavy sun +10%, +600 BTU per person past the second, +4,000 for kitchens - and reports the result three ways: BTU, kW and tons, so it matches whatever label your local seller uses. Inputs are remembered for the next room in the house.
How to use
- Enter room area and ceiling height.
- Set insulation, sun exposure, usual occupancy and whether it's a kitchen.
- Match the BTU (or kW / tons) figure to the unit's cooling rating, not its size.
Frequently asked questions
How many BTU do I need per square meter?
About 215 BTU per m² (20 per sq ft) with a standard 2.7 m ceiling as the starting point - then adjust up for sun, poor insulation, many occupants or kitchen heat, and down for shade and tight modern construction. A 20 m² bedroom lands near 5,000-6,000 BTU; a sunny 40 m² living room near 12,000-14,000.
What happens if the AC is too big?
It short-cycles: the compressor satisfies the thermostat before it has run long enough to dehumidify, leaving a cold, clammy room and doubling on/off wear. Humidity removal - half of comfort - happens in the long, steady runs that oversized units never get. Oversizing wastes money twice: at purchase and in lifetime.
Are kW and tons different from BTU?
Same quantity, different labels: 12,000 BTU/h = 1 ton = about 3.5 kW of cooling. Regions differ in habit - the US sells by BTU, much of Asia by 'HP' or kW, HVAC pros by tons - which is why the calculator shows all three for the same sized unit.
Does this work for heating with a heat pump?
The room-loss logic transfers, but heating loads are usually quoted separately and depend more on climate and insulation than cooling does. Size for whichever season dominates your bill, and remember heat pump ratings list cooling and heating capacities separately - match the right column.