Air conditioner BTU calculator
An oversized air conditioner cools the air to the thermostat setting quickly and then shuts off — before it has run long enough to pull moisture out. The room ends up cold and clammy, the compressor short-cycles, and both comfort and lifespan suffer. Sizing to the actual load and letting the unit run longer at lower output is the correct answer, which is also why inverter units outperform fixed-speed ones of the same rating.
A rule-of-thumb estimate for a single room. A whole-house system or a heat pump should be sized by a proper heat-loss survey.
A rough cooling load is about 25 watts per cubic metre of room, adjusted for insulation, sun and the number of people. A 5 × 4 m room with 2.4 m ceilings needs roughly 1,200 W or 4,100 BTU per hour — which in air-conditioner terms is a small 1.2 kW unit.
How to size an air conditioner
Rules of thumb are fine for a single room and dangerous for a house. The 25 W per cubic metre figure assumes ordinary construction, ordinary glazing and no unusual heat sources. A room with a south-facing glass wall, a server rack or a kitchen in it breaks the assumption completely. So does a conservatory, which can need triple the figure. Where the answer really matters — a heat pump for a whole house, or a system you will live with for fifteen years — the right tool is a room-by-room heat-loss calculation to a recognised method, not a multiplication.
Questions
Roughly 20 BTU per square foot for a standard-height room, which is where the common table of "150 ft² needs 5,000 BTU" comes from.