HVAC / BTU Calculator

How the estimate works

The calculator starts from the usual rule of thumb of about 20 BTU per hour per square foot of floor area (roughly 215 BTU/h per m²), scales it for ceiling height above 8 ft / 2.44 m, and then adjusts it: +10% for a very sunny room and −10% for a shady one, +600 BTU/h for each person beyond the first two, +4,000 BTU/h for a kitchen, and factors for insulation (poor +25%, good −10%) and climate (hot +15%, mild −10%). 1 BTU/h = 0.293 W, and one “ton” of cooling is 12,000 BTU/h (3.517 kW). An air conditioner that is too small never reaches the temperature you want; one that is too large switches on and off constantly, cools unevenly and removes less humidity — so choose a unit close to the result. This is a quick sizing guide for single rooms, not a load calculation: for a whole house, or if you are buying central or ducted systems, ask an HVAC professional for a proper heat-load calculation (for example ACCA Manual J).