BTU calculator for a room
The air conditioner or heater size one room needs, in BTU/h and watts, with the math shown.
Measure your room — these are examples.
Result
Cooling to look for
Type the room's length and width.
Feedback
How it works
Cooling starts from the ENERGY STAR room air conditioner chart, which gives a capacity for each band of floor area and four adjustments: minus 10 percent for a heavily shaded room, plus 10 percent for a very sunny one, plus 600 BTU/h for each regular occupant over two, and plus 4,000 BTU/h for a kitchen. The chart assumes an 8 ft ceiling, so a taller room is scaled by its height; that step is our assumption.
Heating uses King Electric's sizing guide: about 6 W per sq ft in a newer home and 10 W in an older one, 2 W more in a cold climate, and half again for large windows. Garages and sheds with little or no insulation use 50 BTU/h per sq ft, an older farm-shop rule.
The result is rounded up and matched to the next common size. The full method, the sources and the places where we chose between two readings are on How we calculate.
More from Roomtherm
- BTU, watts, kW and tons converter Any capacity in four units, and the room it cools on the ENERGY STAR chart.
- How many square feet each AC size cools The chart read backwards, for shaded and sunny rooms and extra people.
- Radiator size for your flow temperature The ΔT50 rating to buy when a heat pump runs the system cooler.
- Garage heater size Heat for one-, two- and three-car garages by insulation level.