How to use the AC BTU calculator
- Enter the room length, width and ceiling height. Measure only the space the unit will cool; open-plan areas count as one room.
- Choose the sunlight: heavily shaded for rooms with little direct sun, very sunny for west-facing rooms with large windows.
- Choose the insulation level. Older homes with single glazing or thin walls are usually “poorly insulated”.
- Enter how many people normally use the room at the same time, and tick kitchen if you are cooling one.
- Compare the result with unit sizes on sale, which may be listed in BTU/h, kW or tons.
How the size is worked out
× (ceiling height ÷ 8 ft, for ceilings above 8 ft)
× 0.9 if heavily shaded, × 1.1 if very sunny
× 0.9 if well insulated, × 1.1 if poorly insulated
+ 600 BTU/h for each person beyond two, + 4,000 BTU/h for a kitchen
The chart, the sun, people and kitchen adjustments follow ENERGY STAR’s room air conditioner guidance. The ceiling-height and insulation factors are our own planning assumptions, shown openly so you can judge them. One kilowatt of cooling is about 3,412 BTU/h, and one ton is 12,000 BTU/h.
Worked example
A 16 × 14 ft bedroom with 8 ft ceilings, average sun and insulation, used by two people:
- Floor area: 16 × 14 = 224 sq ft, which falls in the chart’s 150–250 sq ft band: 6,000 BTU/h.
- No adjustment for an 8 ft ceiling, average sun, average insulation or two people, so the room needs 6,000 BTU/h.
- In other units that is 1.76 kW or 0.5 ton; the next common size up is 6,000 BTU/h (0.5 ton, 1.8 kW).
Tips and common mistakes
- Bigger is not better. An oversized unit cools the air quickly and switches off before it removes moisture, leaving the room cold and clammy and wasting energy.
- Look at efficiency, not only size. Compare the efficiency rating on the label (such as CEER in the US or the energy label elsewhere); a more efficient unit of the same size costs less to run.
- Central systems need a load calculation. For whole-house air conditioning or heat pumps, ask an HVAC contractor for a proper load calculation such as Manual J.
- Installation is regulated. Split and ducted systems involve refrigerant and electrical work that must be done by licensed technicians.
US, UK and Canada differences
In the US and Canada, window and portable units are sold by BTU/h, and the ENERGY STAR chart used here was written for those markets. UK homes rarely have fixed air conditioning, but portable units are widely sold, usually labelled in BTU/h with a kW figure too. Canadian buyers often choose heat pumps that cool in summer and heat in winter; size those with an installer.
Metric units and other countries
In metric mode, enter the room in metres; the calculator still uses the same chart after converting the area. In Pakistan, India and the Gulf, split air conditioners are usually sold by the ton, such as 1, 1.5 or 2 ton units, and the ton figure here lets you compare directly. Very hot climates and top-floor rooms under a concrete roof often need the “very sunny” and “poorly insulated” settings.