BTU Calculator

What BTU means for air conditioners

A BTU (British thermal unit) measures heat. Air conditioners and heaters are rated in BTU per hour, which is how much heat they can move or produce. Choosing the right capacity matters: too small and the unit runs constantly without reaching the set temperature, too large and it short-cycles, which wastes energy and leaves the air humid. This calculator gives a quick estimate for a single room from its floor area and the conditions that change the load.

How the estimate is calculated

The base figure is 20 BTU per square foot for cooling. The calculator then scales it for ceiling height compared with the standard 8 feet, increases it by 10 percent for very sunny rooms or reduces it by 10 percent for heavy shade, adds 600 BTU for each person beyond two, and adds 4,000 BTU for a kitchen because of the heat from appliances. Heating uses a climate factor between 30 and 60 BTU per square foot, from mild to very cold regions. Results are rounded to the nearest 100 BTU, and you can also see the cooling capacity in tons and kilowatts.

Choosing a unit

Pick a unit at or slightly above the result, within about 10 percent. Window units commonly come in 5,000, 8,000, 10,000 and 12,000 BTU sizes, while mini-splits are sold in steps such as 9,000, 12,000 and 18,000. For open-plan areas, add the connected rooms together. Poor insulation, large west-facing windows and a top-floor location all push the real need higher. Your input values never leave the browser, since the whole calculation runs locally on your device.

Example: a 20 × 15 ft living room with 8 ft ceilings, normal sun and two people needs about 6,000 BTU of cooling by the base rule. Add a sunny exposure and the figure rises to roughly 6,600 BTU, so a 8,000 BTU window unit would be a comfortable choice.