How to work out the BTU you need
- Choose Feet or Metres and enter the room’s length and width.
- Pick how much Sunlight the room gets, enter the number of People who usually use it, and tick Kitchen if it is one.
- Read the suggested cooling capacity in BTU per hour, with the same figure in kilowatts and tons. The chart below shows where your room falls.
Where the numbers come from
ENERGY STAR publishes a sizing chart for room air conditioners that matches cooling capacity to floor area, from 5,000 BTU per hour for 100 to 150 sq ft up to 34,000 for 2,500 sq ft. The calculator looks up your room in that chart and then applies the adjustments ENERGY STAR recommends. Below 100 sq ft it gives the smallest size on the chart. Above 2,500 sq ft it falls back to the common rule of thumb of 20 BTU per sq ft and flags that a space that size needs a professional estimate.
Choosing a unit
Air conditioners are sold in steps (5,000, 6,000, 8,000, 10,000, 12,000 BTU and so on), so pick the nearest size at or just above the result. Labels in Europe and Asia often give cooling in kW instead, which the result also shows. Check the energy label too: a more efficient unit of the same size costs less to run, and the electricity cost calculator shows what a unit’s wattage will cost you per month.