BTU calculator for air conditioners

How many BTU do I need?

The calculator starts from the ENERGY STAR sizing chart, which links the floor area of a room to the cooling capacity it needs. The chart assumes an 8 ft ceiling, so for higher ceilings the calculator raises the capacity in proportion to the height.

Area to cool	Capacity needed
100 up to 150 sq ft	5,000 BTU/h
150 up to 250 sq ft	6,000 BTU/h
250 up to 300 sq ft	7,000 BTU/h
300 up to 350 sq ft	8,000 BTU/h
350 up to 400 sq ft	9,000 BTU/h
400 up to 450 sq ft	10,000 BTU/h
450 up to 550 sq ft	12,000 BTU/h
550 up to 700 sq ft	14,000 BTU/h
700 up to 1,000 sq ft	18,000 BTU/h

Then it applies the ENERGY STAR adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h for each person beyond two, and 4,000 BTU/h for a kitchen. The result is a range of ±10%, because insulation, windows and the floor you live on matter too.

A worked example

In feet (US). The calculator opens with a 15 × 12 ft bedroom, 8 ft ceiling, normal sun, two people.

• Area: 180 sq ft, so 6,000 BTU/h from the chart
• Range: 5,400 to 6,600 BTU/h
• Size to buy: a 6,000 BTU air conditioner (1.8 kW)

With a very sunny room the need rises to 6,600 BTU/h and the calculator moves up to an 8,000 BTU unit.

In metres (UK and elsewhere). A 4 × 4 m room with a 2.7 m ceiling: 16 m² is 172 sq ft, so 6,000 BTU/h, plus 11% for the ceiling, 6,644 BTU/h. In Europe split units start at 7,000 BTU (2.1 kW), which is the size to buy.

BTU per square foot

Rules of thumb say about 20 BTU per square foot. Below 300 sq ft the ENERGY STAR chart works out at 23 to 50 BTU per square foot, and less in big rooms, which have less outside wall for each square foot of floor. A 500 sq ft room needs 12,000 BTU/h; a 1,000 sq ft space, 21,000. Above about 700 sq ft, two units or a ducted system usually cool more evenly than one large window unit.

BTU, kW and tons

AC specs give cooling capacity in BTU per hour, in kW (in Europe) or in tons (central air): 1 kW = 3,412 BTU/h and 1 ton = 12,000 BTU/h. A 12,000 BTU unit is a one-ton, 3.5 kW air conditioner.

Common mistakes

• Buying too big. An oversized AC cools fast and shuts off, but removes little humidity: the room ends up cold and clammy, and the unit wastes energy.
• Looking only at square feet. A 10 ft ceiling holds 25% more air than an 8 ft one.
• Forgetting the kitchen. Stove and oven add heat: ENERGY STAR adds 4,000 BTU/h.
• One unit for the whole home. Closed rooms each need their own unit, so size one room at a time.