BTU Calculator

Find the air conditioner size a room needs in BTU per hour and kW, using the ENERGY STAR sizing chart, plus a BTU, kW and watts converter.

Reviewed by Dany, RightSums team · Updated

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home-energy · Daily life · UK & US

Enter the floor area of a room in square metres or square feet, or its length and width, and the calculator looks up the cooling capacity on the ENERGY STAR room air conditioner sizing chart. It then applies the ENERGY STAR adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h for each regular occupant over two and 4,000 BTU/h for a kitchen. The answer is shown in BTU per hour and kilowatts, using 1 kW = 3,412.14 BTU/h. A separate converter switches between BTU/h, kW and watts. It sizes cooling only and does not size radiators or heaters, and the chart covers rooms from 100 to 2,500 sq ft.

Accepted inputs

  • Floor area in m² or sq ft, or length and width
  • Sun or shade on the room
  • Number of people usually in the room
  • Whether it is a kitchen
  • A value in BTU/h, kW or W to convert

Outputs

  • Cooling capacity in BTU per hour
  • Cooling capacity in kW
  • Step-by-step adjustments
  • BTU/h, kW and W conversion

How to use the BTU Calculator

  1. Enter the room's floor area, or its length and width, in metric or imperial units.
  2. The calculator converts the area to square feet and finds the matching band on the ENERGY STAR room air conditioner sizing chart.
  3. It adjusts that figure by 10% for a heavily shaded or very sunny room, then adds 600 BTU/h for each person over two and 4,000 BTU/h for a kitchen.
  4. The total is shown in BTU per hour and in kilowatts, and the converter below turns any heat output into BTU/h, kW or watts.

Frequently asked questions

How many BTU do I need for my room?

It depends on floor area. On the ENERGY STAR chart, a room of 150 up to 250 sq ft needs 6,000 BTU/h. A 20 m² bedroom is 215.3 sq ft, so it needs 6,000 BTU/h, which is 1.76 kW. Sun, extra people and cooking all add to that.

How many BTU is 1 kW?

One kilowatt is 3,412.14 BTU per hour, based on the NIST factor of 0.2930711 W per BTU/h. So a 2.5 kW air conditioner is about 8,530 BTU/h, and a 12,000 BTU/h unit is about 3.52 kW. Use the converter in the tool for any other figure.

What size air conditioner do I need for a sunny room?

Add 10% to the chart figure. A 30 m² living room is 322.9 sq ft, which the ENERGY STAR chart puts at 8,000 BTU/h. Very sunny adds 800, and a third regular occupant adds 600, giving 9,400 BTU/h or 2.75 kW.

How many BTU do I need for a kitchen?

ENERGY STAR says to add 4,000 BTU/h if the unit cools a kitchen. A 500 sq ft kitchen-diner starts at 12,000 BTU/h. If it is very sunny and four people use it regularly, the total is 12,000 + 1,200 + 1,200 + 4,000 = 18,400 BTU/h, or 5.39 kW.

Can I use this BTU calculator for radiators or heating?

Not for sizing. Heat loss depends on insulation, windows, walls and the outside temperature, and we have not found a simple official UK rule we can cite. The converter will turn a heater or radiator output between BTU/h, kW and watts, but ask a heating engineer for a heat-loss calculation.

Sources

The rates and rules in this tool come from:

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