BTU Calculator

BTU and kW for a room: air conditioner size from ENERGY STAR's chart, or heating from the heat lost through walls, windows, roof, floor and air.

Air conditioners and heaters are rated in BTU per hour in the US, and mostly in kilowatts in the UK and Australia; 1 kW is 3,412 BTU/h. For cooling, this calculator follows ENERGY STAR's chart for room air conditioners: a 12 × 15 ft room (180 ft²) needs about 6,000 BTU/h, or 1.76 kW. It takes 10% off for a heavily shaded room, adds 10% for a very sunny one, and adds 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h for a kitchen. For heating, it adds up the heat the room loses through its outside walls, windows, roof and floor, plus the air that leaks in. A 4.5 × 3.5 m living room with two outside walls (8 m) of unfilled cavity brick, 2.5 m² of modern double glazing and an uninsulated solid floor loses about 1,400 W, or 4,773 BTU/h, at −3 °C outside. Each step of the working is shown.

Units
ft
ft
Cooling capacity
6,000 BTU/h

1.76 kW

Room area
180 ft²

16.7 m²

Tons
0.5

12,000 BTU/h per ton

How it was worked out
  1. Room: 15 × 12 ft = 180 ft² (16.7 m²)
  2. ENERGY STAR's chart for 150–250 ft²: 6,000 BTU/h
  3. Total 6,000 BTU/h ÷ 3,412 = 1.76 kW

This follows ENERGY STAR's sizing guide for room air conditioners. It's a rule of thumb: high ceilings, lots of west-facing glass, poor insulation or a hot climate can call for more, and ducted or whole-home systems should be sized by an installer from a heat-gain calculation. Air conditioners sold in Australia and the UK are rated in kW.

Common use cases

  • Choose the size of a window, portable or split air conditioner for a room
  • Work out how much heat a room needs before buying a heater or radiator
  • Convert a BTU rating to kW, or kW to BTU
  • See how much insulation or new windows would cut a room's heat loss

How to use

  1. Choose cooling or heating, and feet and °F or metres and °C.
  2. For cooling, enter the room's length and width, how sunny it is, how many people use it, and whether it's a kitchen.
  3. For heating, choose the room type, then enter its size, the length of outside wall, the window area and the outside temperature to design for.
  4. Choose the kind of walls, windows, roof and floor.
  5. Read the BTU/h and kW, and check each step in the working.

FAQ

How many BTU do I need per square foot?

ENERGY STAR's chart doesn't use a fixed rate, because small rooms need more per square foot than large ones. 6,000 BTU/h covers 150 to 250 ft², 12,000 BTU/h covers 450 to 550 ft² and 24,000 BTU/h covers 1,400 to 1,500 ft².

How do I convert BTU to kW?

Divide BTU per hour by 3,412: 12,000 BTU/h is 3.52 kW. To go the other way, multiply kW by 3,412. In the US, 12,000 BTU/h is also called one ton of cooling.

What size air conditioner do I need for a 20 m² room?

20 m² is about 215 ft², which falls in ENERGY STAR's 150 to 250 ft² band: 6,000 BTU/h, or 1.76 kW, before adjusting for sun, occupants or a kitchen. High ceilings, lots of glass or a hot climate can call for more, so check with an installer for split and ducted systems.

Why is heating worked out differently from cooling?

Heat loss depends on insulation and the temperature difference as much as on floor area. The example living room loses about 1,840 W with solid walls and single glazing, 1,400 W with unfilled cavity walls and double glazing, and 1,000 W once the cavity is filled.

Can I use this to size a boiler, furnace or heat pump?

Only as a rough guide for one room. To size a heating system for a whole home, an installer adds up every room and allows for heat moving between them, using BS EN 12831 in the UK and Europe or Manual J in the US.

Related tools

Results are estimates for planning, worked out from the figures you enter and typical values (U-values, air leakage, design temperatures) that vary from home to home. They are not engineering or professional advice. Have heating and cooling systems sized and fitted by a qualified installer: gas work must be done by a registered engineer (Gas Safe in the UK), and air conditioning refrigerant work by a licensed or certified technician.