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BTU calculator for UK rooms

Use this BTU calculator to work out the cooling capacity your room needs, and the portable air conditioner size to shop for. It updates as you type and takes about a minute.

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A calculator showing 9,000 next to a dial gauge, a tape measure and a portable air conditioner, for sizing the cooling power of a room

Room details

TV, computer, games console

Enter your room size

Add the length and width in metres. The result updates as you type.

How the BTU calculator works

BTU (British thermal unit) measures how much heat an air conditioner can remove in an hour. One kilowatt of cooling is about 3,412 BTU.

This BTU calculator starts with the room's volume, because the air in the room is what has to be cooled, and adds the heat that people and appliances give off:

  • Room volume: length × width × ceiling height in metres, multiplied by 180 BTU per cubic metre.
  • People: 500 BTU each. A resting adult gives off about 100 to 150 W of heat.
  • Devices: 500 BTU each for a TV, computer or games console.
  • Sun exposure: multiplied by 0.95 for shaded rooms, 1.15 for south-facing ones.
  • Insulation: multiplied by 0.85 for new builds, 1.2 for older, draughty homes.

The result is rounded to the nearest 100 BTU, then matched to the closest standard size sold in the UK.

Worked examples of the BTU calculation for three typical UK rooms
RoomNeedsShop for
Small bedroom3.0 × 3.5 m5,700 BTU7,000 BTU
Living room5.0 × 4.0 m11,500 BTU12,000 BTU
Sunny loft room4.0 × 3.5 m, 2.4 m ceiling10,400 BTU10,000 BTU

BTU calculator results by room size

Typical BTU calculator results for a 2.5 m ceiling, two people, one device, average insulation and normal sun.

Recommended portable air conditioner BTU rating by room size
Room sizeTypical roomUnit to look for
Up to 10 m²Box room, small office5,000 to 7,000 BTU
11 to 16 m²Single or small double bedroom7,000 to 9,000 BTU
17 to 24 m²Double bedroom, small living room9,000 to 12,000 BTU
25 to 32 m²Large living room12,000 to 14,000 BTU
Over 33 m²Open planTwo units or a split system

Frequently asked questions

How do I use a BTU calculator for a room?

Multiply the room volume in cubic metres by about 180 BTU, then add 500 BTU for each person and each heat-producing device such as a TV or computer. Raise the total by about 15% for a sunny south-facing room and 20% for poor insulation. The BTU calculator above does all of this for you.

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

With a 2.5 m ceiling, two people and one device, a 20 m² room needs roughly 10,500 BTU, so a 10,000 to 12,000 BTU portable unit. Manufacturers often claim a 9,000 BTU unit covers 20 m² or more, but that assumes mild weather.

Does the BTU calculator recommend a larger unit to be safe?

Slightly larger is safer than too small, but not much larger, which is why the BTU calculator rounds to the nearest standard size. A unit more than about 20% over what the room needs cools quickly, switches off before it has dried the air and cycles on and off, which wastes electricity and leaves the room clammy.

What if the result falls between two unit sizes?

Choose the larger size if the room is sunny, under a roof or hard to insulate, and the smaller one if it is shaded and well insulated. The BTU calculator suggests the closest standard size within about 8% of the calculated need.

How do I size for a conservatory or loft room?

Both gain far more heat than ordinary rooms. Use high sun exposure and basic insulation, then consider adding another 20% or choosing a dual-hose unit.

Does a higher BTU air conditioner use more electricity?

Yes, a larger compressor draws more power, typically about 0.75 kW at 7,000 BTU and 1.2 kW at 12,000 BTU. Use the running cost calculator to see what that costs per hour.

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