What Size Air Conditioner Do I Need? BTU and kW by Room Size

Air conditioners are sized by how much heat they can take out of a room, in kilowatts (kW) or British Thermal Units per hour (BTU). The room's floor area does most of the work:

1. Measure the floor: length × width in metres gives square metres (m²). For an open-plan space, measure all of it — a unit cools the whole space it can reach.
2. Multiply by about 100 W per m² for a typical room, or 125–150 W per m² if it's sunny, on the top floor or busy.
3. If the ceiling is higher than about 2.4m, scale up in proportion — a 3m ceiling adds about a quarter.
4. Multiply the watts by 3.412 for BTU, and choose the next unit size up.

Worked example: a 5m × 4.5m living room is 22.5 m². It faces south with patio doors, so use 125 W: 22.5 × 125 = 2,810 W, or about 9,600 BTU. A 9,000 BTU unit would be slightly short on hot afternoons, so choose 12,000 BTU.

Here are the same sums for common room sizes. The common split unit sizes are 9,000 BTU (about 2.6 kW), 12,000 BTU (about 3.5 kW), 18,000 BTU (about 5 kW) and 24,000 BTU (about 7 kW).

Treat the chart as a starting point. A room-by-room heat gain calculation by your installer is the accurate way, and it's worth having for anything unusual — a glass extension, a server cupboard or a commercial kitchen.

• Sun on big windows. South- and west-facing glass, bi-fold doors and roof lights let in a lot of heat. Conservatories and glass extensions can need far more than 150 W/m² — get those assessed.
• Top floors and lofts. Rooms under a roof, especially a flat roof, heat up through the ceiling all afternoon.
• People and equipment. Each person gives off roughly 100 W of heat, and computers, TVs, lighting and cookers add their own. Offices, salons, gyms and kitchens need the higher figure.
• High ceilings. There's more air to cool; scale by the ceiling height ÷ 2.4.

Things that bring it down: a shaded, north-facing room, good loft insulation, blinds or solar film on the windows, and keeping doors to hotter parts of the house shut.

Both measure the same thing. One kilowatt is 3,412 BTU per hour, so a "12,000 BTU" air conditioner cools at about 3.5 kW. Makers round the BTU figure, so check the kW rating on the spec sheet — our AURA 12,000 BTU unit is rated at 3.4 kW cooling and 3.42 kW heating.

The "typical room" column uses 100–125 W per m². For a sunny or top-floor room, take the lower end of that range — or the next unit up.

Too small and the unit runs flat out on the hottest days without ever reaching the temperature you set, and it's louder doing it. This is the more common mistake.

Too big costs more to buy. Older on/off units also cooled in short bursts that left rooms clammy, because they switched off before taking the moisture out. Inverter units — all of the AURA range — slow the compressor down instead of switching off, so going one size up is rarely a problem.

Yes, if it's a heat pump model, as every AURA unit is. The heating output is about the same as the cooling output, and because a heat pump moves heat in from outside rather than making it, it typically gives several kilowatts of heat for every kilowatt of electricity in mild weather — much cheaper to run than a plug-in electric heater, which gives one for one.

Heat pump output does fall as it gets colder outside, so in a poorly insulated room on a freezing day it may not keep up on its own. For spring and autumn, and as a top-up to the central heating, it works well.

• Installer: split units use R32 refrigerant, so the refrigerant connections must be made and commissioned by an F-Gas certified engineer. We supply the units and installation parts but don't fit air conditioning.
• Outdoor unit: decide where it goes — a solid wall or the ground, with space around it for air, and not right under a neighbour's bedroom window.
• Pipe run: every AURA unit comes with 5 m of copper pipe and cable; a 5 m extension kit adds more.
• Drainage: the indoor unit makes condensate water that has to drain away by gravity or through a condensate pump.
• Electrics: larger units often need their own circuit, which is work for a qualified electrician.
• Permission: outdoor units can need planning permission — check with your council, especially for a listed building, a conservation area or a flat, where you'll also need the freeholder's consent.

The air conditioning buying guide covers types, noise and running costs in more detail.