What size air conditioner do I need?

Work out the size for your room

The size calculator gives you a starting size in kW and BTU from your room’s measurements. Use it for one room at a time, anywhere in the UK.

kW and BTU explained

kW and BTU both measure how much heat a unit can move out of a room. kW is the standard UK measure, and the one on energy labels. BTU stands for British thermal unit, and on air con it really means BTU an hour.

One kW equals 3,412 BTU an hour. Here’s how the usual sizes convert:

BTU labels are rounded and vary by maker, so compare kW when you weigh up two units. Some makers also sell smaller units and sizes in between, such as 1.5 kW and 4.2 kW.

A unit’s kW rating is its cooling output, not the electricity it uses. A 2.5 kW unit might draw about 0.6 kW at full power. What it costs to run.

The sizes on this page are for fixed units. Portable units are sold by BTU too, but they’re less efficient, so the same room needs more. Portable or fixed?

Sources: Daikin UK’s size guide, NIST’s conversion tables, Mitsubishi Electric’s data sheets, Orion Air Sales’ BTU labels, and Heatable on portable units.

What makes a room need more cooling

A room needs more cooling when more heat gets in or builds up. The calculator starts at 0.1 kW per square metre, Daikin UK’s rule of thumb for average ceilings and insulation. It then adjusts for the four things below.

Sun and windows

Large south or west-facing windows often add 10 to 20%, Daikin says. The calculator adds 20% for a very sunny room, because a unit has to cope on the hottest afternoons. It takes off 10% for a room that’s heavily shaded.

Top floors and lofts

Rooms under a roof take in heat from above, so they need more than the same room lower down. The calculator adds 20%.

UK guides agree on the point but give no figure, so the 20% is a rule of thumb. The one table with numbers is from US maker Kenmore. It shows 12 to 33% more than a room with a home above it, up to about 23 square metres.

High ceilings

Ceilings above 2.4 metres mean more air to cool, so the calculator scales up with the height. A 3 metre ceiling adds 25%, the example Daikin gives. A lower ceiling gets no reduction.

People in the room

People add heat, so the calculator adds 0.18 kW for each person beyond the first two. That’s Daikin’s figure, and ENERGY STAR’s 600 BTU a person is almost the same.

What the calculator can't see

Some rooms need more than the calculator shows, so treat its answer as a lower limit:

• Kitchens: ovens and hobs often add 10 to 20%, Daikin says. Cooling an open-plan room.
• Home offices: several computers add heat too.
• Glass rooms: a conservatory needs far more than its floor area suggests. Cooling a glass room.
• Poor insulation: Daikin says a 20 square metre bedroom might need 2 kW if well insulated, and 2.5 kW if poorly insulated and sun-facing.
• London and the South East: Daikin says a 30 square metre room could need 3 kW in Scotland and 3.6 kW in the South East.

The calculator has no step for these, which is one reason it rounds up. A survey checks them all. What we check at the survey.

Sources: Daikin UK’s size guide, ENERGY STAR (US government), a Kenmore manual’s sizing table, and Air Con Centre on lofts and glass rooms.

Sizes for common rooms

Bedrooms usually come out at 2.0 to 2.5 kW, and living rooms at 2.0 to 5.0 kW. Which unit suits a bedroom.

The table assumes two people and a 2.4 metre ceiling. Room names are a rough guide, so go by your own measurements.

A figure just over a size moves up to the next one, because the calculator rounds up. Above 7.0 kW, two units or a multi-split may suit better than one.

Here’s the sum for a top-floor room of 5 by 4 metres. It has a 2.7 metre ceiling, big south-facing windows and three people:

1. Floor area: 5 × 4 = 20 square metres.
2. Starting figure: 20 × 0.1 = 2.0 kW.
3. Ceiling: 2.0 × (2.7 ÷ 2.4) = 2.25 kW.
4. Very sunny: 2.25 × 1.2 = 2.7 kW.
5. Top floor: 2.7 × 1.2 = 3.24 kW.
6. Third person: 3.24 + 0.18 = 3.42 kW.
7. Size: round to 3.4 kW, then up to the next size, 3.5 kW. That’s usually sold as 12,000 BTU.

Source: Daikin UK’s size guide. The room names and the rounding rule are ours.

Why too big or too small is a problem

A unit that’s too small can’t keep up, and one that’s too big wastes money and can leave the room clammy.

A unit that’s too small:

• struggles to reach a comfortable temperature
• runs constantly, which raises your bills
• wears out sooner

A unit that’s too big:

• costs more to buy
• switches on and off too often, which adds running costs and wear
• can be noisier
• cools the room before it removes the damp, so the air feels clammy

Inverter units can slow down to match the room, which is why the calculator rounds up and not down. A 3.5 kW Mitsubishi Electric wall unit runs anywhere from 0.8 to 4.0 kW.

Sometimes the best size is none. If a room gets hot on a few afternoons a year, shade and cooler night air may be enough. Cheaper ways to stay cool.

Sources: Daikin UK’s size guide, ENERGY STAR (US government) and Mitsubishi Electric’s MSZ-LN data sheet.