Air conditioning BTU calculator
This air conditioning BTU calculator works out the cooling each room needs in kW and BTU, then picks the smallest standard unit that covers it. Add up to five rooms for a multi-split, and you get a guide price and a running cost with the answer.
Most UK bedrooms and living rooms need 2.5kW (9,000 BTU) to 3.5kW (12,000 BTU) of cooling. Start from about 135 watts per square metre of floor at a 2.4m ceiling, then add for a sunny window, a roof overhead or a tall ceiling. A 14 square metre double bedroom facing south comes to 2.1kW, so it takes a 2.5kW unit; a 22 square metre living room facing south comes to 3.3kW, so a 3.5kW unit.
We check the numbers in the room
At a free survey anywhere in London we measure the room, find where the condenser can go and put a fixed price in writing.
Guide ranges for budgeting, excluding VAT. We confirm a fixed price after a free survey; the price we agree is the price you pay.
Get a fixed quote for these rooms
Two quick steps. We attach your calculator result, so the surveyor starts from your numbers.
No account, no obligation. We only use this to prepare your quote.
How the air conditioning BTU calculation works
We start from floor area: length times width, multiplied by 135 watts per square metre. We took that figure from inside the 100 to 150 W/m² band UK sizing guides use, for a room with a 2.4m ceiling, one or two people and another room above.
Then we adjust for the three things that move the number most: which way the main window faces, what sits above the room and the ceiling height. Loft conversions, top-floor flats and glazed kitchen extensions overheat first in London, so each gets its own factor.
We round the heat load up to the smallest standard wall-mounted size that covers it: 2.0, 2.5, 3.5, 4.2, 5.0, 6.0, 7.1kW. Some ranges skip 4.2 and 6.0kW, so a quote may step to the next size.
Why we stop at the next size up
A unit a size too big reaches temperature fast, then stops and starts. Short cycling wears the compressor and leaves the room clammy, because a unit dries the air only on long, steady runs. If a quote goes two sizes above the calculation, ask why.
How we work out the running cost
We divide the unit size by an efficiency (EER) of 3 for budget units to 4 for premium ones, which gives the electricity draw at full output, then price it at 26.32p per kWh, the Ofgem price cap average for Direct Debit customers, 1 October to 31 December 2026. For a heatwave month we assume eight hours a day for 30 days, at about half power once the room is cool. Our FAQ on what air conditioning costs to run goes further.
What the survey adds
The calculator gives a starting figure. At survey we measure the glazing, check the insulation and allow for the heat a flat roof stores through a July afternoon, then size the final unit from those.
| What changes the number | Adjustment |
|---|---|
| Main window faces northLittle direct sun at the hottest time of day | × 0.85 |
| Faces eastMorning sun, gone by the afternoon peak | × 1.00 |
| Faces south, or not sureSun for most of the day | × 1.10 |
| Faces westLow afternoon sun arrives with the day's peak temperature | × 1.20 |
| Roof above the roomThe roof heats the ceiling all afternoon | × 1.15 |
| Loft conversionSloping ceilings under the slates, often with roof windows | × 1.35 |
| Glazed extension or roof lightsGlass roofs and walls let in more sun than any other room type | × 1.50 |
| Ceiling heightMore air and more wall to cool | × height ÷ 2.4 |
| Each person beyond twoA seated adult gives off about 100 watts | + 100W |
| Home officeComputers and screens running all day | + 200W |
| Kitchen or open-plan kitchenHob, oven and fridge heat | + 300W |
What size room does each air conditioner cool?
The largest floor area each standard unit covers, by the direction of the main window, at a 2.4m ceiling with two people in the room. One kilowatt is 3,412 BTU per hour.
| Unit | Faces north | Faces south | Loft, south |
|---|---|---|---|
| 2.0kW7,000 BTU | 17m² | 13m² | 9m² |
| 2.5kW9,000 BTU | 21m² | 16m² | 12m² |
| 3.5kW12,000 BTU | 30m² | 23m² | 17m² |
| 4.2kW14,000 BTU | 36m² | 28m² | 20m² |
| 5.0kW18,000 BTU | 43m² | 33m² | 24m² |
| 6.0kW21,000 BTU | 52m² | 40m² | 29m² |
| 7.1kW24,000 BTU | 61m² | 47m² | 35m² |
BTU and kW questions, answered
How many BTU do I need for my air conditioner?
Multiply the room's floor area in square metres by about 135 watts, adjust for sun, a roof above and ceiling height, then multiply the kilowatts by 3,412 to get BTU. A 14 square metre bedroom with a south-facing window comes to about 7,100 BTU, so it takes a 9,000 BTU (2.5kW) unit, the next standard size up.
What size room will a 12000 BTU air conditioner cool?
A 12,000 BTU (3.5kW) unit cools a room of up to about 30 square metres with a north-facing window, or 23 square metres facing south, at a standard 2.4m ceiling. In a loft conversion that falls to about 17 square metres, because the roof adds heat all afternoon.
Is 7000 BTU enough for a bedroom?
For a small bedroom, yes. A 7,000 BTU (2.0kW) unit covers up to about 13 square metres with a south-facing window, or 17 facing north. Under a roof that limit drops to about 11 square metres, or 9 in a loft conversion, so a larger double or a loft bedroom needs 9,000 BTU (2.5kW). A single-hose portable rated at 7,000 BTU cools less than a split system of the same rating, because pushing room air out through the hose pulls warm outside air in around doors and windows.
What size room will a 7kW air conditioner cool?
A 7kW (24,000 BTU) unit, listed by manufacturers as 7.1kW, cools up to about 61 square metres with a north-facing window or 47 facing south at a 2.4m ceiling, and about 35 square metres in a loft conversion. It is the largest common wall-mounted size; a bigger open-plan space takes two indoor units or a ducted system.
How do I convert BTU to kW?
Divide BTU per hour by 3,412 to get kilowatts, or multiply kilowatts by 3,412 to get BTU. Manufacturers sell air conditioners in rounded classes: 9,000 BTU is a 2.5kW unit, 12,000 BTU a 3.5kW unit, 18,000 BTU a 5kW unit and 24,000 BTU a 7.1kW unit.
Ready to price your air conditioning?
Tell us the rooms and the rough plan. We reply within one working day with an honest cost bracket, then a free survey and a fixed written quote.