Air Conditioning Size Calculator — kW and BTU
Work out what cooling capacity your room actually needs before you price it. Enter the floor area, ceiling height, sun, occupants, equipment and insulation for a recommended kW and BTU range — then confirm it with a survey.
Size your room in 30 seconds
Six questions. The result is a cooling-load range in kW and BTU, the standard wall-unit size that covers it, and an indicative running cost.
How the sizing calculator works
Installers size domestic air conditioning from a rule of thumb of about 100–150 W of cooling per square metre of floor area, then adjust for the things that push a room outside the norm. This tool starts at 125 W/m² for a standard 2.4 m ceiling and scales it: up for higher ceilings (more air to cool), up for south- or west-facing rooms and big glazing (solar gain is the single biggest swing), up for poor insulation, and down for shaded, well-insulated rooms. It then adds roughly 100 W per person normally in the room and the wattage of any equipment that runs while you want it cool — a desktop PC, a TV, kitchen appliances.
The output is deliberately a range — 10% either side of the mid-point — rather than a single number, because the inputs are estimates and because wall units come in fixed steps: 2.0, 2.5, 3.5, 5.0 and 7.0 kW are the common domestic sizes. The calculator names the smallest standard unit that covers the top of your range. If the bottom and top of the range straddle two sizes, it says so, because that is exactly the call a survey is for.
kW is what UK installers quote in; BTU per hour is the older imperial unit you will see on portable units and American sites. The conversion is simple — 1 kW is about 3,412 BTU/h — and both figures are shown so you can compare like with like. For running cost the tool assumes the July 2026 price cap of around 26p/kWh and a seasonal efficiency of roughly 2.8–3.6 at full output, which is why a 2.5 kW unit comes out at about 18–23p per hour flat out and a good deal less once the room is at temperature and the inverter modulates down.
Air conditioning size by room area (2026)
Standard 2.4 m ceiling, average sun and insulation, two people. Use the calculator above for anything unusual.
| Room area | Recommended unit | Typical room |
|---|---|---|
| Up to 10 m² | 2.0 kW (~7,000 BTU) | Box room, small study, nursery |
| 10–15 m² | 2.5 kW (~9,000 BTU) | Double bedroom, home office — the most common UK size |
| 15–25 m² | 3.5 kW (~12,000 BTU) | Lounge, master bedroom, large office |
| 25–40 m² | 5.0 kW (~18,000 BTU) | Open-plan living room or kitchen-diner |
| 40–55 m² | 7.0 kW (~24,000 BTU) | Very large open-plan space |
| Over 55 m², or several rooms | Two units, multi-split or ducted | See the multi-split and ducted calculators |
Ranges at roughly 100–150 W/m². Conservatories, loft rooms and glass-heavy extensions can need double the figure for their floor area; always survey those.
Why getting the size right matters
Undersized is the obvious failure: the unit runs flat out on the hottest afternoons and never quite gets there. Oversized is the one people do not expect. A unit with far more capacity than the room needs hits the setpoint in minutes, shuts down, and then cycles on and off all afternoon. That short-cycling wears the compressor, costs more to run than a smaller unit working steadily, and — because the coil never runs long enough to condense much moisture out of the air — leaves the room cold but clammy. A correctly sized inverter unit instead modulates down to 30–40% output and holds temperature quietly, which is also the cheapest way to run it.
Size also drives the price. Each step up the capacity ladder adds £200–£500 to the installed cost of a single split, so sizing a 3.5 kW room as 5 kW costs more on day one and every hour after. Once you have a size, the split system cost calculator prices it including pipe run and condenser siting, and the running costs guide shows what it costs per summer and per winter.
The same unit heats in winter. Every modern domestic split is a reversible air-to-air heat pump, and the heating output at rated conditions is usually a little higher than the cooling figure, so a room sized for summer will normally heat well too. If heating is the main reason you are looking, read air conditioning vs a heat pump — air-to-air systems are efficient (SCOP 3.5–4+) but are not eligible for the Boiler Upgrade Scheme, which covers air-to-water only.
Finally, a survey is not a formality. An F-Gas certified engineer will check wall construction, glazing and orientation, where the indoor head and outdoor condenser can physically go, the pipe route, and whether your electrics need a dedicated circuit. Most surveys land within one size step of this calculator; where they differ, trust the survey.
Sizing calculator FAQs
Related guides and calculators
Split system cost calculator
Now you have a size, price the install including pipe run and siting.
Multi-split calculator
Several rooms off one condenser — priced per room.
Which system?
Split, multi-split or ducted — the trade-offs explained.
Running costs
Pence per hour and pounds per summer for each unit size.
Installation cost guide
Full 2026 pricing for every system type.
Planning permission
When the outdoor unit needs consent.
Sources & methodology
- Cooling-load rule of thumb (100–150 W/m² for UK domestic rooms, ~100 W sensible heat per occupant) as used in installer sizing practice and CIBSE domestic guidance; this tool is a simplified estimator, not a CIBSE heat-gain calculation.
- Unit capacity steps (2.0 / 2.5 / 3.5 / 5.0 / 7.0 kW) from manufacturer domestic ranges (Daikin, Mitsubishi Electric, Fujitsu, Panasonic, Toshiba, Hitachi, LG, Samsung).
- Conversion 1 kW = 3,412 BTU/h. Electricity price context from the Ofgem price cap, July 2026 (~26p/kWh).
- Results are indicative ranges for guidance only. Sizing is confirmed by survey.
Get your room sized and priced by an engineer
Air Con Surrey is a free matching service. Tell us about the room and we'll match you with vetted, F-Gas certified engineers who survey it, confirm the size and quote for exactly your job.