How to size an air conditioner
Getting the size right matters more than almost any other decision. Too small, and the unit struggles on the hottest and coldest days. Too big, and it short-cycles, controls humidity poorly and wastes energy and money. Capacity is measured in kW (with BTU as the older imperial equivalent), and the right figure depends on your room’s size plus a handful of other factors. The tool below gives a quick estimate; a proper survey confirms it.
Sizing tool
A guide estimate. Answer a few questions about the room.
Enter a floor area to see a suggested size.
Start with the room
The biggest single factor is the room’s floor area. As a rough starting point, a standard, reasonably insulated UK room needs in the region of 0.12 kW of cooling per square metre — so a 25 m² (≈ 270 ft²) living room starts at around 3 kW, rounding up to a 3.5 kW (≈ 12,000 BTU) unit. But area is only the beginning: the factors below can move the answer up or down by a fair margin, which is exactly why a flat “kW per m²” figure on its own is unreliable.
The factors that change the size
- Sun and orientation — a south- or west-facing room, or one that catches a lot of sun, needs more; a shaded or north-facing room needs a little less.
- Glazing — large windows, patio doors and lots of glass add to the load.
- Occupancy — people give off heat, so a room that’s regularly busy needs more than the same room used by one person.
- Heat-generating equipment — kitchens, home offices full of kit, and AV gear all add heat.
- Ceiling height — a tall or vaulted ceiling means more air to condition.
- Insulation and age — a well-insulated or new-build room holds temperature better; an older, solid-wall or poorly insulated room needs more.
- Top-floor rooms — a room directly under the roof gains more heat in summer.
The method behind the estimate
The sizing tool starts from your room’s floor area, applies a base figure of about 0.12 kW per square metre for a standard room, then adjusts up or down for the factors above — sun, glazing, occupancy, equipment, ceiling height, insulation and roof exposure — before rounding up to the nearest available unit size. It shows the result in kW with the BTU equivalent. It’s deliberately a guide: the aim is to get you into the right ballpark and rule out obviously wrong sizes, not to replace a professional calculation.
Why a survey still matters
A free-standing calculator can’t see your home. A proper survey measures the room, accounts for construction, glazing, aspect and how you’ll use the space, and sizes the unit — along with the pipe runs and outdoor-unit position — precisely. For anything beyond a simple single room, and certainly for multi-split and ducted systems, that’s the step that gets it right. Larger and commercial spaces need a full heat-load calculation.
Conservatories are a special case
They gain and lose heat far faster than ordinary rooms because of all the glazing, so the standard sizing rule understates what they need. Treat them separately — see air conditioning for conservatories — and get a survey.