How to Size an Air Conditioner (UK): kW, BTU & Heat Load

Correctly sizing a split or multi-split system is the difference between a customer who's comfortable and efficient and one who complains the unit "doesn't work". Sizing means matching the unit's capacity to the room's heat load — and that's driven by far more than floor area.

What drives the heat load

• Floor area and ceiling height — the volume of air to condition.
• Building fabric — a solid-wall period property loses and gains heat far faster than a modern insulated build.
• Glazing and orientation — south- and west-facing glass adds large solar gains on a sunny afternoon.
• Occupancy and equipment — people, computers, kitchens and server kit all add sensible and latent heat.
• Air infiltration — draughty buildings lose conditioned air continuously.

The rule-of-thumb starting point

For a quick sanity check, many UK installers use roughly 0.10–0.15 kW per m² of floor area for a typical room. So a 20 m² living room lands around 2.0–3.0 kW as a first guess. Treat this as a starting point only — it's useful to catch an obviously wrong specification, not to size the final unit. The air conditioning size calculator does the same sum on a phone; the room-by-room heat loss calculator does the real one.

The proper method: room-by-room load

A real calculation works each room individually, adding up the component loads — fabric loss/gain, solar gain through glazing, infiltration and internal gains from people and equipment — then applies a modest safety margin. The result is a required capacity in kW, which you match to the nearest suitable unit, allowing a little headroom without grossly oversizing.

Oversizing is a real trap: an oversized inverter unit short-cycles, controls humidity poorly and wears faster; an undersized one runs flat-out and never satisfies the room. The aim is a close capacity match.

Multi-split considerations

On a multi-split you size each indoor unit to its room, then check the outdoor unit against the combined load and the manufacturer's diversity/simultaneity data — you rarely need the outdoor unit to cover 100% of every indoor running at once. Pipe runs, height differences and refrigerant charge limits all feed back into the design.

Do it in seconds with Vento

Vento runs this room-by-room calculation the moment you enter each room's dimensions, glazing, insulation and orientation, then matches a capacity from a SEER/SCOP catalogue — so you never over- or under-size, and the sizing flows straight into the drawing and proposal. You can also try the public air conditioning size calculator for a quick estimate.