# What size air conditioning do I need for a room?

*The Datum team · Heating, cooling & plumbing engineers · Datum · updated 2026-10-19 · Air con*

Allow about 0.1 kW per square metre as a starting point. Daikin's guide puts a well-insulated 20 m² bedroom at about 2 kW and a 35 m² living room at about 3.5 kW, with large south or west windows adding 10–20%. BestBuilders says an undersized unit runs flat out and never cools the room, while an oversized one short-cycles and wastes money. We confirm the size at the survey.

## Why floor area isn't enough

Two rooms the same size can need very different units. A north-facing bedroom with modest windows is easy; a south or west-facing living room with big glazing takes a lot of afternoon solar gain and needs more capacity. Daikin adds 10–20% for large south or west windows.

A conservatory is the extreme case — mostly glass, so heat load is high. We size to the glazing and aspect, not a square-metre rule of thumb.

## The cost of getting it wrong

An undersized unit is the classic false economy: it looks cheaper on the quote, but BestBuilders says it runs flat out and never cools the room. An oversized one short-cycles and wastes money. Right-sized is the aim.

## How we size it

At the free survey we look at the room's dimensions, glazing and orientation, insulation and how you use the space, then spec the unit that holds temperature efficiently. It's ten minutes that saves you money every summer the system runs — and it's part of the fixed quote, not an add-on.

> An undersized unit fails on exactly the days you bought it for. We size to the glass and the aspect, not the floor plan.
>
> — The Datum team, Heating, cooling & plumbing engineers

## Related questions

### What size AC unit for a bedroom?

Daikin puts a well-insulated 20 m² bedroom at about 2 kW and a poorly insulated or sun-facing one at about 2.5 kW; a loft or south-facing room can need more. We size to the actual room rather than a generic figure so it's quiet and efficient at night.

### Is a bigger AC unit always better?

No — an oversized unit short-cycles (cools fast, switches off, repeats), which is inefficient and less comfortable. Correct sizing beats bigger every time.

### Can one unit cool the whole house?

Usually not well — a single indoor unit cools the room it's in. Whole-home comfort means a unit per key room (multi-split), which we'll size and price honestly at the survey.

## Sources & verification

- [Daikin UK — Air conditioner size guide (about 0.1 kW per m²; a well-insulated 20 m² bedroom about 2 kW, 2.5 kW if poorly insulated or sun-facing; a 35 m² living room about 3.5 kW; large south or west glazing often 10–20% more; accessed 19 Oct 2026)](https://www.daikin.co.uk/en_gb/residential/inspiration/articles/air-conditioner-size-guide.html)
- [BestBuilders — Air conditioning cost UK 2026 (single-room split £1,500–£3,000; Daikin or Mitsubishi single-room unit plus install £1,800–£3,200; brand choice can move a single-room price by £400–£800; two-room multi-split £2,800–£4,500; three-to-four-room £3,500–£7,000; pipe runs over about 5 m add £150–£400; budget £70–£150 a year for a service; an undersized unit "runs flat out and never cools the room" while an oversized one "short-cycles and wastes money"; accessed 19 Oct 2026)](https://bestbuilders.co.uk/costs/air-conditioning-cost-uk)

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