How to size air conditioning for a room (BTU guide)

An air conditioner that is too small runs constantly and never quite cools the room. One that is too big cools too fast, cycles on and off, and leaves the room humid. Sizing matters. Here is how installers do it.

The rough rule of thumb

For a typical UK room:

• Small bedroom (10-14 sq metres): 5,000 to 7,000 BTU (about 1.5 to 2 kW cooling)
• Standard bedroom (15-20 sq metres): 7,000 to 9,000 BTU (about 2 to 2.6 kW cooling)
• Living room (20-30 sq metres): 9,000 to 12,000 BTU (about 2.6 to 3.5 kW cooling)
• Big open-plan lounge/kitchen (30-45 sq metres): 12,000 to 18,000 BTU (about 3.5 to 5 kW cooling)
• Home office (10-14 sq metres): 6,000 to 8,000 BTU (about 1.7 to 2.3 kW cooling)

BTU/hr and kW convert like this: 1 kW cooling equals about 3,412 BTU/hr. So a “2.5 kW split unit” is a “9,000 BTU unit” - same thing, two labels.

The formula installers use

The rule of thumb works for a typical room. If your room is unusual (huge windows, south-facing, high ceiling, cathedral roof) the formula matters.

Base sizing: floor area in square metres, times 135 BTU per square metre.

Then adjust:

• South-facing with big windows: +20%
• North-facing with small windows: -10%
• Ceiling over 2.7 metres: +15% per 30 cm above 2.4 m
• Top-floor room under an uninsulated loft: +15%
• Two adults regularly in the room: +1,200 BTU total
• Kitchen with cooking equipment in the room: +4,000 BTU
• Home office with several computers or a games console setup: +2,000 BTU

Example: 4 metre by 5 metre south-facing living room with big windows, 2.7 metre ceiling, two people sitting there most evenings.

• Base: 20 sq m x 135 = 2,700 BTU
• South-facing +20%: 3,240 BTU
• Ceiling +15%: 3,726 BTU
• Two people +1,200: 4,926 BTU

That is way too low - the base multiplier is too small for a UK room where you want proper cooling on a hot day. Real installers use 200 BTU per sq metre as a base for cooling-first rooms.

• Base: 20 sq m x 200 = 4,000 BTU
• South-facing +20%: 4,800 BTU
• Ceiling +15%: 5,520 BTU
• Two people +1,200: 6,720 BTU

Actually still low for a summer heatwave. In practice the installer sizes a 9,000 BTU or 12,000 BTU unit for this room. The formula is guidance, not gospel.

The three sizing mistakes we see

1. Sizing for cooling only, ignoring heating. If you plan to use the unit for winter heating too, size it to the heating requirement not the cooling one. Heating in a UK winter usually needs 30% more capacity than cooling in a UK summer.
2. Sizing for peak demand. A single hot week does not justify an oversized unit for the rest of the year. Better to size for typical hot summer conditions and accept the room stays at 22 not 20 on the worst day.
3. Sizing per room in a multi-room install without checking the outdoor unit. Multi-split systems have a total-capacity limit. Two 9,000 BTU indoor heads sharing an outdoor unit rated for 12,000 BTU will never both run at full output.

What “BTU” means when the label says kW

Every UK-sold unit lists both figures. The BTU number is the imperial/US standard. The kW number is the metric standard. They mean the same thing - cooling capacity in an hour.

Nominal cooling capacity is the “at 35 degrees outside” number. Cooling drops off as it gets hotter outside. A 3.5 kW nominal unit produces about 3.2 kW at 42 degrees outside. Fine in a UK summer.

Heating capacity is a separate number. Many budget units heat at 80% of their cooling capacity - a 2.5 kW cooling unit only heats at 2 kW. Premium units heat at 100%+ of the cooling rating.

Rule of thumb we use in practice

• Small bedroom or home office: 2.5 kW cooling (9,000 BTU) is the default. Undersizes rarely fail.
• Big lounge or open-plan: 3.5 kW cooling (12,000 BTU) is the default. Anything smaller in a south-facing UK living room disappoints.
• Multi-room install: talk to the installer. Total kW capacity across all indoor heads should not exceed 90% of outdoor unit capacity, or the whole system runs sub-optimally.

Get an installer to size the room for you

Every survey a Cooler Spaces installer does includes a proper BTU calculation for each room. That is what they are trained for. If you specify a room size and get a quote without a survey, that quote is a guess.

Fill in the quote form with your rooms and rough sizes. Three fixed quotes back within 24 hours, each with a proper capacity calculation.

Or check the prices page for a rough bracket first.