What Size Air Conditioner Do I Need? UK Sizing Guide

To size an air conditioner, start with the room's floor area. Daikin's rule of thumb for typical UK homes is about 0.1 kW of capacity per square metre, so a 20 m² bedroom needs around 2 kW and a 35 m² living room around 3.5 kW. Then adjust for ceiling height, insulation, sunny windows, heat from appliances and how many people use the room.

This guide walks through the calculation, the adjustments that matter, how kW and BTU compare, and when a rule of thumb is not enough.

At a glance

Room	Floor area	Approximate capacity (Daikin)
Small bedroom	20 m², 2.4 m ceiling	About 2 kW
Living room	35 m², 2.4 m ceiling	About 3.5 kW
Open lounge or kitchen	40 m², 2.4 m ceiling	About 4 kW
Open-plan area	50 m²	One 5 kW unit may work

Source: Daikin UK, air conditioner size guide. Figures are a starting point for a typical, reasonably insulated home.

Step 1: measure the room

Daikin's UK size guide says to multiply the length by the width to get the floor area in square metres, so a 5 m by 4 m room is 20 m². If several rooms need cooling, measure each one separately, because each needs its own capacity.

Daikin's rule of thumb is about 0.1 kW per square metre for average ceiling heights and insulation in UK homes. That is the starting figure before the adjustments below.

Step 2: adjust for the room

• Ceiling height. Daikin notes that most sizing rules assume a 2.4 m ceiling. A 3 m ceiling increases the room volume by 25%, and the capacity should rise to match. For high ceilings, work from the volume in cubic metres.
• Insulation and glazing. Daikin gives the example of a well-insulated 20 m² bedroom with double glazing needing only about 2 kW, while a poorly insulated, sun-facing room of the same size could need 2.5 kW.
• Sunny windows. Large south or west-facing windows add heat. Daikin says they often call for 10% to 20% more capacity.
• How the room is used. Kitchens, with ovens and hobs, often need 10% to 20% more than a living room of the same size, according to Daikin. Home offices with several computers and media rooms need extra too.
• People. Every person adds heat, so a busy meeting room or shop needs more than its floor area suggests.
• Where you are. Daikin gives the example of a 30 m² room needing about 3 kW in Scotland but around 3.6 kW in the South East of England, because of higher outdoor temperatures and longer cooling seasons.

kW and BTU

In the UK, air conditioning capacity is usually given in kilowatts. Daikin explains that 1 kW is the ability to move 1,000 joules of heat every second. Portable units and some older systems are rated in BTU per hour instead. One kilowatt is about 3,412 BTU per hour, so:

kW	Approximate BTU/h
2.0 kW	6,800 BTU/h
2.5 kW	8,500 BTU/h
3.5 kW	11,900 BTU/h
5.0 kW	17,100 BTU/h
7.1 kW	24,200 BTU/h

Wall-mounted units are commonly sold in steps such as 2.0, 2.5, 3.5, 5.0 and 7.1 kW. Pick the size closest to your adjusted figure, not the next one up "just in case". For portable units, Meaco's help centre suggests 7,000 to 9,000 BTU for small rooms such as bedrooms and home offices, and 10,000 to 12,000 BTU for larger bedrooms and living rooms.

Why the right size matters

Daikin calls oversizing one of the biggest pitfalls: buying bigger "just in case" increases running costs and makes the unit noisier. An oversized unit also cools the room too quickly, switches off, and does a poor job of taking moisture out of the air. Undersizing is the opposite problem. Daikin says it forces the unit to run constantly, raising energy bills and shortening its life, and the room may still not reach the set temperature on hot days.

Inverter units cope with a little oversizing better than fixed-speed ones, because they can slow the compressor down, but they are still most efficient when sized properly. Our guide to inverter air conditioning explains why, and our guide to SEER ratings covers efficiency at part load.

One unit or several

Daikin notes that one 5 kW unit may handle an open-plan 50 m² area, but that in a multi-room home, separate smaller units give better comfort control, because one large unit struggles to reach distant rooms. If you need, for example, 3.5 kW for a lounge and 2 kW for a bedroom, a multi-split system with one outdoor unit may suit better than two separate systems. Our guide to cassette and wall-mounted units covers the indoor unit options.

Offices, shops and commercial spaces

Rules of thumb work for typical homes. Commercial spaces vary far more, because of people, computers, lighting, kitchens, glazing and opening hours. A server room, a restaurant dining room and a quiet office of the same size can need very different capacities. For commercial jobs, a heat load calculation from a site survey is the right way to size the system. Our guides to office air conditioning and server room cooling cover those cases, and our commercial installation team surveys sites across London.

Questions people ask about air conditioner sizing

What size air conditioner do I need for my room?

Start with about 0.1 kW per square metre, Daikin's rule of thumb for typical UK homes. A 20 m² room needs around 2 kW. Then adjust for high ceilings, poor insulation, sunny windows and heat from appliances or people.

How many BTU do I need for a room?

Work out the kW first, then multiply by about 3,412. A room needing 2.5 kW needs about 8,500 BTU per hour. Portable units are usually rated in BTU, so check the manufacturer's room size guidance too.

Is it better to oversize an air conditioner?

No. Daikin says oversizing raises running costs and makes the unit noisier, and an oversized unit cycles on and off and removes less moisture. Choose the size closest to your calculated need.

What happens if my air conditioner is too small?

It runs constantly, uses more energy and wears faster, according to Daikin, and may not keep the room cool on the hottest days.

Does ceiling height affect air conditioner size?

Yes. Sizing rules usually assume a 2.4 m ceiling. Daikin says a 3 m ceiling increases the room volume by 25%, so the capacity should increase too.

Can I use one air conditioner for several rooms?

Only in open-plan spaces. For separate rooms, Daikin recommends separate indoor units, often on a multi-split system, because one unit struggles to cool distant rooms evenly.

The short version

Measure the floor area, start at about 0.1 kW per square metre, and adjust for ceiling height, insulation, sun, use and location. Choose the nearest standard size, not a bigger one. For offices, shops and anything unusual, get a proper heat load survey.

Last updated: 4 November 2026