How many BTU for a 4m x 4m room?

A 4m x 4m room is 16m². Using the rough guide of 400–600 BTU per m², that points to roughly 6,400–9,600 BTU. A unit around 9,000–10,000 BTU is a sensible starting point, but adjust upwards for a sunny, south-facing room or a high ceiling. Run your own figures through our BTU calculator before buying.

Is it better to oversize or undersize an air conditioner?

Neither extreme is ideal. An undersized unit runs constantly and never quite cools the room. A heavily oversized unit cools the air quickly but cycles on and off, which can leave the room feeling cold and clammy because it has less time to remove humidity. Aim to match the unit to the room, then nudge up slightly if there are strong heat sources.

BTU stands for British Thermal Unit. For air conditioners it describes cooling capacity — how much heat the unit can remove from a room per hour. A higher BTU rating means more cooling power, which is why room size and heat sources determine the BTU figure you need.

What size air conditioner do I need?

As a rough guide, UK rooms need roughly 400–600 BTU per square metre of floor area. So a typical 12m² bedroom needs somewhere around 5,000–7,000 BTU, while a 25m² living room is closer to 10,000–14,000 BTU. That range exists because the exact figure depends on how much heat your room gains — and a few common factors can move the number up or down significantly.

What does BTU mean for sizing?

BTU (British Thermal Unit) measures cooling capacity: how much heat the unit can shift out of the room each hour. Get it roughly right and the unit cools comfortably without overworking. Get it badly wrong and you either never feel cool enough, or the unit short-cycles and leaves the room cold and damp. The aim is a sensible match, not a precise science.

How do I work out the basic number?

Start with floor area. Measure the length and width of the room in metres and multiply them together to get the area in square metres, then multiply by roughly 500 BTU/m² as a middle-of-the-road figure. For a 4m x 5m room (20m²) that gives about 10,000 BTU as a starting point. From there, adjust for the factors below.

Which factors increase the number I need?

Several things mean a room gains more heat and therefore needs more cooling capacity:

Sun exposure. A south- or west-facing room with large windows can need 10–20% more capacity than a shaded north-facing room of the same size.
Ceiling height. Standard sizing assumes ceilings around 2.4m. Rooms with high or vaulted ceilings have more air volume to cool, so add capacity.
Occupants. Each person adds body heat. A room used by several people — a busy living room or home office — needs a little more.
Kitchens. Cooking appliances throw out a lot of heat, so kitchens and open-plan kitchen-diners need a meaningful uplift.
Insulation and glazing. A well-insulated room with modern double glazing holds its temperature better and may need slightly less; an older, draughty or single-glazed room needs more.

What about undersizing or oversizing?

It is tempting to buy big “to be safe”, but a heavily oversized unit cools the air fast and switches off before it has removed much humidity, which can leave the room cold and clammy. An undersized unit, on the other hand, runs flat out and never catches up on a hot day. Match the unit to the room first, then add a modest margin for strong heat sources rather than doubling up.

How do I get a number for my exact room?

Rules of thumb get you in the right ballpark, but the easiest way to account for your room’s size, aspect and use is to put the details into our BTU calculator. It applies the same factors above and gives you a tailored figure, so you can shop with a clear target capacity in mind.