Air conditioning is sized in kilowatts (kW) of cooling. Some brands also quote BTU, where 1 kW is roughly 3,400 BTU. Get the size right and the system runs quietly and efficiently. Get it wrong and it either struggles on hot days or switches on and off too often, leaving the room clammy.
A simple guide by room size
| Room size | Typical room | Typical unit |
|---|---|---|
| Up to 15 m² | Bedroom, home office, small garden room | 2.0 to 2.5 kW |
| 15 to 25 m² | Main bedroom, living room, larger garden room | 3.5 kW |
| 25 to 40 m² | Large living room, kitchen-diner | 5.0 kW |
| Over 40 m² | Open-plan space, shop or office floor | two units, or larger |
To work out your room size, multiply the length by the width in metres. A 4 m by 3.5 m bedroom is 14 m².
What makes a room need a bigger unit
- Sun and glass. South or west-facing rooms, conservatories and rooms with bifold or patio doors gain a lot of heat.
- Loft and top-floor rooms. Heat from the roof keeps these rooms warm well into the evening.
- High ceilings. There is more air to cool than the floor area suggests.
- Kitchens and busy rooms. Cookers, computers and people all add heat.
- Insulation. Older or poorly insulated rooms need more capacity, especially if you also want to use the system for heating.
Why bigger is not always better
An oversized unit cools the room quickly, then switches off before it has taken the moisture out of the air. The room can feel cool but damp, and frequent stopping and starting uses more energy. Modern inverter units are good at running gently, but the starting size still matters.
How we size your system
At your free survey we measure each room and look at the windows, orientation, insulation and how you use the space, then recommend the right size for each room. You can get an instant estimate now with our quote builder, where you choose each room's size, or read how much air conditioning costs.