Choosing a system · 6 min read
What Size Air Conditioner Do I Need for My Room?
Capacity is about heat gain, not floor area. Here is what a surveyor measures and why two identical-looking rooms can need different systems.
Published 2026-07-20 · Updated 2026-07-20 · WDI Group (Surrey) Ltd
What capacity actually means
Air conditioning capacity is expressed in kilowatts, and it describes how much heat the system can move per hour. Domestic wall mounted units commonly range from around 2kW for a small bedroom up to 5kW or more for a large open-plan space.
The aim is to match capacity to the room's peak heat gain, with a sensible margin, and no more.
What a surveyor assesses
Two rooms with the same floor area can need different capacities. A north-facing bedroom with one small window is a very different proposition to a south-facing loft room with two roof windows.
- Room dimensions including ceiling height, giving volume rather than area
- Glazing area, glazing type and orientation
- Shading from trees, blinds, overhangs or neighbouring buildings
- Insulation levels, roof construction and whether the room is above or below other rooms
- Number of people typically in the room
- Heat from computers, televisions, lighting and equipment
- Whether the space is open to adjoining rooms or hallways
Why bigger is not better
An oversized system reaches the set temperature quickly and then stops, so it spends its time cycling rather than running steadily. That reduces dehumidification, can make the room feel clammy, and often makes the unit more noticeable because it runs at higher fan speeds in short bursts.
An undersized system never catches up on the hottest days. Correct sizing is where comfort actually comes from.
Online calculators and rules of thumb
A rule of thumb based on room volume gives a rough starting point and is useful for budgeting. It cannot account for a glazed roof, a poorly insulated flat roof above, or a rack of computer equipment in the corner.
If you use our quotation form to give us room dimensions and details, any capacity we mention beforehand is a guide only. Final system sizing and installation requirements are confirmed following a survey.
Sizing multi-room systems
On a multi-split system, each indoor unit is sized for its own room, and the outdoor unit is sized for the realistic combined load rather than the simple sum of every indoor unit at full output. How many rooms you expect to run at once matters, so we ask.
Frequently asked questions
- Can I just tell you the room size over the phone?
- It helps us have a useful conversation, but we will not commit to a capacity on that basis. Glazing, orientation and construction change the answer significantly.
- Should I allow for future extensions?
- Tell us at survey if you plan to knock through or extend. It can change both the sizing and the sensible position for the outdoor unit.