Air con sizing · Room-by-room cooling load
What Size Air Conditioner Do I Need?
The right air conditioner makes all the difference in your home. What it depends on, is how much heat your room really needs to get rid of, not just the size of the room, it's important to take into account things like where windows sit, and how much sun the room gets. For example, a 14 m² bedroom with a 2 m² west-facing window needs around 1.3 kW of cooling, so a 2.0 kW wall-mounted split would comfortably cover it. A larger 25 m² living room with 4 m² of west-facing glass needs closer to 2.6 kW, which pushes it up to a 3.5 kW unit. Windows often make a bigger difference than people expect, particularly if they get strong afternoon sun. That's why our calculator starts by asking about the glass in the room.
Air conditioner size by room
The examples you find here are common room sizes with standard windows, and we ran them all through our calculator using standard double glazing, with 2.4m ceilings. We recommend you take a quick look at your room, and maybe eyeball a few measurements to see which you have, and what unit size you might need.
| Room | Floor area | Windows | Cooling load | Unit size |
|---|---|---|---|---|
| Box bedroom, flat | 9 m² | 1.2 m² north | 0.45 kW | 2.0 kW |
| Double bedroom, flat | 13 m² | 2 m² east | 1.26 kW | 2.0 kW |
| Home office with gaming PC, flat | 10 m² | 1.5 m² west | 1.17 kW | 2.0 kW |
| Living room, house | 20 m² | 3 m² south | 1.86 kW | 2.0 kW |
| Living room, house | 25 m² | 4 m² west | 2.57 kW | 3.5 kW |
| Living room, top-floor flat | 25 m² | 4 m² west | 2.82 kW | 3.5 kW |
| Loft bedroom, dormer window | 15 m² | 1.5 m² south | 1.48 kW | 2.0 kW |
| Open-plan living and dining, house | 35 m² | 6 m² south | 3.39 kW | 3.5 kW |
For most of these single rooms, the output is 2.0 kW (the smallest common wall-mounted split size). The jump to 3.5 kW tends to happen when you combine a larger room with lots of glass, particularly west-facing glass. For example, the 25 m² living room with a 3 m² south-facing window can use a 2.0 kW unit. Increase the glass to 4 m² and face it west, and the recommended size jumps to 3.5 kW.
kW and BTU: what the size means
When people talk about the size of an AC unit, they are really talking about its cooling capacity. Put simply, it's how much heat it can move out of a room in each hour. UK wall splits are sold in kW, which is what we used here. Portable units on the other hand, are usually sold in BTU (British thermal units per hour). If you see a listing using one over the other, this is the formula you can use to translate it.
2.0 kW ≈ 6,800 BTU · 2.5 kW ≈ 8,500 BTU · 3.5 kW ≈ 12,000 BTU · 5.0 kW ≈ 17,000 BTU
Now, these figures are not the same as the electricity each unit pulls, the measurement is simply the amount of heat it can move. Roughly speaking, as the numbers increase, so too will the amount of power they draw, but it's not exact and there will be outliers. The air conditioning running cost calculator works out what each size costs per hour.
Where the heat in a room comes from
Your room's cooling load is simply all the different sources of heat that come into the room, added together. The calculator looks at four main areas.
Sun through the windows
Sunlight coming through the glass can add a surprising amount of heat to a room. The amount depends on the area of glass, how much solar energy the glass allows through and which direction the window faces.
2 m² × 0.7 × 500 W/m² (west) = 700 W
| Window faces | Summer peak sun on the glass | Gain per m² of standard double glazing |
|---|---|---|
| East | 500 W/m² | 350 W |
| West | 500 W/m² | 350 W |
| South | 400 W/m² | 280 W |
| North | 100 W/m² | 70 W |
Example peak values and g-values (standard double glazing 0.70–0.76, low-E 0.55–0.65, solar-control 0.25–0.40) from h2x Engineering's solar heat gain guide. Peak sun varies with latitude, date and cloud.
People
People are sources of heat too! Someone doing light activity at around 22°C gives off roughly 90 W of heat, while somebody sitting at rest will produce around 80 W (according to CIBSE Guide A). The calculator uses 90W per person, three people in the room adds 270 W to the cooling load.
Electrical equipment
Almost all the electricity used by equipment in the room eventually becomes heat. A gaming PC and monitor drawing 450 W therefore add around 450 W of heat. That's why a small home office or gaming room can need more cooling than you might expect from its floor area.
Walls, roof and air
The calculator uses an allowance per m² of floor that rises with exposure:
- 25 W per m² for a flat with another flat above
- 30 W per m² for a house
- 40 W per m² for a top-floor flat
- 55 W per m² for a loft room
These are planning figures rather than measurements of your particular property. A proper air conditioning survey will replace them with an assessment of the actual room.
Why west-facing rooms need bigger units
West-facing windows tend to cause problems because they catch strong sun in the late afternoon and early evening. At that point, the sun is lower in the sky and hits vertical glass more directly than the high midday sun hits a south-facing window. That is why the calculator uses a peak figure of 500 W/m² for west-facing glass compared with 400 W/m² for south-facing glass.
By late afternoon, heat has already built up in the walls and roof, and more people are often at home. A west-facing living room or bedroom can therefore reach its hottest point at exactly the time you want to use it. That is why a west-facing room may need the next unit size up even when a south-facing room of the same floor area does not. East-facing bedrooms have the same issue earlier in the day, which can matter if you sleep late during hot weather. Heathrow reached an insanely high 40.2°C on 19 July 2022, and on very hot days like that, the amount and direction of the glass can make a huge difference to how uncomfortable a room becomes.
Top-floor flats, loft rooms and extensions
A room under the roof gains heat through the ceiling as well as the walls and windows, because the sun heats the roof directly. The calculator's allowance rises from 25 W per m² for a flat with a flat above to 40 W for a top-floor flat and 55 W for a loft room.
- Loft conversions have sloping ceilings close to the roof covering. Roof windows face the sky and take sun for more of the day than a wall window, so they need a survey rather than this calculator.
- Rear extensions with rooflights or bi-fold doors combine a large glass area with a flat roof. Enter the doors as a window, then treat the result as a minimum.
- Conservatories are mostly glass, and their load is outside the range this calculator handles.
What happens if the unit is too small or too big
An undersized unit runs at full output for long periods on hot days and still may not reach the set temperature. Running flat out also costs more per hour: a 3.5 kW Daikin wall split costs about 5p an hour at half output on a warm day and about 20p an hour flat out, at 26.32p per kWh.
An oversized unit cools the room quickly, then turns down or switches off. Inverter splits can slow down a long way, so modest oversizing is less of a problem than it was with fixed-speed units. A much larger unit still costs more to buy, and short running periods remove less moisture from the air, so the room can feel clammy. Choosing the next standard size above the room's load avoids both problems.
Sizing air conditioning for several rooms
Work out each room separately with the calculator and add it to your list. Each room gets its own indoor unit, sized to that room. With a multi-split, all the indoor units connect to one outdoor unit, which the installer sizes from the combined load. A Daikin 3MXM68A9 outdoor unit, for example, has 6.8 kW of cooling capacity shared between up to three rooms.
Our air conditioning installation costs for West London list prices for one, two and three rooms. Before anything is fitted, check the planning rules for the outdoor unit.
Air conditioner size: common questions
What size air conditioner do I need for a bedroom?
A bedroom up to about 15 m² with an ordinary window comes out at 2.0 kW in the calculator. A bedroom with a large west-facing window, or one in a top-floor flat or loft, can need 2.5 kW. Put your bedroom's details into the calculator to check.
How many BTU do I need for a room?
Work out the room's cooling load in kW and multiply by 3,412 to get BTU per hour. A room with a 1.3 kW load needs about 4,400 BTU, so the smallest common units of 2.0 kW (about 6,800 BTU) cover it.
What size air conditioner do I need for 20 m²?
A 20 m² room needs a 2.0 kW unit with a modest window, and 2.5 kW to 3.5 kW with a large east- or west-facing window, several people or a top-floor position. Floor area alone doesn't settle it.
Can one air conditioner cool a whole flat?
A single wall unit cools the room it's in well, and air moves poorly into rooms beyond a closed door. To cool a bedroom and a living room, plan an indoor unit in each, on a multi-split or two separate systems.
Do I need a bigger unit to heat the room in winter?
Many reversible splits have a slightly higher heating output than cooling output. The Daikin FTXM25A is rated at 2.5 kW cooling and 2.8 kW heating, and the FTXM35A at 3.5 kW and 4.0 kW. Winter heat loss is a separate calculation, so ask the surveyor to check both if you plan to heat with it.
Get your rooms measured properly
Hounslow Air Conditioning arranges surveys and installations across Hounslow, Ealing, Hillingdon, Hammersmith & Fulham, Richmond and Kensington & Chelsea through F-Gas certified installers. The survey measures each room, its windows and its exposure, and sizes the unit to the room before you're quoted.