The short answer
As a rough starting point, allow about 0.1–0.15 kW of cooling per square metre of floor area — so a typical 16 m² bedroom often lands around a 2.0–2.5 kW unit. That is only a guide: the real figure depends on glazing, which way the room faces, occupancy, heat from appliances and insulation. A qualified installer should do a proper heat-load calculation before specifying a system, because an undersized unit never copes and an oversized one wastes money and dehumidifies poorly.
Getting the size right is the single most important decision when buying air conditioning. Too small and the unit runs flat out yet never reaches temperature on a hot day; too large and it cycles on and off, costs more to buy and run, and leaves the room clammy. This guide explains the rough rule of thumb people use to ballpark a size, the factors that change it, and why the figure that actually goes on the order should come from a proper survey.
Sizing at a glance
- Rough rule ~0.1–0.15 kW cooling per m² of floor
- Small bedroom (~12 m²) Often around 1.5–2.0 kW
- Living room (~25 m²) Often around 3.5 kW
- Raises the load South-facing glass, kit, people
- Decided by A proper heat-load calculation
- Wrong size costs Comfort, efficiency and money
The rough rule of thumb
People usually start sizing with a simple ratio: roughly 0.1 to 0.15 kW of cooling capacity for every square metre of floor area. Multiply your room’s floor area by that figure and you get a ballpark for the capacity (in kilowatts) a unit needs to deliver. On a 20 m² room that points to somewhere around 2.0–3.0 kW. Capacity is sometimes quoted in BTU/h instead of kW — the two are interchangeable, and 1 kW is about 3,412 BTU/h, so the same 20 m² room works out near 7,000–10,000 BTU/h. For how that conversion works, see the BTU calculator explained.
This rule is genuinely useful for a sanity check, but it is deliberately crude. Floor area alone ignores ceiling height (you are really cooling a volume) and every source of heat in or around the room. Treat it as the opening bid, not the answer.
What changes the figure
The heat that air conditioning has to remove is called the heat load (or cooling load). Two rooms of identical floor area can need very different capacities once you account for what is actually heating them up:
- Glazing and aspect — large windows, especially south- or west-facing, let in a lot of solar gain and push the load up.
- Occupancy — every person adds heat; a busy office or a bedroom for two needs more than a quiet study.
- Appliances and lighting — computers, kitchens, servers and TVs all dump heat into the room.
- Insulation and construction — a well-insulated modern room holds temperature; a single-skin extension or loft conversion loses and gains heat fast.
- Roof and floor position — a top-floor room under a hot roof, or a conservatory, runs much warmer.
| Room (typical) | Floor area | Rough capacity |
|---|---|---|
| Small bedroom / study | ~10–12 m² | ~1.5–2.0 kW |
| Double bedroom | ~14–18 m² | ~2.0–2.5 kW |
| Living room | ~22–28 m² | ~3.5 kW |
| Open-plan / kitchen-diner | ~30–40 m² | ~5.0 kW+ |
These are starting points only — a sunny, glass-heavy living room can need the same capacity as an open-plan space a third larger.
Why a proper heat-load calculation matters
A competent installer will not size from floor area alone. They carry out a heat-load (cooling-load) calculation that adds up solar gain through the glazing, gains from people and equipment, fabric gains through walls and roof, and ventilation, then specifies a unit with the right capacity at your design conditions. This is the figure that should go on the quote. When you get an installation quote, ask how the capacity was worked out — a one-line “2.5 kW should do it” without any survey is a warning sign.
Cooling several rooms
Sizing gets more involved when more than one room is in play. You do not simply add the floor areas together and buy one big unit, because cool air does not flow freely between rooms and each space has its own heat load, glazing and use. Instead, each room is sized individually, and then the question becomes how to serve them: separate single splits, or one multi-split outdoor unit feeding several indoor heads. The per-room calculation is the same either way; the difference is how the capacity is delivered. On a multi-split, the outdoor unit must be rated to meet the combined demand of the indoor units likely to run at once, so the design has to account for diversity — whether every room is really cooled simultaneously or only some at a time.
Open-plan spaces bring their own wrinkle. A kitchen-diner or a knocked-through living space behaves as one large volume with mixed heat sources — cooking heat, big windows, several occupants — so it often needs more capacity than its floor area alone suggests, and sometimes more than one indoor unit to distribute the cool air evenly. Long, thin or L-shaped rooms can leave a single unit struggling to reach the far corners. These are exactly the judgements a survey is for, which is why a measured heat-load calculation, not a rule of thumb, decides the final specification.
Single room or several?
If you are cooling more than one room, the choice of system changes the sizing conversation. A single wall unit suits one room; for several you compare a split versus multi-split system, where one outdoor unit feeds several indoor heads, each sized to its own room. Whichever route you take, each indoor unit still needs to match the load of the space it serves, so the per-room calculation does not go away — it just gets repeated. This is general guidance and not a substitute for a site survey; a qualified, F-Gas-certified installer should confirm the final specification.
Not sure what capacity your room needs?
A qualified installer can carry out a proper heat-load calculation and recommend the right size unit for your room. A quick survey avoids buying something too big or too small.
Frequently asked questions
How many kW of air con do I need per square metre?
As a rough starting point, allow about 0.1–0.15 kW of cooling per square metre of floor area. A 20 m² room therefore points to roughly 2–3 kW — but glazing, aspect and heat load can shift that, so an installer should confirm it with a heat-load calculation.
What happens if my air con is too small?
An undersized unit runs continuously on hot days but never reaches the target temperature, which wastes energy, wears the compressor and leaves the room uncomfortable. Sizing correctly from the start avoids this.
Is it bad to oversize air conditioning?
Yes. An oversized unit cools the air fast then shuts off before removing enough humidity, so the room feels cold and damp, and the compressor short-cycles — raising running cost and wear. Match capacity to the calculated load.
Do I need a survey to size air con?
For anything beyond a rough estimate, yes. A proper heat-load calculation accounts for glazing, orientation, occupancy and equipment, and is what a reputable installer uses to specify the unit that goes on your quote.
Sources & further reading
- Energy Saving Trust — home cooling and air conditioning guidance
- GOV.UK — Approved Document L (conservation of fuel and power)
- Daikin — technical sizing and product data (used as factual specification)
- Mitsubishi Electric — heat-load and capacity guidance (used as factual specification)
This guide is general information, not a site-specific survey or a substitute for a quote from an F-Gas-certified installer. Installation, servicing and refrigerant handling are legally restricted to F-Gas-certified engineers.