Every figure checked against its source on 7 November 2026
What size of air conditioner does a room need?
Start from the floor area, then adjust for the glass, the aspect and the floor the room is on. A 15 square metre bedroom with a north window and a 15 square metre loft with two Veluxes facing south are not the same job. This guide gives the starting figures and the adjustments; the survey gives the answer.
Get a price
Three details and we call you back with the next step.
What is the starting figure by floor area?
| Room area | Cooling output |
|---|---|
| 10 to 15 m² | 1.5 to 2.0kW |
| 15 to 25 m² | 2.0 to 2.5kW |
| 25 to 35 m² | 2.5 to 3.5kW |
| 35 to 50 m² | 3.5 to 5.0kW |
These are iHeat's figures for rooms with standard ceilings, updated in May 2026. Multiply length by width in metres for the area. Source: iHeat.
What is a BTU, and how does it convert?
A British thermal unit per hour is the older measure that unit labels still carry. A kilowatt of cooling is about 3,412 BTU an hour, so the common 12,000 BTU wall unit is roughly 3.5kW. Glow Green's table gives 9,000 BTU for 14 to 23 square metres, 12,000 for 28 to 33 and 18,000 for 37 to 42, which lines up with the kilowatt figures above once converted. Source: Glow Green.
What pushes the figure up?
- Glass. Large windows, especially south- or west-facing ones, add to the load, and a conservatory is almost all glass.
- Loft rooms. A room under the roof takes heat from above as well as through its windows.
- People and equipment. A home office with two screens, or a kitchen-diner, runs warmer than a bedroom.
- Ceiling height. The tables assume standard ceilings; a Victorian room with a high ceiling holds more air.
- Doors. An open-plan room has to be sized as a whole.
Source: iHeat.
Why is bigger not safer?
Because an oversized unit short-cycles. It drops the air temperature quickly, switches off before it has removed the moisture, and then starts again a few minutes later, which leaves the room clammy, wears the compressor and wastes electricity. A unit matched to the load runs longer at a lower speed, which is quieter and dries the air properly. Modern inverter units can turn down, but not without limit.
How do we size a room at the survey?
We measure the floor and the ceiling height, measure the glass and note its direction, ask how many people use the room and when, and look at what is above and beside it. For heating we also check the room's insulation and the coldest morning it has to cope with: Greenwich Observatory, the nearest Met Office station to Dartford, averages about 23 nights of air frost a year. You get the figure and the reasoning in the written quote, and the air conditioning in Dartford page explains the rest of what we check. Source: Met Office.
Questions people ask
Will a single unit cover two rooms?
Only where no door separates them. Cooled air does not pass through a closed door, so two rooms generally means two indoor units on a multi-split.
My room is 20 square metres but faces south with a glass door. What size?
Probably the top of the 2.0 to 2.5kW band or a step above it. The survey settles it; the table is a starting point.
Does the heating output match the cooling output?
Not exactly. Heating output is usually a little higher on the label but falls as the outdoor temperature drops, so a unit relied on in winter is sized for the cold mornings.
Sources we checked
Worth knowing before you ring
With these we can usually tell you on the first call whether the job is simple.
- The postcode
- Whether it is a house, a flat or business premises, and roughly its age
- If you rent or lease, who owns the external wall
- Which room, how big, and the direction its window faces
- Any management company, conservation area or listing that applies
Decided? Our air conditioning in Dartford page covers what we install, what we maintain and where we go.
The services this guide leads to
Air conditioning installation
The whole job from the first measurement to the handover of the remote, with every consent checked on the way.
Home air conditioning
The back bedroom, the new flat with the glass wall, the loft nobody can sleep in: cooled in summer, warmed in winter.
Conservatory air conditioning
For the glass room that is a greenhouse in July and a fridge in January, and the garden office beside it.
Other guides
What does air conditioning cost to fit and to run?
Where the money goes: the unit, the labour, the wiring, the electricity, and the extras a flat or a new estate brings.
Air conditioning and planning permission: the rules for England
Every permitted development condition in one table, then the twists Dartford's new estates and conservation areas add.
Air conditioning with no outdoor unit: how monoblocs work
The through-the-wall units for flats and blocks that cannot take an outdoor box.
Air conditioning in a flat: who has to agree, and in what order
Lease, freeholder, management company and planning, in the order to tackle them.
Bedroom air conditioning: what it does for sleep, and what it costs
The sleep threshold, noise at night, sizing and the price of a quiet bedroom.
Cooling a loft conversion: the options that work
Why lofts cook, the free fixes first, then where a unit goes on a dormer.
How to check an air conditioning installer on the F-gas register
The company certificate, the engineer's category and where to look them up.
Portable or fixed air conditioning: which is right for you?
Price, power, noise and running cost, and when a portable is the right call.
Want us to ring you?
A name, a number and a postcode is enough. Say which room if you can. Or ring 01322 952910.