Checked against the rules on 3 November 2026
What size air conditioning do you need?
For a typical room with a standard ceiling, plan on about 2 to 2.5kW of cooling for 15 to 25 square metres and 2.5 to 3.5kW for 25 to 35 square metres. Sun, glass, ceiling height, insulation and how the room is used can all move that figure, so a floor-area chart is a starting point rather than an answer.
Get a quote
Name, number and postcode are all we need. We phone you back.
Cooling output by room size
These bands come from iHeat's 2026 guide, which calls them only a guide, for rooms with standard ceiling heights:
| Room size | Suggested cooling |
|---|---|
| 10 to 15 square metres | 1.5 to 2.0kW |
| 15 to 25 square metres | 2.0 to 2.5kW |
| 25 to 35 square metres | 2.5 to 3.5kW |
| 35 to 50 square metres | 3.5 to 5.0kW |
Other published charts run higher: Glow Green suggests 12,000 BTU, about 3.5kW, for 28 to 33 square metres. Units are sold in set sizes, and 2.5kW, 3.5kW and 5kW are common, so the real choice is usually between two.
kW and BTU: the same thing in different units
Sellers quote cooling in either kilowatts or British thermal units per hour. One kilowatt is about 3,412 BTU per hour, so a 12,000 BTU unit is about 3.5kW and a 9,000 BTU unit about 2.6kW. Check that you are comparing cooling output rather than the electricity the unit uses, which is a much smaller number.
What moves the answer
- Sun and glass. Large windows facing south or west can push you up a size. Conservatories are in a class of their own.
- Ceiling height. A vaulted room holds more air than its floor area suggests.
- The roof above. Loft rooms often run hotter and may need more.
- Insulation. Solid walls and older houses lose heat in winter, which matters for heating as well as cooling.
- People and equipment. A home office with two screens or a busy kitchen adds heat of its own.
- How you use it. A bedroom used at night has a different pattern from an office used all day.
Why bigger is not always better
Modern inverter units slow down to match the room, so being slightly over-sized is less of a problem than it once was. A unit much too big for the room still costs more to buy and may cycle on and off in mild weather, cooling the air without removing much moisture, which can leave the room feeling clammy. One too small runs flat out on the hottest day and may not get there, which is the time you bought it for.
If you want it for heating too
Check the heating output as well as the cooling output. Heat loss through walls and windows in January is a different calculation from heat gain in July, particularly in an older Maidstone house with solid walls. Our guide to heating with air conditioning explains more. We work out both at the survey.
Common questions
What size do I need for a double bedroom?
For an average double of about 12 to 15 square metres, a small unit of around 2kW to 2.5kW is usually about right, unless it is in the loft or gets strong afternoon sun.
Can I work it out myself?
You can get close with the chart, and it helps to measure the room before you ring us. A survey picks up the things a chart misses.
What about an office?
Offices are sized on people, screens and sun as well as floor area. See office air conditioning.
Sources
Have these to hand
A quote is quicker when we know a little about the building.
- Postcode and type of property
- The rooms, and their rough size
- Which way the windows face
- Where an outdoor unit might go
- Whether it is listed or in a conservation area
Looking for someone to do the work? See air conditioning in Maidstone for what we install, service and repair, and where.
Related services
Home air conditioning
Bedrooms, loft rooms, living rooms and home offices.
Multi-split air conditioning
One outdoor unit serving several rooms, each with its own control.
Air conditioning installation
From the survey to the handover: what fitting a split system involves.
More guides
How much does air conditioning cost?
Installation, electrics, servicing and running costs, with the sums shown.
Do you need planning permission for air conditioning?
Permitted development, noise limits and Maidstone's conservation areas.
Can air conditioning heat your home?
Reversible systems, the running cost against gas and electric, and the limits.
F-gas rules: what to check about your installer
The certificates to ask for, leak-check duties and the 2025 refrigerant ban.
Portable or fixed air conditioning?
Cost, noise, cooling and which one suits your room.
Ask for a quote
Leave your number and we will phone you back to talk through the job. Or ring 01622 239046.