# What Size Air Conditioning Do I Need? | kW and BTU

> Work out what size air conditioning unit your room needs, in kW and BTU. Free calculator with adjustments for aspect, floor level, glazing and room use.

*Climate Shield London — https://climateshieldlondon.co.uk/what-size-air-conditioning-do-i-need/. Verbatim from the page; this is the same content, not a summary.*

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![Wall-mounted air conditioning unit fitted in a London living room](https://climateshieldlondon.co.uk/assets/img/jobs/job-08-1200.webp)

# What size air conditioning do I need?

Work out the kW and BTU figure for your room in about twenty seconds, with the adjustments that actually matter in London: which way the room faces, whether it is a top floor or a loft, and how much glass it has.

- Bedroom, 12 square metres: usually **2.0 to 2.5kW**
- Double bedroom, 20 square metres: usually **3.5kW**
- Living room, 30 square metres: usually **5kW**

Size my room [WhatsApp us](https://wa.me/447875564437?text=Hi%20Climate%20Shield%2C%20I%20need%20help%20sizing%20an%20air%20conditioning%20unit.)

## Book a free survey

We come out, measure, and send a written price. No charge, no obligation, and no deposit before you have seen the number.

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## Size your room

Give it the floor area and the four things that change the answer. It returns both the kW figure your installer will use and the BTU figure the shops advertise.

## Air conditioning we have fitted

Our own jobs, photographed on site. Real rooms, real outdoor positions, nothing from a catalogue.

[All our installations](https://climateshieldlondon.co.uk/installations/)

[![Outdoor unit on wall brackets, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-04-800.webp) Air conditioning **Outdoor unit on wall brackets** Above brick, isolator and cabling run tidy](https://climateshieldlondon.co.uk/installations/)[![Condenser going up on straps, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-06-800.webp) Air conditioning **Condenser going up on straps** Lifted into a wall position, no scaffold](https://climateshieldlondon.co.uk/installations/)[![Wall unit, commissioned, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-07-800.webp) Air conditioning **Wall unit, commissioned** Set to 24 degrees and handed over](https://climateshieldlondon.co.uk/installations/)[![Black wall unit in a sitting room, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-08-800.webp) Air conditioning **Black wall unit in a sitting room** Chosen to sit with the room, not against it](https://climateshieldlondon.co.uk/installations/)[![Ceiling cassette, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-09-800.webp) Air conditioning **Ceiling cassette** Recessed flush into the ceiling](https://climateshieldlondon.co.uk/installations/)[![Samsung Wind-Free, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-11-800.webp) Air conditioning **Samsung Wind-Free** Cooling without a draught on the room](https://climateshieldlondon.co.uk/installations/)[![Rooftop commissioning, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-12-800.webp) Air conditioning **Rooftop commissioning** Pressure tested and charged on gauges](https://climateshieldlondon.co.uk/installations/)[![Outdoor unit at ground level, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-14-800.webp) Air conditioning **Outdoor unit at ground level** On paving beside the house, easy to service](https://climateshieldlondon.co.uk/installations/)[![Black wall unit, running, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-15-800.webp) Air conditioning **Black wall unit, running** Quiet enough for a bedroom](https://climateshieldlondon.co.uk/installations/)[![One-way ceiling cassette, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-16-800.webp) Air conditioning **One-way ceiling cassette** Only the grille shows](https://climateshieldlondon.co.uk/installations/)[![Outdoor unit on paving, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-17-800.webp) Air conditioning **Outdoor unit on paving** Against render, clear of the boundary](https://climateshieldlondon.co.uk/installations/)[![Condenser high on a wall, a job we fitted in London](https://climateshieldlondon.co.uk/assets/img/jobs/job-18-800.webp) Air conditioning **Condenser high on a wall** On brackets where the ground was not an option](https://climateshieldlondon.co.uk/installations/)

As a rule of thumb, a UK room needs about 0.14kW of cooling per square metre of floor area, so a 16 square metre bedroom needs about 2.5kW, which is a 9,000 BTU unit.

That baseline then moves with aspect, floor level, glazing and what the room is used for. A south facing loft bedroom with large roof windows can need 50 percent more than the same floor area on a shaded ground floor.

### Read the full guide: 5 sections, about 3 minutes

## kW to BTU, and what the shops call it

| Cooling output | BTU per hour | Marketed as | Typical room |
| --- | --- | --- | --- |
| 2.0 kW | 7,000 BTU | 7k | Small bedroom, up to 14 square metres |
| 2.5 kW | 9,000 BTU | 9k | Bedroom or office, 14 to 18 square metres |
| 3.5 kW | 12,000 BTU | 12k | Double bedroom or small living room, 18 to 25 square metres |
| 5.0 kW | 18,000 BTU | 18k | Living room, 25 to 35 square metres |
| 7.0 kW | 24,000 BTU | 24k | Open plan, 35 to 50 square metres |
| 10.0 kW | 34,000 BTU | 34k | Large open plan or light commercial |

## Sizing a specific room

The calculator above works for any room. These four come up often enough, and behave differently enough, to be worth their own page: each one runs this same model and shows the working for a handful of real examples.

[Bedrooms Where noise is the specification, and why an oversized unit is the thing that wakes you up. Bedrooms](https://climateshieldlondon.co.uk/bedroom-air-conditioning/) [Loft conversions Thirty per cent more output than the same room downstairs, and the outdoor unit is the expensive question. Loft conversions](https://climateshieldlondon.co.uk/loft-conversion-air-conditioning/) [Home offices Equipment heat, running cost for a working day, and staying quiet enough for a call. Home offices](https://climateshieldlondon.co.uk/home-office-air-conditioning/) [Conservatories The one room this calculator does not size properly, and what to do about the roof before buying anything. Conservatories](https://climateshieldlondon.co.uk/conservatory-air-conditioning/)

## The adjustments that matter in London

| Factor | Adjustment | Why |
| --- | --- | --- |
| South or west facing | +15 to 20 percent | Afternoon sun is the biggest single gain |
| North facing | -10 percent | No direct solar gain |
| Top floor | +10 percent | Heat from the roof and from the floors below |
| Loft conversion | +25 to 35 percent | Roof, rising heat and usually big roof windows |
| Large glazing or patio doors | +15 to 25 percent | Glass is the weakest part of any wall |
| Kitchen or kitchen diner | +25 percent | Cooking equipment is a large heat source |
| Home office with two people and screens | +10 to 15 percent | Each person and machine is a heater |
| Well shaded, small window | -10 percent | Less solar gain to remove |

## Why bigger is not safer

The instinct is to buy one size up to be sure. In air conditioning that instinct is wrong, and it is the reason a lot of rooms end up feeling cold and damp rather than comfortable.

An oversized unit hits the set temperature fast and then stops. Removing humidity takes *time* at the coil, not power, so a unit that runs in short bursts never dehumidifies properly. You get a room at 22 degrees that still feels sticky, plus short cycling that wears the compressor and wastes energy on every restart.

A correctly sized unit runs for longer at lower power, holds the temperature steadily, removes moisture as it goes and costs less to run. That is the whole argument for sizing it properly instead of generously.

## Sizing a multi-split for several rooms

When several indoor units share one outdoor unit, the outdoor unit does not need to equal the sum of the indoor units, because you will not run every room at full output simultaneously. Manufacturers allow a degree of oversizing on the indoor side, typically up to about 130 percent of the outdoor capacity.

What that means in practice is that three 2.5kW bedrooms, totalling 7.5kW indoors, are normally served by a 6.8 to 7.1kW outdoor unit rather than an 8kW one. Getting that ratio right is worth several hundred pounds and it is the part a homeowner cannot reasonably work out alone.

### What will it cost?

Once you know the size, the price follows. Full breakdown by room and job size.

[Installation costs](https://climateshieldlondon.co.uk/air-conditioning-cost-london/)

### Not sure it is allowed?

Worth checking the permission position before you go any further, especially in a flat or a conservation area.

[Planning permission](https://climateshieldlondon.co.uk/air-conditioning-planning-permission/)

![An outdoor air conditioning unit we fitted on brackets on a London rear wall](https://climateshieldlondon.co.uk/assets/img/svc/ac-install-700.webp)

Your estimate

£2,400*to*£3,200

Supplied, fitted, commissioned and certificated. Includes VAT.

Send this on WhatsApp

## Now price the size you landed on

The output you need is what drives the unit price, so the sizing answer above is also the first half of the price.

How big is the room?

Which indoor unit?

Last step: where do we send your written price?

Every figure on this site is an estimate built from the quotations we issued in 2026 and our own installation experience. It is not a quotation. The price you are given after the free survey is the price you pay.

## Sizing questions

### What size air conditioning unit do I need?

As a starting point, allow about 0.14kW of cooling per square metre of floor area for a standard UK room with average glazing, then adjust up for a south or west facing room, a top floor or loft, large glazing, or a room with several people and computers in it.

A 12 square metre bedroom typically needs 2.0 to 2.5kW, a 20 square metre double bedroom or small living room 3.5kW, and a 30 square metre living room 5kW. The calculator on this page does the adjustments for you.

### What is the difference between kW and BTU?

They measure the same thing in different units. 1kW is about 3,412 BTU per hour. A 2.5kW unit is sold as 9,000 BTU, a 3.5kW unit as 12,000 BTU, a 5kW as 18,000 BTU and a 7kW as 24,000 BTU.

UK installers usually talk in kW and retailers usually advertise in BTU, which is why the same unit appears to have two different sizes.

### Is it better to get a bigger unit to be safe?

No, and this is the most common and most expensive mistake. An oversized unit cools the room quickly, hits the setpoint, and shuts down before it has removed the humidity, so the room feels cold and clammy rather than comfortable.

It also short-cycles, which wastes energy and wears the compressor faster. A correctly sized unit runs longer at lower power, which is both cheaper and more comfortable.

### Does a loft conversion need a bigger unit?

Usually yes, often one size up from what the floor area alone suggests. A loft sits directly under the roof, has heat rising into it from the whole house, and frequently has large roof windows facing the sun.

A 15 square metre loft bedroom that the floor area calculation puts at 2.5kW will often actually need 3.5kW. It is the single most common room we are called back to because someone else undersized it.

### Can one outdoor unit cool the whole house?

Up to a point. A multi-split outdoor unit can typically serve four or five indoor units, and its total capacity is what limits it. Beyond that, or where the rooms are far apart, two outdoor units usually costs less than forcing one and gives better performance.

The catch with a multi-split is that all the indoor units share a compressor, so they cannot run in heating and cooling at the same time. In a house where one person wants the bedroom cold in October while another wants the office warm, that matters.

## Send us the room and we will size it properly

Floor area, which way the window faces and what the room is used for is enough for a solid answer. A photo helps even more.

Call 020 3393 1661 [WhatsApp us](https://wa.me/447875564437?text=Hi%20Climate%20Shield%2C%20can%20you%20help%20me%20size%20an%20air%20conditioning%20unit%20for%20my%20room%3F)

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Page updated 19 September 2026