What size air conditioning do I need? BTU and kW sizing guide

Most UK homes need roughly 0.1 kW of cooling per square metre of floor space for a standard room, rising to about 0.15 kW per square metre for kitchens and sun-traps. In the units you will see on product listings, that is around 350 to 550 BTU per square metre. So a typical 25 square metre bedroom needs somewhere near 2.2 kW (about 7,500 BTU), and you round up to the next standard size.

That is the short answer. The right number depends on more than floor area, though, and getting it wrong in either direction costs you comfort and money. Below you will find a room-size-to-BTU table you can use as a starting point, the adjustments that change the result, a plain-English explanation of BTU and kW, and a worked example.

Prefer the maths done for you? Our free BTU calculator applies exactly this method to your room and rounds the answer to standard unit sizes.

What does BTU mean in air conditioning?

BTU stands for British Thermal Unit, a measure of heat energy. In air conditioning it is used as BTU/hr: how much heat a unit can remove from a room in an hour. The "/hr" is almost always dropped on retail listings, so a "9,000 BTU" portable really means 9,000 BTU/hr.

The bigger the BTU number, the more cooling power the unit has. A small bedroom unit might be rated at 5,000 to 7,000 BTU; a large open-plan space could need 18,000 BTU or more.

BTU vs kW: what is the difference and how do I convert?

They measure the same thing. kW (kilowatts) is the standard UK unit and is what you will see on energy labels, wall-mounted split specifications and government efficiency ratings. BTU tends to appear on portable and imported units and in online sizing tools.

The conversion is fixed:

• 1 kW = 3,412 BTU/hr

• To go from kW to BTU, multiply by 3,412.

• To go from BTU to kW, divide by 3,412.

So a 2.0 kW unit is about 6,824 BTU, which rounds to a standard 7,000 BTU model. A 3.5 kW unit is roughly 12,000 BTU.

Air conditioning sizing table: room size to BTU and kW

This is the table to start from. It is built on the conservative, internally consistent UK floor-area method (roughly 0.1 to 0.15 kW per square metre), which holds up far better than the much lower "100 BTU per square metre" rule that floats around some sites. That lower figure is a US "BTU per square foot" rule mis-converted to metres, and it sizes rooms far too small, so we do not use it here.

The BTU figures are the kW values multiplied by 3,412 and rounded. The table assumes ceilings of about 2.4 to 3 metres, average insulation and a typical UK summer. Apply the adjustments below on top, then round up to the next standard size.

If a sizing guide also offers an on-page calculator, its answer should match a table like this one. A calculator that uses a different multiplier will quietly contradict the published figures, which is a common fault on competing pages.

The adjustments that change your answer

Floor area gets you a baseline. These factors push the requirement up or down, and a good estimate accounts for them.

• Sun exposure. Add 10 to 15% for large south-facing windows or bi-fold doors. Subtract around 10% for north-facing or heavily shaded rooms.

• Ceiling height. The table assumes about 2.4 to 3 metres. Add roughly 10% for each extra metre of height, because you are cooling more air.

• Insulation and glazing. Add 15 to 20% for an older, single-glazed, poorly insulated home. Use the lower end of every range for a modern, cavity-insulated, double-glazed home.

• Occupancy. Add around 600 BTU (about 0.1 kW) for each regular occupant beyond two, a typical allowance installers use rather than an exact rule. People are a steady source of heat.

• Kitchens and electronics. Kitchens typically need about 20% more, or roughly 4,000 BTU extra, again a typical installer allowance rather than a precise rule. Add 0.3 to 0.6 kW for heat-heavy gear such as an oven, a large fridge-freezer, a gaming PC or a big TV.

• Portable units. Add 10 to 15% to the calculated figure. Portables vent some of their waste heat back into the room and are less efficient than a fixed split, so they need more rated capacity to do the same job.

Worked example: a 30 square metre south-facing lounge

Take a 30 square metre lounge in an older property, facing south, with a TV and a couple of regular occupants.

1. Baseline: 30 m2 at 0.1 kW/m2 = 3.0 kW (about 10,200 BTU).

2. South-facing windows: add 15%, so 3.0 kW becomes about 3.45 kW.

3. Older, less-insulated property: add 15%, taking it to about 3.97 kW.

4. Round up to the next standard size: a 5.0 kW (18,000 BTU) unit, or a 4.0 to 4.5 kW model if one is available and the room is well shaded for part of the day.

The same room in a modern, well-insulated home with shading might land closer to 3.5 kW. That spread is exactly why a survey matters for a fixed install.

What size for a bedroom, living room or conservatory?

• Bedroom. Most bedrooms are quiet, lower-activity rooms, so they sit at the lower end of the scale. A medium double bedroom of 15 to 25 m2 usually needs 1.6 to 2.2 kW (around 5,500 to 7,500 BTU).

• Living room. Lounges run warmer thanks to electronics, occupants and often more glazing. A 35 to 45 m2 living room typically needs 2.9 to 4.1 kW (around 9,900 to 14,000 BTU).

• Conservatory. Conservatories are the hardest case: lots of glass, high solar gain and often poor insulation. Treat them as a sun-trap, size at the top of the range, and budget for the adjustments to stack up. A professional heat-load survey is strongly advised here.

• Home office. A small office of 10 to 15 m2 usually needs 1.2 to 1.6 kW, but add for a gaming PC or several monitors.

What happens if I get the size wrong?

Both directions cause problems, so accuracy beats guesswork.

• Too big (oversized). An oversized unit cools the air quickly, then shuts off before it has pulled enough moisture out of the room. The result is a space that feels cold but clammy. The constant start-stop cycling, known as short-cycling, wastes energy and wears the components out faster.

• Too small (undersized). An undersized unit runs continuously and still struggles to reach the target temperature. That drives up your bills and the constant full-load running shortens its life.

If you genuinely have to choose, slight oversizing is usually the safer error, but aim to stay within about 10% of the calculated figure. Rounding up to the next standard size is fine; jumping two sizes up is not.

How do I size more than one room or a whole home?

You cannot cool several closed-door rooms with one large unit. Air conditioning cools the space it is in, so each room needs its own capacity.

Size each room individually using the method above, then either fit a separate unit per room or use a multi-split system, where several indoor units run from one outdoor condenser. For a multi-split, the outdoor condenser is matched to the summed kW of the indoor units it serves. An installer will confirm the right combination during a survey.

A rule of thumb is not a heat-load survey

The table and adjustments here are a sound planning estimate, and they are all you need to shop for a portable unit, which is plug-in and needs no special installation.

For a fixed wall-mounted split or multi-split, treat your figure as a starting point. A professional on-site survey measures the actual heat loads, glazing, wall construction and airflow, and will refine the size. Installing a refrigerant-charged split is not a DIY job either: anyone fitting or servicing one must hold F-Gas certification (the qualification for working with fluorinated refrigerants), and reputable installers are REFCOM-registered. Always have a registered installer do the final heat-load calculation before you commit. Our guide to how air conditioning is installed explains what the survey and fitting day involve.

Sizing figures are estimates for planning. Energy rates correct as of July 2026; the Ofgem price cap changes quarterly, so check current rates before budgeting running costs.

Related reading: air conditioning running costs, how to choose an air conditioning system and how many air conditioning units you need for your home.