Air Conditioning Calculator

UK summers are getting warmer, and more homes and offices are turning to air conditioning to stay comfortable. But buying the wrong size unit is a common and costly mistake. Too small, and it'll run constantly without cooling the room. Too big, and it'll switch on and off frequently, waste energy and leave the air feeling damp.

Our Air Conditioning Calculator estimates the cooling power your room needs in BTUs and suggests a suitable unit size. This guide explains how air con sizing works and what else to consider before you buy.

What does BTU mean?

Air conditioners are usually rated in British Thermal Units per hour (BTU/h), a measure of how much heat they can remove from a room. You'll also see cooling capacity in kilowatts (kW). One kilowatt is about 3,412 BTU/h.

Common portable and wall-mounted unit sizes include 7,000, 9,000, 12,000 and 18,000 BTU.

How to use the calculator (Air Conditioning Calculator)

1. Enter the room's length and width in metres.
2. Add the ceiling height. Taller ceilings mean more air to cool.
3. Choose how sunny the room is. South-facing rooms with large windows need more cooling.
4. Enter how many people usually use the room. Each person adds heat.
5. Tell us if it's a kitchen or a loft room, both of which get hotter.

The calculator gives the cooling you need in BTU/h and kW, and suggests the nearest standard unit size.

A worked example

A 5m by 4m living room with a standard 2.4m ceiling and average sunlight, used by two people, needs about 6,800 BTU/h, so a 7,000 BTU unit would be a good fit.

A sunny 6m by 5m open-plan kitchen would need about 15,200 BTU/h, so an 18,000 BTU unit is more suitable. Try your own rooms in the Air Conditioning Calculator.

How the estimate works

The calculator starts with about 100 watts of cooling per square metre, which is a common rule of thumb for a typical UK room with a 2.4m ceiling. It then adjusts for:

• Ceiling height, in proportion to the volume of air
• Sunlight, adding around 15% for very sunny rooms or reducing slightly for shaded ones
• People, adding about 100 watts for each person beyond two
• Kitchens, adding extra for heat from cooking appliances
• Loft rooms, which gain heat through the roof

Rooms with poor insulation, lots of glazing, or several computers and TVs may need more. If in doubt, an installer can carry out a proper heat gain calculation.

Portable vs split air conditioning

Portable units

Portable air conditioners are freestanding and vent hot air through a hose out of a window. They're cheaper and need no installation, which makes them popular for rented homes. However, the hose lets some warm air back in, so they're less efficient and can be noisy.

Split systems

A split system has an indoor unit on the wall and an outdoor unit outside. They're quieter, more efficient and often work as heat pumps, providing heating in winter too. They must be installed by an F-gas certified engineer, because they contain refrigerant gases that are regulated in the UK.

Running costs

Air conditioners use electricity, so running costs depend on the unit's efficiency, how long you use it and your tariff. Look for the energy rating and the EER (energy efficiency ratio) or SEER (seasonal efficiency). A higher number means more cooling for each unit of electricity.

To keep costs down:

• Close curtains or blinds on sunny windows. Our blinds cost calculator can help if you're thinking of adding them.
• Set a sensible temperature, such as 24 to 26°C, rather than very cold.
• Keep doors and windows closed while it's running.
• Clean or replace filters regularly.

Air conditioning at work

There's no maximum workplace temperature in UK law, but employers must keep temperatures reasonable. The Health and Safety Executive suggests employers assess the risks when it's hot and take action, which could include air conditioning, fans, flexible hours or extra breaks.

Alternatives to air conditioning

• Fans cost far less to run and help you feel cooler, though they don't lower the temperature.
• External shading, such as awnings or shutters, stops heat entering in the first place.
• Insulation keeps homes cooler in summer as well as warmer in winter.
• Ventilation at night, when it's cooler outside, can flush heat out of the house.

Measuring your room accurately

Measure the length and width of the room at floor level, and the ceiling height from floor to ceiling. For L-shaped rooms, split the room into rectangles, work out each area and add them together. If a room is open to another space, such as a kitchen-diner or an open staircase, include that space too, as the air conditioner will need to cool all of it.