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Air conditioning BTU calculator

What size air conditioner do I need? Tell us about the room and get a kW and BTU range, the nearest standard unit, and every step of the sum.

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What is the room?

Room length
Room width
Ceiling height
People who use it

How well insulated is it?

Well insulated: modern build or retrofitted, double glazing. Poor: single glazing, solid walls, uninsulated loft.

How much sun does it get?

Very sunny: big south or west-facing windows with no shade.

Where is it in the building?

About 2.1 kW · 2.5 kW unit

You probably need about

2.1kW

7,200 BTU/h of cooling. Range 1.9 to 2.4 kW (6,400 to 8,200 BTU/h).

Closest standard unit

A 2.5 kW unit is the smallest standard size that covers it. Bigger is not better: an oversized unit switches on and off too often, cools unevenly and wastes electricity.

Find installers in Brighton & HoveWhat will this cost to install? →What would a 2.5 kW unit cost to run?
Show the sums
  1. Floor area: 5.0 × 4.0 = 20.0 m²
  2. Base load: 20.0 m² × 90 W/m² = 1800 W
  3. Adjust for ceiling height (×1.00), sun (×1) and floor (×1) = 1800 W
  4. Extra people: (3 − 2 included, min 0) × 100 W = 100 W
  5. Heat-giving kit: 200 W
  6. Total: 2100 W = 2.1 kW (÷ 1,000), or × 3412 = 7,200 BTU/h
  7. Range: 90% to 115% of that, to allow for what a survey would find.
Room and windows1800 W
Extra people100 W
Heat-giving kit200 W

How the air conditioning BTU calculator works

It is a rule-of-thumb sizing method for UK rooms, built so you can check it by hand. It is not a replacement for a survey, which measures your actual windows, walls and roof. The steps:

  1. Floor area (length × width).
  2. Base load per m². 70 W/m² for a well insulated room, 90 W/m² for an average one and 115 W/m² for a poor one. This already includes typical glazing, ventilation, lighting and the first two people.
  3. Adjustments to the room itself: ceiling height compared with 2.4 m, then sun (shaded × 0.9, average × 1, sunny × 1.15, very sunny × 1.3) and floor (top floor or loft × 1.15, otherwise × 1).
  4. Add-ons: 100 W for each person beyond two, plus the watts of heat-giving kit such as a kitchen or a computer setup.
  5. Convert: watts ÷ 1,000 is kW, and kW × 3,412 is BTU per hour. Then we pick the smallest of the common unit sizes (2, 2.5, 3.5, 5, 7 kW) that covers the load.

What this calculator leaves out

Large areas of glass, rooms with open staircases, a very hot loft above, lots of people (a meeting room), commercial kitchens and server rooms all need a proper load calculation by an engineer. The same goes for sizing a heat pump to heat a room in winter. If the answer is above 7 kW, ask installers about two units, a multi-split or a ducted system.

Next

Once you know the size, estimate what it will cost to run and compare installers in Brighton & Hove: check they hold an F-gas certificate and ask for a written quote after a survey.

FAQ

Questions

What size air conditioner do I need for my room?
It depends on floor area, ceiling height, how well insulated the room is, how much sun it gets and who and what is in it. For a typical well-used 20 m² living room, expect a unit of about 2 to 2.5 kW. Use the calculator for your room, then let an installer confirm it at a survey.
How many BTU is 1 kW?
1 kW is 3,412 BTU per hour, so a 2.5 kW unit is about 8,500 BTU/h. Older advice and online shops often quote BTU; UK installers usually talk in kW.
Is it bad to buy an air conditioner that is too big?
Yes, within reason. An oversized unit cools the room quickly, switches off, then restarts again and again. That wastes electricity, cools unevenly, removes less moisture and wears the compressor. Installers size to the load, not to the biggest box.
Does this cover heating as well as cooling?
No. The calculator sizes for cooling, which is the usual limit. A reversible (heat pump) split system also heats; its heating output is listed on the unit's data sheet, and sizing for winter is a separate calculation your installer should do.
Why does the calculator give a range?
Because the inputs are estimates. A survey measures glazing, shading, insulation and room use properly. We show 90% to 115% of the central figure so you can see how much a different assumption would change the answer.

Next steps

Find installers in Brighton & HoveAir conditioning running cost estimatorAir conditioning installation cost calculatorHow we check installers