BTU calculator: what size air conditioner do I need?
Enter your room and get a recommended capacity in kW and BTU, using the same conservative UK method as our sizing guide.
You need roughly
Calculated cooling load: .
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This is an estimate from your room details, using the same method as our sizing guide. A fixed installation always needs a proper site survey: oversizing wastes money and short-cycles, undersizing never keeps up.
How this calculator works
The calculator uses the conservative UK floor-area method from our sizing guide: roughly 0.1 kW of cooling per square metre for a standard room, rising to about 0.15 kW per square metre for kitchens and conservatories. In BTU, that is around 350 to 550 BTU per square metre. It then applies the adjustments that genuinely change the answer, such as strong sun, poor insulation, high ceilings, extra occupants and heat-generating equipment, and rounds the result up to the standard unit sizes you will find in shops.
Some sizing tools use a much lower figure of about 100 BTU per square metre. That 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. The full method, the sizing table and a worked example are in our guide to what size air conditioning you need.
Sizing matters because both directions cost you. An oversized unit cools the air quickly but short-cycles, leaving the room cold and clammy while wasting energy. An undersized unit runs flat out, never quite copes, and wears itself out. For a fixed installation, treat the result as a starting point for a proper site survey, not a substitute for one.
Sizing questions, answered
What does BTU mean in air conditioning? +
BTU stands for British Thermal Unit, a measure of heat energy. On air conditioners it describes how much heat a unit can remove from a room in an hour, so a "9,000 BTU" unit really means 9,000 BTU per hour. The bigger the number, the more cooling power. kW is the standard UK unit for the same thing: 1 kW equals 3,412 BTU per hour.
How many BTU do I need per square metre? +
Around 350 to 550 BTU per square metre for UK homes, which is roughly 0.1 to 0.15 kW per square metre. Quiet, shaded rooms sit at the lower end; kitchens, conservatories and sunny south-facing rooms sit at the top. Ignore the "100 BTU per square metre" figure some sites quote; it is a mis-converted US rule that sizes rooms far too small.
Is it better to oversize or undersize? +
Neither is good, but if you genuinely have to choose, slight oversizing is the safer error. Round up to the next standard size and aim to stay within about 10% of the calculated figure; jumping two sizes up causes short-cycling, which wastes energy and wears the unit out.
Can one unit cool my whole home? +
No. Air conditioning cools the space it is in, so several closed-door rooms each need their own capacity. Size each room individually, then either fit a unit per room or use a multi-split system, which runs several indoor units from one outdoor unit.
Get an accurate quote
Every home is different, and a fixed installation needs a proper survey. Compare quotes from local F-Gas registered installers.