Air Conditioner BTU Calculator — What Size Do I Need?
Getting the size right is the single most important decision when buying an air conditioner. Cooling power is measured in BTU (British Thermal Units): too few and the unit runs constantly without ever cooling the room; too many and you pay more upfront and lose efficient dehumidification. Our free air conditioner BTU calculator works out exactly what size portable air conditioner you need in seconds.
As a guide, allow about 600 BTU per square metre, then adjust up for sunny rooms, busy rooms and top-floor or loft spaces. Enter your room details below for a precise recommendation matched to units we have in stock.
What size do I need?
Too small and it never cools; too big and you waste money. Answer three quick things and we'll match the right unit.
You need around
7,000
BTU · 2.1 kW
Our recommendation
9000 BTU (2.6 kW) Mini Split Air Conditioner A++ Energy Efficient AC Unit
Air conditioner size chart (BTU by room size)
| Room size | Typical room | Recommended BTU |
|---|---|---|
| Up to 12 m² | Small bedroom, study | 5,000 – 7,000 BTU |
| 12 – 18 m² | Bedroom, home office | 7,000 – 9,000 BTU |
| 18 – 25 m² | Living room, kitchen | 9,000 – 12,000 BTU |
| 25 – 35 m² | Open-plan, large lounge | 12,000 – 14,000 BTU |
| 35 m²+ | Conservatory, big spaces | 14,000 – 18,000 BTU |
Figures are a guide based on ~600 BTU/m². Sunny, south-facing, top-floor or heavily-glazed rooms should size up one step.
BTU calculator — frequently asked questions
How many BTU do I need per square metre?
As a rule of thumb, allow around 600 BTU of cooling per square metre of floor area. So a 15 m² room needs roughly 9,000 BTU. Add extra capacity for sunny south-facing rooms, top-floor or loft spaces, rooms with lots of glazing, and rooms used by several people or full of heat-generating electronics.
What size air conditioner do I need for a bedroom?
Most UK bedrooms are well served by a 5,000–9,000 BTU portable air conditioner. A small single room of around 10 m² is comfortable on 5,000–7,000 BTU, while a larger double or warm loft bedroom benefits from 9,000 BTU. For bedrooms, also check the noise level in decibels and pick a model with a quiet sleep mode.
Is it better to oversize or undersize an air conditioner?
Neither extreme is ideal. An undersized unit runs flat-out and never quite cools the room, wasting electricity. A heavily oversized unit costs more to buy, can cool too quickly without dehumidifying properly, and short-cycles. Sizing within about 10–15% of the calculated BTU gives the best comfort and running cost.
Does the BTU I need change in summer heatwaves?
Your room doesn't change size, so the base BTU figure stays the same — but during extreme heat a correctly sized unit simply runs more of the time. If your room is south-facing or poorly insulated and regularly bakes in a heatwave, it's worth sizing up one step and adding blinds or shading to reduce the heat load.