Air conditioner size calculator
A rule of thumb for what size unit to look for, adjusted for the things that actually change the answer.
Your figures
Result
- Por superficie2.000100,0 %
- In BTU per hour (BTU/h)
- 7,936
- In kW (kW)
- 2.33
- In watts (W)
- 2,326
- Room volume (m³)
- 50.0
- Capacity per unit of floor area
- 100
What we assume
- This is a rule of thumb, not an engineering calculation. It tells you what range to shop in.
- It sizes for cooling. Heating needs are usually similar but depend far more on your climate.
- Orientation, glazing type, the floor you are on and what is above and below all matter and are only roughly captured here.
- Oversizing is a real mistake, not a safe margin: an oversized unit short-cycles, dehumidifies poorly and leaves the room clammy.
How it is calculated
The starting point is the volume of the room rather than its floor area, because a high-ceilinged room holds far more air to cool. That base figure is then adjusted by insulation, sun exposure and the heat given off by the people in it.
Roughly 50 units of cooling per cubic metre is the usual baseline, moved up or down by up to about 30% for the factors above.
Why bigger is not safer
An oversized unit cools the air quickly, hits the thermostat and switches off before it has pulled any moisture out. You get a cold, damp room and a compressor that starts and stops all day, which is both inefficient and hard on the machine. Sizing close to the real need is better than over-specifying.
An example
A 20 m² room with 2.5 m ceilings, average insulation, moderate sun and two people works out at roughly 2,500 units of cooling, about 10,000 BTU/h or 2.9 kW — a standard small split unit.
Frequently asked questions
Is it better to go bigger to be safe?
No. An oversized unit short-cycles: it hits the target temperature fast, shuts off, and never runs long enough to dehumidify. The room ends up cold and clammy, and the compressor wears out faster.
Does the same size work for heating?
Roughly, but heating depends much more on your climate and on how cold it gets. If you are relying on it as your main heat source, get it sized properly.
What about an open-plan space?
One unit rarely serves an open plan well, because the air does not circulate evenly. Two smaller units usually beat one large one.
Keep calculating
All tools →- Paint calculator for a roomRoom dimensions, number of coats and the area of doors and windows. We give you litres, tins and cost.
- Floor tile calculatorRoom size and tile size. We give you tiles, boxes and cost, with the wastage that cuts and breakages actually need.
- Appliance running cost calculatorPower rating, hours of use and your electricity price. That is enough to know what any appliance is really costing you.
Updated on 2026-08-27. Calculations run in your browser; nothing you type is sent to a server.