Heater size calculator
Enter the floor area and ceiling height, then pick how well the room is insulated and how cold your climate is. You get an estimated heating power in watts, kilowatts and BTU per hour.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose metric or US units and enter the floor area and ceiling height.
- Pick the insulation level and the winter climate that best describe the room.
- Read the estimated power. The last line rounds it up to the next 100 W as a heater size to look for.
The rule of thumb behind it
For a room with a 2.5 m (about 8.2 ft) ceiling in a temperate climate, the calculator uses:
| Insulation |
Watts per m2 |
| Good |
40 |
| Average |
60 |
| Poor |
80 |
Power = area x watts per m2 x (ceiling height / 2.5 m) x climate factor, with climate factors 0.8 (mild), 1.0 (temperate) and 1.25 (cold). US inputs are converted with 1 ft = 0.3048 m and 1 ft2 = 0.09290304 m2.
These are common rules of thumb. The assumptions are: one room with a normal number of windows, no extreme drafts and a target indoor temperature of a typical comfortable level.
Worked example
A 20 m2 room with a 2.5 m ceiling, average insulation and a temperate climate: 20 x 60 x 1 x 1 = 1200 W, which is 1.2 kW or about 4095 BTU/h. A heater of 1200 W or slightly more is a sensible starting point.
Tips
- If a heater is the only heat source in a cold climate, choose the next size up.
- An oversized heater with a thermostat is usually fine; an undersized one never reaches the set temperature.
- For a whole house or a heat pump, use a proper heat-loss calculation.
Frequently asked questions
How many watts per square meter does a room need?
A common rule of thumb is about 40 W per square meter for a well-insulated room, 60 W for average insulation and 80 W for a poorly insulated one, at a 2.5 m ceiling in a temperate climate. This calculator uses exactly those figures.
How are ceiling height and climate handled?
Power is scaled in proportion to ceiling height compared with 2.5 m, since there is more air to heat. The climate factor is 0.8 for mild, 1.0 for temperate and 1.25 for cold winters. These factors are simple planning assumptions, not a standard.
Is this a heat-loss calculation?
No. A real heat-loss calculation uses wall, window and roof areas, materials and the design temperature difference. This is a quick estimate for choosing a heater size, so treat it as a rough guide.
Why does it show BTU per hour?
Some heaters and boilers are rated in BTU/h. The conversion used is 1 W = 3.412141633 BTU/h.
Does sun, floor type or a drafty window change the result?
They matter but are not included. If the room has large single-glazed windows or sits above an unheated space, choose poor insulation or add some margin.