Heater Size Calculator
Enter the room length, width and height, how much warmer you want it than outside and how well it is insulated. The calculator gives the heat needed in watts and BTU an hour, a heater size to buy and the cost an hour at full power.
- Heater watts
- BTU an hour
- Running cost
Temperature rise is the gap between the coldest outdoor temperature you expect and the room temperature you want.
Bigger rooms, colder weather and poor insulation all need more watts. A 4 × 3.5 m bedroom with a 2.4 m ceiling is about 34 m³. Raising it 20 °C with average insulation needs about 1,140 W, so a 1,500 W heater is a good fit. A 2,000 W heater suits most living rooms.
Heater size chart
2.4 m ceiling, 20 °C rise, average insulation. Search for a room.
| Room (m) | Volume (m³) | Watts | Heater |
|---|---|---|---|
| 2 × 2 | 9.6 | 326 | 500 |
| 3 × 2.5 | 18 | 612 | 750 |
| 3 × 3 | 21.6 | 734 | 750 |
| 4 × 3.5 | 33.6 | 1142 | 1500 |
| 4 × 4 | 38.4 | 1306 | 1500 |
| 5 × 4 | 48 | 1632 | 2000 |
| 6 × 4 | 57.6 | 1958 | 2000 |
| 6 × 5 | 72 | 2448 | 2500 |
Watts needed.
Room volume and watts
20 °C rise, average insulation. Slide the room volume.
The heater grows with the heat the room needs.
How to size a heater
Room to watts. Step through it.
Measure
L × W × H.
Use a heater with a thermostat and tip-over cut-out for safety and lower bills.
Convector, oil-filled or fan heater
Heater types. Compare them.
Fast.
Steady.
Even.
All electric heaters turn every watt into heat. The type changes how the heat feels, not the cost per kWh.
Before you buy
Heater checks. Tick each item.
Tick each item as you go.
Plug heaters straight into the wall, not into an extension lead.
What you get
Heater sizing. Each tile explains one result.
Needed.
An hour.
Size.
An hour.
Room.
A 2 kW heater at full power uses 2 kWh an hour, but a thermostat often halves that.
Frequently Asked Questions
No question matches that search.
What size heater do I need?
Multiply the room volume by the temperature rise and an insulation factor of about one point seven watts.
Is a two kilowatt heater enough for a living room?
Usually, for a room up to about fifty cubic metres with average insulation.
How much does a heater cost to run?
Its kilowatts times your electricity price per kilowatt hour, for each hour at full power.
Are some electric heaters cheaper to run?
Not per kilowatt hour. They all turn electricity into heat at the same rate.
How do I convert watts to BTU?
Multiply by three point four one two.
What size heater for a bedroom?
About one to one and a half kilowatts for most bedrooms.
Do high ceilings need a bigger heater?
Yes, as there is more air to heat.
Warm, without waste
Choose the heater that fits the room and let the thermostat do the rest.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.