Heater Size Chart by Room Area

Multiply the floor area by 60-100 W and you have the heater size — this page does the multiplication for the rooms people ask about most.

The sizing rule for electric heaters is 60-100 W per square metre of floor area. The bottom of the band suits insulated rooms; the top suits standard rooms; uninsulated walls push the requirement above the band entirely.

Worked out, the chart looks like this: 6 m² needs 360-600 W, 10 m² needs 600-1,000 W, 12 m² needs 720-1,200 W, 15 m² needs 900-1,500 W, and 20 m² needs 1,200-2,000 W. Each figure is the area multiplied by the same 60-100 W rule.

For the 12 m² bedroom that brings most visitors here, the answer is 720-1,200 W. In practice that means a 1,000-1,500 W heater with a thermostat: it reaches temperature, then cycles on and off instead of running continuously, so the average draw settles near the room's real heat loss.

Insulation is the correction factor that matters most. Two bedrooms with identical floor area can sit at opposite ends of the band, and an uninsulated 12 m² room can need more than 1,200 W to hold temperature on a cold night.

Once you have the wattage, the next questions are which type delivers it best and how to run it safely — the comparison page and the safety checklist on this site take those in order.

• Measure the floor area in square metres.
• Multiply by 60 W for an insulated room or 100 W for a standard one.
• Add margin above the band if the walls are uninsulated.
• Choose a heater whose thermostat can cycle at that load, then check it against the type comparison and safety checklist.

Further reading

• Fan Heater vs Oil Filled vs Halogen
• Electric Heater Safety Checklist