What size sauna heater do I need? kW by room size

Heater makers size heaters by the volume of the hot room in cubic metres, not by the number of people. Work out the volume, add extra for glass and log walls, then pick a heater whose published range covers it.

Step 1: the room volume

Multiply the inside length, width and height of the hot room in metres. The answer is in cubic metres (m³). A room 2.0 m by 1.8 m and 2.0 m high is 7.2 m³. A barrel is round, so as a rough guide multiply its inside length by its inside width twice, then by 0.8 (our approximation).

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Step 2: add for surfaces that soak up heat

Glass, stone and tiles soak up heat, so the heater has to work harder. Harvia's manuals add 1.2 m³ for each square metre of them: a 2 m² glass front adds 2.4 m³. Solid log walls with no insulation also soak up heat, so makers often say to add about a quarter to the volume. Sources: Harvia electric heater range, HUUM heater sizing guide.

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Step 3: match it to a heater

Every heater's spec sheet gives the room sizes it suits, such as "5 to 8 m³". Pick one where your room sits in the middle of the range, not at the top. A rough guide from those ranges:

• 4.5 kW: small cabins up to about 6 m³
• 6 kW: about 5 to 9 m³
• 8 kW: about 7 to 12 m³
• 9 kW: about 8 to 14 m³

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What it means for your electrics

A bigger heater needs a bigger electricity supply. Most heaters up to 8 kW run on a dedicated circuit. Many 9 kW heaters need a much bigger supply (50 amp, or three-phase), which adds around £700 more by our figures. Electrics explained. A wood-burning stove avoids the electrics, but check the smoke rules.

Every sauna page shows the heater's power where the seller publishes it, and whether it is included in the price.