How to size a sauna heater

Start from the room volume

Every heater page on Saunakivi lists the room size its maker rates it for. This guide explains where those ranges come from.

Harvia says a sauna needs, on average, about 1 kW of heater power for every cubic metre of room volume. HUUM also works on one kilowatt of heater power per calculated cubic metre.

Finnleo sizes a heater from the room's length times width times height in cubic feet and suggests a dealer check location and materials.

Glass, stone and log walls count extra

Harvia says windows and heat-storing surfaces such as brick, concrete or massive logs raise the power a room needs.

Harvia counts each square metre of stone, glass or other uninsulated surface as if it added 1.2 cubic metres to the room. Harvia gives a factor of 1.5 where the interior walls are uninsulated logs.

HUUM adds the area of uninsulated surfaces such as a glass door, glass wall or uninsulated log wall to the room volume before choosing a heater. In HUUM's own worked example, uninsulated surfaces made up over 40% of the heater capacity the room needed.

Aim for the middle of the range

Harvia recommends choosing a heater whose rated room-volume range puts your calculated volume near its midpoint.

Harvia says a heater that is too small has to run longer and harder, which shortens its life.

Read next

• How big should a sauna be?
• Wood-burning vs electric sauna heater
• Sauna electrical supply and permits