Sauna heater size calculator

Enter your room size to see the effective volume and a rough heater output. Glass, tile and log walls all raise the size you need. Then check the result against the room range on the heater's spec sheet.

Size your heater

7 to 10 kW

Effective volume about 9.9 m³ (349 cu ft).

How the calculation works

Heater makers rate each model for a range of room volumes, for example 8 to 13 cubic metres. The calculator turns your room into an effective volume you can check against that range:

  1. Room volume: length × width × height.
  2. Cold surfaces: glass, tile, concrete and stone absorb heat, so about 1.2 m³ is added for every square metre of such surface (roughly 4 cu ft for every sq ft).
  3. Log walls: an uninsulated log room takes much longer to heat, so the volume is multiplied by about 1.5.
  4. Output: the kW range shown assumes roughly 1 kW for every 1 to 1.4 m³ of effective volume.
Diagram of a sauna room showing length, width and height, the three measurements used to size a heater
Measure inside the finished room, from bench wall to bench wall and floor to ceiling.

Use the result the right way

The kW figure is a guide, not a purchase decision. Always check the room volume range printed on the heater's spec sheet; it already includes the maker's own allowances. If your effective volume sits between two models, choose by how you use the sauna:

  • Choose the larger heater for a cold climate, an outdoor sauna or a room with a big window.
  • Choose the smaller heater if the room is well insulated and you like gentler heat.
  • Remember the electrical side. A larger heater needs a larger breaker and wire; see wiring a 240 V sauna heater in Canada.

Typical sizes for common rooms

Room (inside)VolumeTypical heater
4 × 4 × 7 ft, one or two peopleabout 3.2 m³ (112 cu ft)3 to 4.5 kW
6 × 5 × 7 ft, small familyabout 5.9 m³ (210 cu ft)4.5 to 6 kW
7 × 6 × 7 ft, familyabout 8.3 m³ (294 cu ft)6 to 8 kW
8 × 8 × 7 ft, large familyabout 12.7 m³ (448 cu ft)9 to 12 kW

For wood stoves, makers usually give a recommended room volume instead of kW; use the effective volume from the calculator the same way. Not sure which type to buy? Read electric vs wood-burning sauna heaters, then compare models in the brand comparison.

Questions buyers ask

What size sauna heater do I need?

Start from the room volume. As a rough guide, an insulated sauna needs about 1 kW for every 1 to 1.4 cubic metres (35 to 50 cubic feet). Add volume for glass, tile or stone surfaces and for log walls, then pick a heater whose rated room range includes the result.

Does a glass door change the heater size?

Yes. Uninsulated surfaces such as glass, tile, concrete or stone soak up heat. Many manufacturers add about 1.2 cubic metres to the room volume for every square metre of such surface.

What happens if the heater is too small?

It will run for a long time, may never reach the temperature you want and gives weak steam. A heater that is far too large heats quickly but can make the air harsh and may cycle on and off often.

Should I size up for a log sauna?

Usually. Solid log walls take a long time to warm through. A common rule is to multiply the room volume by about 1.5 for an uninsulated log sauna.

Last checked November 8, 2026. Facts come from the brands' own sites and documents. How we check