Sauna Heater Size Calculator
Enter your sauna room's inside size and any glass or uninsulated walls. You get the heater output to look for, worked out two ways from published heater-maker rules.
Result
About 1 kW for every calculated cubic metre. Pick the heater with the closest output; for a wood stove, leave some reserve so it isn't run flat out.
Harvia lists a room-size range in cubic feet for each heater. Choose one whose range covers this number.
Always check the heater's manual for its room-size range and clearances. An electric heater needs an electrician to confirm your breaker and cable size first.
How sauna heater sizing works
Heater makers size heaters by the volume of air in the room, not the floor area. Measure the inside length, width and height and multiply them. Then add extra volume for surfaces that soak up or leak heat, because the heater has to work harder to warm a room with a glass front or bare masonry.
The HUUM method (metric)
HUUM adds 1 m³ to the room for every square metre of glass, log wall or other non-insulated surface, then allows about 1 kW of heater output per calculated cubic metre. Their worked example: a 2 × 1.8 × 2.2 m room is 7.92 m³; a 1.8 × 2.2 m glass wall adds 3.96 m³; the calculated volume is 11.88 m³, so a heater of about 12 kW.
The Harvia method (imperial)
Harvia's US sizing sheet adds 2 cubic feet for every square foot of cold surface (glass, tile, brick, concrete or stone, not counting the door) and matches the total to the cubic-foot range printed for each heater.
| Room (inside) | Volume | HUUM estimate, no glass |
|---|---|---|
| 4 × 6 × 7 ft | 168 ft³ (4.8 m³) | about 5 kW |
| 6 × 6 × 7 ft | 252 ft³ (7.1 m³) | about 7 kW |
| 6 × 8 × 7 ft | 336 ft³ (9.5 m³) | about 9.5 kW |
| 8 × 8 × 7 ft | 448 ft³ (12.7 m³) | about 12.7 kW |
FAQ
What size heater do I need for a 6 × 6 × 7 ft sauna?
The room is 252 cubic feet, about 7.1 m³. With no glass or bare masonry, the HUUM rule of about 1 kW per cubic metre points to a heater of around 7 kW. Glass adds volume, so a glass front pushes that up.
Does a glass door count?
Harvia's sizing sheet leaves the door out. HUUM counts every glass surface. If your door is all glass, run the numbers both ways and look at heaters whose range covers both.
Is a bigger heater always better?
No. Each heater lists a minimum and maximum room size. Too small and the room never gets hot enough; too big for the room and you are outside the maker's range. Match the range in the manual.
Sources: HUUM, How to dimension a sauna heater; Harvia, Sizing Your Harvia (US sizing sheet).