Sauna heater size calculator

Measure the inside of the sauna room in feet.

Walls
Location
Winter climate

Example result for a 6 x 5 x 7 ft insulated room with an 11 sq ft glass door

7.2 kW minimum heater size

Room volume
210 cu ft
Log wall adder
0 cu ft
Cold surface adder
43.3 cu ft
Effective volume
253.3 cu ft

Heaters rated for this room size

Closest to the middle of their rated range first, as Harvia advises.

The formula
  1. Room volume = length x width x height, in cubic feet, measured at the highest point of the ceiling.
  2. Log or barrel walls: multiply the room volume by 1.5 (Harvia).
  3. Cold surface adder: Harvia adds 1.2 cubic meters for each square meter of glass, stone, tile or concrete, which is about 3.94 cubic feet per square foot. Glass doors count.
  4. Effective volume = room volume (after any log wall multiplier) + cold surface adder.
  5. Minimum heater size = effective volume / 35.3. Harvia and HUUM both size heaters at 1 kW per cubic meter, about 1 kW per 35.3 cubic feet.
  6. Then pick a heater whose manufacturer-published room range includes your effective volume, ideally near the middle of the range.

Sources:Harvia, How do I select the correct heater power (updated May 11, 2026); HUUM, How to dimension a sauna heater (modified Jan 13, 2026).

STOP sizing a sauna heater by floor area or price alone, because one glass door can push your room into the next heater size. Harvia and HUUM both size heaters by volume, and this calculator does that math for you.

How to use it

Measure the inside of the room in feet, with the height taken at the highest point of the ceiling. Count any glass, tile, brick, concrete or stone surface in square feet, including a glass door. Choose “log or barrel staves” if the walls are solid wood with no insulation behind them. Then compare the heaters rated for your effective volume, or browse our electric sauna heater picks.

Why glass, stone and log walls change the answer

Glass and masonry soak up heat that an insulated wall would hold. Harvia handles this by adding 1.2 cubic meters of volume for every square meter of cold surface, about 3.9 cubic feet per square foot. Solid log walls and barrel staves also lose more heat, so Harvia multiplies the room volume by 1.5. Our barrel sauna guide shows why a 6 kW heater can be stretched in a 4-person barrel.

Outdoor saunas in cold climates

Harvia says outdoor saunas and cold climates need a larger heater but publishes no number. Pick a heater whose rated range puts your room at or below the middle, not at the top. Wood-fired heaters follow the same volume logic; see our wood burning sauna stove picks.

Next steps

Estimate what the heater will cost to run with the sauna running cost calculator, or start at the home sauna buyer’s guide.