Sauna building
Sauna volume and heater sizing
Works out the effective volume a heater has to be rated for, including the allowance uninsulated glass and stone add — the number to take to a manufacturer’s specification rather than a kilowatt figure to trust.
The question this answers: “What size heater does this room need, and does the glass door change the answer?”
Result
- Effective volume for heater sizing
- 11.76 m³
- The number to compare against a heater’s rated volume range.
- Actual room volume
- 10.08 m³
- 2.4 × 2 × 2.1 m.
- Added by uninsulated surfaces
- 1.68 m³
- 17% more than the room itself, from 1.4 m² glass and 0 m² stone.
- Floor area
- 4.8 m²
- For bench planning rather than heater sizing.
Worth knowing about this answer
- This gives a volume to check against a heater’s rated range. It does not choose a heater, state a kilowatt figure or say that any installation is safe — the rating belongs to the manufacturer, and a wood stove is a notifiable installation with clearance distances that are theirs to specify.
- Measure to the ceiling you will actually have. A void above a dropped ceiling is not heated volume, and counting it is the commonest way a heater ends up oversized.
Show the working
- Room volume
2.4 × 2 × 2.110.08 m³ - Glass allowance
1.4 m² × 1.21.68 m³ - Uninsulated stone allowance
0 m² × 1.20 m³ - Effective volume
10.08 + 1.68 + 011.76 m³
Effective volume for heater sizing: 11.76 m³. Actual room volume: 10.08 m³. Added by uninsulated surfaces: 1.68 m³. Floor area: 4.8 m²
Room volume is length by width by height. Uninsulated surfaces behave like additional volume because they absorb and lose heat rather than reflecting it, and the conventional trade allowance is 1.2 cubic metres per square metre of glass or exposed stone. Effective volume is the room volume plus those allowances, and it is the figure to compare against a heater’s rated volume range.
- The 1.2 factor is the conventional allowance used across sauna heater literature. It is an industry convention rather than a measured constant, and it is stated here as such.
- An otherwise insulated timber room. A building insulated below current practice needs more heater than this suggests.
- It outputs a volume, never an appliance and never a kilowatt figure. Heater ratings embed each manufacturer’s assumptions about stone mass, element layout and their own testing, and substituting a rule of thumb for a specification on a high-power or combustion appliance is precisely the decision this catalogue refuses to make for a reader.
- It makes no claim that any installation is safe, and none about clearances to combustibles, flue components or ventilation. Those are the manufacturer’s figures and, for a wood stove, a notifiable installation — both covered on the sauna building page.
- It says nothing about how long a room takes to heat, which depends on stone mass and thermal mass rather than on volume.
Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-17. More on Sauna building.