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Sauna Heater Size Calculator
How many kW your sauna needs before you buy, with the corrections for glass, cold surfaces, insulation and outdoor installs that the rough divide-by-50 rule quietly skips.
Measure the inside of the finished room. Most saunas use a low ceiling of about 7 ft on purpose — it holds heat where you sit and needs a smaller heater.
How it is calculated
- Base = L×W×H ÷ 45 cubic feet per kW.
- Add for glass (+1.2 cf/sq ft), cold surfaces (+25% of area), uninsulated wall (+15%), high ceiling (+10%/ft over 7), outdoor (+20%).
- Round up to the nearest heater sold.
What it assumes, and when to size up
The number assumes a properly insulated, sealed and vented room. If yours leaks heat, size up. A few reasons to take the next size: you run the sauna daily and want fast reheats, you like it at the hot end (190°F+), or you have a lot of glass or a masonry wall the corrections only roughly capture. Oversizing slightly is cheap insurance; undersizing means a heater that runs flat out and never quite gets there.
Once you have the kW, two things follow from it: the circuit it needs (a 6 kW and an 8 kW heater are different breakers and wire), and which units actually come in that size, which you can filter in the heater database.