Why sauna heater calculators disagree
Put the same room into three calculators and you can get three different heater sizes. The difference comes from how each rule counts cold surfaces.
Everyone starts from room volume
All the common rules begin with the inside volume of the room: length × width × height. Nordic manufacturers work in cubic metres and use roughly 1 kW per m³. North American retailers work in cubic feet and use roughly 1 kW per 45 to 50 ft³. These two starting points are close; the disagreement comes after.
Glass and masonry: how much extra?
Glass, tile, brick and concrete absorb heat or let it escape, so every rule adds something for them. Harvia adds 1.2 m³ of calculated volume for each square metre. HUUM adds 1 m³ per square metre. US retailers add about 1 kW for every 11 ft² of glass. For a single glass door the three rules land within one heater size of each other.
Log and uninsulated walls: the big split
This is where answers diverge. Harvia multiplies the volume by 1.5 for solid log or uninsulated walls. HUUM treats the whole uninsulated shell like glass and adds its full area. US retailers typically add 10 to 20%. For a 6 ft × 6 ft barrel sauna with a glass door, that gives 8 kW under the Nordic rule, 15 kW under the surface rule and 6 kW under the US rule.
How to choose when they disagree
Look at the middle result rather than the highest or lowest. A heater that is far too large cycles on and off and can make the steam harsh; one that is too small runs constantly and takes a long time to heat. Then check the specific model's rated room range: the calculated volume should fall inside it. The calculator does this and also estimates warm-up time, which is the most practical way to compare two sizes.
Sources: Harvia support, HUUM sizing guide, Sauna Marketplace on HUUM sizing.