Heater makers and retailers use different sizing rules, so their calculators disagree. Enter your room once to see all three rules side by side, how long it takes to heat, and which heaters are rated for it.
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Only what a working sauna needs, plus a few things worth having.
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Each bar is one sizing rule applied to your room. The amber line is the recommendation, the middle of the three.
Start from 1 kW per m³. Each m² of glass, stone or other uninsulated surface adds 1.2 m³ of calculated volume. Solid log or uninsulated walls multiply the volume by 1.5. The calculated volume is matched to heaters whose rated room range covers it.
Vc = (V + 1.2 × cold m²) × wall factor
Source: Harvia support
Every m² of non-insulated surface adds 1 m³. A log wall or uninsulated wall counts the same as glass, so for log and barrel saunas the whole shell is added. This is the strictest rule and can recommend a much larger heater.
VA = V + 1.0 × non-insulated m²
Source: HUUM sizing guide
1 kW per 50 ft³, plus 1 kW per 11 ft² of glass and per 22 ft² of masonry. Uninsulated walls add 20%, solid log 15%, and an outdoor or unheated location another 10%. The result is rounded up to the next standard size.
kW = ft³ × factors ÷ 50 + glass ft² ÷ 11
Based on North American retailer guidance, e.g. Saunaplace
The recommendation is the smallest standard size that reaches the middle (median) of the three rules, so one unusually strict or lenient rule cannot move it alone. Heaters are listed when the manufacturer's rated room range covers your Nordic calculated volume, closest to the recommended power first.
The room is modelled as one heat store: the air, the outer 15 mm of wood paneling, exposed masonry, the stones and the heater body. Heat leaves through walls, glass, floor and ventilation at six air changes per hour. The result is an estimate; real rooms vary with stone placement, vents and how cold the walls start.
Current is power divided by voltage for 230 V, 400 V three-phase and 240 V. US circuits are commonly sized at 125% of a continuous load. Energy is warm-up at full power plus the heat the room loses at target temperature during your session. A licensed electrician decides the actual breaker and cable.