Sauna Heater Size Calculator
What size sauna heater (kW) do you need: room volume plus 1.2 m³ for every m² of glass or uninsulated wall, log-wall factor, recommended standard heater and current draw.
How it works
effective volume = length × width × height + 1.2 m³ per m² of glass, stone or uninsulated surface (× 1.5 for solid log walls) · heater ≈ 0.75–1 kW per effective m³ · current = W ÷ V (single-phase) or W ÷ (√3 × 400 V)
Sauna heaters are sized by the volume of air and the heat soaked up by surfaces that are not insulated. Harvia’s method adds 1.2 m³ to the room volume for each square metre of glass, stone, tile or other uninsulated surface, and multiplies by 1.5 for bare log walls; about 1 kW per cubic metre is the rule of thumb, and each heater model is rated for a range of volumes. In Harvia’s own example an 8.5 m³ effective volume needs at least 6 kW, with a 6.8 kW heater the best fit, which this calculator reproduces by choosing the first standard size at or above 0.75 kW per m³. An undersized heater takes too long to reach 70–90 °C; an oversized one cycles and dries the air.
Worked example
A 2 × 1.8 m sauna 2.1 m high is 7.56 m³; a 1.5 m² glass door adds 1.2 × 1.5 = 1.8 m³, giving 9.36 m³ effective. At 0.75–1 kW per m³ that is 7.0–9.4 kW, so the suggested heater is 8.0 kW, drawing 34.8 A at 230 V single-phase.
Sources: Harvia — How do I select the correct heater power? · HUUM — How to dimension a sauna heater
Frequently asked questions
What size sauna heater do I need?
Work out the room volume in m³, add 1.2 m³ for every m² of glass or uninsulated wall, multiply by 1.5 for log walls, and pick a heater rated for that volume — roughly 0.75–1 kW per cubic metre. A 2 × 1.8 × 2.1 m sauna with a glass door needs about 8 kW.
How many kW for a 2-person sauna?
A small 2-person sauna of 4–5 m³ needs a 3–4.5 kW heater; add about 1.5–2 kW for a full glass door or glass front.
Does a glass door affect the sauna heater size?
Yes. Glass loses heat fast, so each square metre counts like 1.2 extra cubic metres of room. A typical full-glass door adds about 1.8 m³, roughly 1.5 kW more heater.
What circuit does an 8 kW sauna heater need?
At 240 V single-phase an 8 kW heater draws about 33 A, usually on a 40 A circuit; at 400 V three-phase it draws about 11.5 A per phase. Follow the heater manual and local electrical code.
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This calculator is for educational purposes. Double-check important results before acting on them.