Sauna Heater kW Calculator
Get a correctly sized heater in under a minute. Adjust dimensions, glass and stone — we work out effective volume, fuse rating and recommended power.
Dimensions and materials
How the calculator works
Heater manufacturers rate every heater for a range of room volumes — Harvia's 9 kW KIP90E, for example, is rated for 8–14 m³. We work out the room volume from your dimensions (with a 2.1 m ceiling), add 1.2 m³ for each m² of glass or exposed stone, and pick the smallest standard heater (4.5 / 6 / 8 / 9 / 10.5 / 12 / 15 kW) whose rated range covers it.
How many kW does my sauna heater need?
Go by the room-volume range the manufacturer states for each heater, not a flat kW per m³. A 2.5 × 2.5 m sauna with a 2.1 m ceiling is about 13 m³, which a 9 kW heater (rated 8–14 m³) covers. Add 1.2 m³ per m² of glass door or exposed stone, and count uninsulated log walls as 1.5 times the volume.
Why does glass increase the power needed?
Glass has a U-value around 1.1–1.4 W/m²K, versus around 0.2 for an insulated sauna wall, so a glass door loses 5–7 times as much heat as the same area of timber. Heater manufacturers account for it by counting each m² of glass as 1.2 m³ of extra room volume, and so do we.
What fuse rating is needed?
At 400 V three-phase, a 4.5–6 kW heater needs a 10 A fuse, 8–10.5 kW needs 16 A, 12 kW needs 20 A and 15 kW needs 25 A. The installation must always be done by a licensed electrician.