How the heater size is worked out
- Room volume = inside width × depth × height (for a barrel: the round section above the floor boards × length).
- Effective volume. Heater makers add 1.2 m³ for every m² of uninsulated surface such as glass, tile, stone or concrete, and multiply the volume by 1.5 for massive log walls (Harvia's installation rule). We apply the log rule to barrel saunas too, because their staves are solid, uninsulated wood.
- Match. Every heater in our catalog carries its maker's own minimum and maximum room volume. A heater fits when your effective volume lies inside that range.
| Room (effective volume) | Typical heater output |
|---|---|
| 3–5 m³ (105–175 ft³) | 3.5–4.5 kW |
| 5–8 m³ (175–280 ft³) | 6 kW |
| 7–12 m³ (250–425 ft³) | 8 kW |
| 8–14 m³ (280–495 ft³) | 9 kW |
| 10–18 m³ (350–635 ft³) | 10.5–12 kW |
The table shows typical maker ranges, not a rule: ranges differ by brand and model, so the calculator checks each heater individually.
Why not just "1 kW per m³"?
The old rule of thumb (about 1 kW per m³, or 1 kW per 45–50 ft³) works for a well-insulated indoor room with a wooden door. It undersizes a sauna with a full glass front, and it undersizes barrels and log cabins in winter. A heater that is too small has to run flat out for longer, heats up slowly and wears out sooner; one that is too big for a small room overheats the benches before the stones are hot enough for good steam.
What the heater also needs
- Electrics: a dedicated circuit sized from the heater's manual (see the electrical requirements tool).
- Clearances: the minimum distances to walls, benches and the ceiling from the manual; the room must be at least as tall as the heater requires.
- Ventilation: fresh air in near the heater, exhaust low on the opposite side (sauna ventilation guide).