Sauna heater size calculator
Measure the hot room only, estimate the glass and uninsulated area honestly, and the calculator does the rest: effective volume by Harvia's published rules, matched against each heater's own rated range rather than a generic formula. Everything runs in your browser.
Common rooms, pre-measured
Each preset is a scenario we sized in advance, with the answer already in the page.
- Heater size for a 2 person barrel sauna
A two person barrel hot room at these dimensions lands in the middle of the 4.5 kW window, with the 6 kW class opening right at its edge.
- Heater size for a 4 person barrel sauna
A four person barrel at these dimensions sits near nine effective cubic meters, which is 8 kW territory with the 6 kW class at its upper edge.
- Heater size for a 6 person cabin sauna
A six person cabin at these dimensions needs the 8 kW class as a floor, and window area decides whether even that holds.
- Heater size for a shed sauna conversion
An eight foot shed converts to a far bigger heating job than its tape measure suggests: uninsulated surface inflates the effective volume into the largest heater class we track.
The rules this tool applies
Manufacturer rules we apply
Base power per volume: 1 kW per m3 of hot-room volume
Harvia publishes 1 kW per cubic meter as the average requirement, and notes the calculated figure is in most cases the minimum. An undersized heater runs longer and harder, which shortens its life.
Read from Harvia's published guidance on 2026-08-30.
Glass and non-insulated surface: +1.2 m3 of effective volume per m2
Harvia treats each square meter of glass, stone or similar non-insulated surface as if the room were 1.2 cubic meters larger, because those surfaces store and leak heat.
Read from Harvia's published guidance on 2026-08-30.
Non-insulated log walls: multiply room volume by 1.5
Harvia applies a 1.5 factor when the hot-room interior walls are non-insulated logs.
Read from Harvia's published guidance on 2026-08-30.
Where in a heater’s range to buy: aim for the middle of the maker’s min-max m3 range
Harvia advises against buying at the minimum or maximum of a heater’s stated volume range. We sort matches by distance from the midpoint of each heater’s own range for this reason.
Read from Harvia's published guidance on 2026-08-30.
Framing that is ours
Circuit sizing framing: amps at 240 V, then 125% for a continuous load
This is our conservative framing, not an installation instruction. A sauna heater runs for hours, and sizing a circuit at 125% of a continuous load is common North American practice. Every figure we show is paired with the same advice: have a licensed electrician confirm the circuit, breaker and wiring for your installation.
More on sourcing at how we verify. For the circuit conversation, read the electrical guide before calling an electrician.