Quick answer: Measure the sauna’s interior volume, then allow for exposed glass, stone and other uninsulated surfaces. A useful first estimate is about 1 kW per cubic metre of adjusted room volume. The final choice must fit the heater manufacturer’s stated room-volume range and installation requirements.
This calculator is for a conventional electric sauna with insulated walls and ceiling. It is not for infrared cabins, steam generators, wood-burning stoves or uninsulated log buildings.
Estimate your electric sauna heater size
Planning estimate only. Check the specific heater manual for room range, ceiling height, clearances, ventilation and electrical supply. This tool does not select a breaker, cable or safe installation location.
How the calculation works
Adjusted volume (m³) = length × width × height (in metres) + 1.2 × exposed uninsulated area (in m²). The approximate kW starting point uses 1 kW per adjusted cubic metre. The conversion is 1 ft = 0.3048 m; area and volume are converted using the square and cube of that factor.
The sizing rule and surface allowance follow Harvia’s heater-power guidance. Harvia also specifies that individual heaters have their own minimum and maximum room-volume ratings. Those product-specific ranges take precedence over treating this rough kW result as an exact model specification.
Worked example
A 2 × 2 × 2 metre room has a physical volume of 8 m³. Adding 1 m² of exposed glass produces an adjusted volume of 8 + (1.2 × 1) = 9.2 m³. The rule-of-thumb power is about 9.2 kW, but that does not mean you must buy a heater labeled exactly 9.2 kW. Compare products whose stated room ranges comfortably cover the adjusted volume.
What to measure before shopping
- Finished internal dimensions: measure inside the planned wall and ceiling finish, using one unit system. For an irregular room, calculate its sections separately and consult the supplier.
- Exposed surfaces: measure the glass door or window width and height, multiply them, then add the areas of relevant uninsulated surfaces.
- Construction: confirm the insulation and wall type. This calculator deliberately excludes uninsulated log rooms, which need a different adjustment.
- The candidate heater’s manual: note its permitted room-volume range and all installation constraints before ordering.
Heater size is not electricity use
A 9 kW rating describes power, not a fixed cost per session. For example, running 9 kW continuously for half an hour consumes 4.5 kWh. Actual session energy also depends on warm-up and thermostat cycling. Once you have a candidate model, use its rated power in our sauna running-cost calculator.
Frequently asked questions
Can I use this for a barrel sauna?
The form assumes a rectangular interior. A curved room needs its actual interior volume and construction considered. Use the sauna supplier’s stated volume and heater compatibility information rather than entering an outside bounding box here.
Should I automatically round up to the next heater size?
No. Match the adjusted volume to the manufacturer’s range and check the complete installation requirements. This page supplies an initial comparison number, not a substitute for the product manual or an installer reviewing your room.
Does the result determine my electrical circuit?
No. Electrical supply, protection and wiring must be selected for the specific heater and local installation requirements by a qualified installer. A room-volume estimate cannot establish those details.
Continue with the home sauna planning guide or browse all sauna tools.
Methodology checked September 16, 2026. Calculator and explanatory examples by SaunaHelper. The estimates are calculations, not measurements from a tested installation.