Sauna room volume and heater size calculator
A sauna heater is sized by room volume, not by floor area. Glass, stone and tile walls take extra heat, and log walls take more. Enter the inside size of the room and its cold surfaces to get the calculated volume and the heater power to ask a wellness room builder about.
How we calculated this
Room volume is inside length times width times height. Feet are converted at 1 cu ft = 0.0283168 m³ and 1 sq ft = 0.092903 m².
Cold surfaces: the Harvia Spirit manual (version 08122023 B) says to add 1.2 m³ to the volume for each square metre of non-insulated wall such as brick, glass block, glass, concrete or tile, and to multiply the volume of a log sauna by 1.5. Its example: a 10 m³ room with a glass door needs about the output of a 12 m³ room.
Power: the tool selects among the cited Harvia models only when their published room range holds your volume. Ranges: KIP45B 4.5 kW, 2.8 to 6.0 m³ (Harvia data sheet); Spirit HSPE60 6 kW, 5 to 8 m³, and HSPE80 8 kW, 5 to 12 m³ (Harvia Spirit manual); KIP90 9 kW, 8 to 14 m³ (Harvia data sheet, printed 2025-04-30). Harvia advises a heater whose range puts your volume near the middle, not at an end. Below 2.8 m³ or above 14 m³, the tool reports an out-of-range result and gives no power or current recommendation. Ask your builder for applicable manufacturer sizing guidance.
Other makers use their own factors and ranges. Use the manual of the heater you buy. Sources fetched 2026-10-29: resources.saunaplace.com/pdfs/Harvia-Spirit-IOM.pdf and harvia.com/en/products/HB900400.
Method checked on 2026-10-29.
An estimate for planning, not a design. The heater manual and a licensed electrician decide the heater, the cable and the circuit for your room.
Questions
- Why does a glass door change the heater size?
- Glass, stone and tile do not hold back heat like an insulated wood wall. Harvia adds 1.2 m³ to the room volume for each square metre of such a surface, so a room with a large glass front needs a stronger heater.
- Is a bigger heater always better?
- No. Each heater has a minimum and a maximum room volume. A heater that is too large for the room can overheat it quickly and cycle off often. Harvia advises a heater whose range puts your volume near the middle.
- Who sizes the electrical supply?
- A licensed electrician. The heater manual lists the current, the cable size and the circuit. The local code decides the rest, for example NEC Article 424 in the US or BS 7671 Section 703 in the UK.
- Does this work for an infrared sauna?
- No. An infrared cabin is sold as a unit with its own emitters and power need. Use the maker's specification for it.
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