Allow roughly a kilowatt per cubic metre of sauna space as a starting point, not a finished specification. Glazing and heat-storing surfaces increase the power needed, while insulation and ventilation also matter. Measure the room, including its height, then use Harvia or HUUM’s calculator to account for its construction before comparing heaters.
Measure the whole room, not just the floor
Heavenly Saunas calculates sauna volume by multiplying length × width × height. Take those measurements in metres to get a result in cubic metres.
Its example measures 2 metres long by 1.75 metres wide, with a height of 2 metres. That gives a room volume of 7 cubic metres, before allowing for features that affect heater power.
Keep the individual dimensions handy. Harvia and HUUM both ask for width, depth and height, along with information about the room’s construction. Floor area alone leaves out information their calculators need.
Use the rough power rule with care
Heavenly Saunas suggests around a kilowatt per cubic metre. For the room above, that means roughly 7 kW. Its guide also says insulation and ventilation can change the requirement.
Harvia and HUUM take a different approach. Neither gives a universal kilowatt-per-cubic-metre figure in its calculator information; both base recommendations on room size and construction. Volume matters across all these sources, but a simple power rule cannot account for the whole room.
Glass and wall materials change the requirement
Glazing and heat-storing surfaces increase the power needed, according to both Harvia and HUUM. Brick and concrete matter here, as do substantial log surfaces. Look beyond the dimensions to what the walls and doors are made from.
Harvia’s calculator has options for log construction and a glass door. It also asks for the uninsulated wall area in square metres. HUUM requests that area too, with a reminder to include a glass door.
For windows or a glass door, Heavenly Saunas suggests adding roughly a kilowatt to 2 kW. It gives the same allowance for poor insulation. Harvia and HUUM do not confirm that fixed allowance in their calculator information, so treat it as broad guidance rather than a substitute for recording the room’s actual features.
Working with Harvia and HUUM’s calculators
Alongside the room dimensions, have the uninsulated wall area in square metres ready. Note any glass door, log construction or heat-storing surfaces so you can include them where requested.
HUUM also asks whether the sauna is indoors or outdoors, and whether you want an electric or wood-burning heater. For a garden sauna, use its outdoor option. Its calculator information does not give a fixed outdoor power allowance.
Results are presented slightly differently. Harvia gives a calculated sauna volume with recommended power. HUUM says its results include suitable heater power and a model.
Match the heater rather than sizing up
More power is not automatically better. Heavenly Saunas warns that heaters can perform poorly when they are too small or too large for the room.
Among Zauna’s checked products, the PYRY Electric Sauna Heater by Tulikivi and the HUUM DROP Electric Sauna Heater are both rated at 4.5 kW. The Narvi NC Domestic Electric Sauna Heater (Wi-Fi) has a 6 kW rating. Those figures help you compare products once you have assessed the room; they do not establish which model suits it.
Use the calculator’s recommendation to guide that comparison, with the room’s measurements and relevant construction features accounted for. For electrical work, use a qualified electrician.
Questions people ask
- What size heater do I need for a 7 m³ sauna?
- Using the rough kilowatt-per-cubic-metre rule gives around 7 kW. That is not a final specification: glazing and heat-storing surfaces can increase demand, and insulation matters too. Check the room in a manufacturer’s calculator before choosing.
- Does a glass door affect sauna heater size?
- Yes. Harvia and HUUM identify glass as a factor that increases the power requirement. Include the door in their sizing tools rather than relying on room volume alone.
- Do garden saunas need a fixed amount of extra power?
- There is no fixed extra allowance established by these sources. HUUM lets you select an outdoor sauna alongside entering its dimensions and construction features.
- Can I choose a heater from floor area alone?
- No. These sizing methods need room volume, so height matters as well as width and depth. You also need to account for glazing and uninsulated surfaces.
- Is a higher-kW sauna heater always better?
- No. Heavenly Saunas warns that either an undersized or oversized heater can perform poorly. Match the power to the room’s size and construction instead of choosing the highest rating.
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