Most sizing calculators apply one coefficient and hand you a single number. That number cannot be right for every brand, because the manufacturers do not publish the same rule. This one applies each method separately, shows you the arithmetic, and quotes the source it came from.
What each manufacturer’s own method gives
Harvia’s published method
Published for Harvia, Tylö, Finnleo
Calculated volume6.9 m³245 ft³
Implied output6.9 kW
Show the arithmetic
Room volume: 7 × 5 × 7 = 245.0 ft³
Calculated volume: 6.94 m³ → 6.9 kW at 1 kW per m³
“An average of 1 kW of heater power is required for each cubic metre of sauna volume.”
“One square metre of stone, glass or similar non-insulated surface increases the heater power requirement as much as if the sauna room volume were increased by 1.2 cubic metres.”
“In the case that there are uninsulated walls made of bricks, tiles, glass or logs in the steam room, an additional volume of 3.3 ft³ per each such square feet of the wall should be added to the cubic size of the steam room.”
Matched on published output against the higher of the two calculated requirements, smallest sufficient model first. This is not a ranking, no commission is involved in the order, and models no affiliate retailer carries are listed on exactly the same terms as models that earn us something. Always confirm the model’s own room-volume range in its manual — several manufacturers publish a minimum room size as well as a maximum, and an oversized heater in a small room is its own problem.
Retailer names above are affiliate links — we may earn a commission if you buy, at no extra cost to you. They were attached after this list was computed and had no influence on which heaters appear in it or in what order. Full disclosure.
This is a planning estimate produced by applying each manufacturer’s own published sizing method to the dimensions you entered. It is not a specification and not an electrical instruction. Confirm the final heater choice against that model’s manual and have the circuit specified by a licensed electrician against your local code.
What size sauna heater do I need?
The starting point both manufacturers agree on is roughly 1 kW of heater output per cubic metre of sauna room volume — measure the finished interior length, width and ceiling height, and multiply. A 2.0 × 1.8 × 2.1 m room is 7.56 m³, which puts you in the 7.5–8 kW bracket before any adjustment.
The adjustment is where it stops being simple. Any surface that does not hold heat the way an insulated wall does — glass, tile, stone, exposed masonry, uninsulated log — raises the requirement, and the two manufacturers who publish their rule in plain terms do not raise it by the same amount. That is what the calculator above is for: it runs both published rules against your room and shows you both answers rather than averaging them into one number that matches neither.
Sizing is a planning calculation, not an installation approval. The exact model manual, its required clearances, the controls arrangement and your local review still determine whether a heater belongs in a specific room.
Why two calculators disagree
Harvia’s heater-selection guidance says: “An average of 1 kW of heater power is required for each cubic metre of sauna volume.” On cold surfaces it says: “One square metre of stone, glass or similar non-insulated surface increases the heater power requirement as much as if the sauna room volume were increased by 1.2 cubic metres.” For interior walls of non-insulated logs it gives a separate instruction — “the corresponding factor is 1.5.”
HUUM’s heater-selection guidance says: “As a rule of thumb, heating 35.3 ft³ / 1 m³ of the steam room requires 1 kW of power.” It says uninsulated brick, tile, glass or log wall calls for an additional 3.3 ft³ for each square foot of that wall. Its European support page expresses the same idea as an additional 1 m³ per square metre of uninsulated brick, tile or glass wall (HUUM’s EU support guidance).
These are not interchangeable calculator settings. Harvia adds 1.2 m³ per m² of cold surface and separately applies a 1.5 multiplier for non-insulated log walls. HUUM adds 1.0 m³ per m² and treats log walls within the same additional-surface rule. The same room therefore produces different adjusted volumes under the two approaches.
Input
Harvia published approach
HUUM published approach
Basic room volume
About 1 kW per m³
1 kW per 1 m³ / 35.3 ft³
Glass, tile, stone, similar uninsulated surface
Add 1.2 m³ per m²
Add 1.0 m³ per m² (3.3 ft³ per ft²)
Non-insulated log walls
Apply 1.5 factor
Included in additional-surface rule
Worked example in metres
Take a finished room measuring 2.0 m by 1.8 m by 2.1 m. The base volume is 7.56 m³. Suppose it has a 0.8 m² glass door and no exposed log interior.
Under Harvia’s published method the glass adjustment is 0.96 m³ — 0.8 m² multiplied by 1.2 — giving an adjusted planning volume of 8.52 m³. Under HUUM’s published method the glass adjustment is 0.8 m³, giving 8.36 m³. The gap is modest here, but it is real. Treat it as a prompt to consult each candidate model’s own selection material, not as permission to round casually.
Now change the construction: same enclosure, but with non-insulated log interior walls. Harvia’s stated log factor makes its planning result 11.34 m³ before any product-specific review. HUUM’s method instead asks you to count the qualifying log surface and add its area-based volume. The two results cannot be reconciled without deciding which manufacturer’s rule governs the heater in question.
Worked example in feet
For a room measured in feet, calculate cubic feet first and convert only when comparing against a metric rule. A room 6.5 ft long, 6 ft wide and 7 ft high has a base volume of 273 ft³ — approximately 7.73 m³. With a 7 ft² glass door, HUUM’s stated adjustment is 23.1 ft³, producing a planning volume of 296.1 ft³.
For Harvia’s metric surface rule, convert the 7 ft² door to approximately 0.65 m². The published 1.2 m³-per-m² addition is about 0.78 m³; added to 7.73 m³ that is roughly 8.51 m³. The conversion is not the point. The point is to preserve the manufacturer’s method from start to finish, and to label which method produced the number.
What the volume figure leaves out
A heater’s nominal output does not settle the rest of the room design. Door opening direction, heater-to-bench relationships, glass placement, the ventilation path, control location, service access and the manufacturer’s clearance diagram can each disqualify an otherwise plausible choice. Rooms also change after sizing, when finishes, a larger glass panel or a different ceiling get selected.
Do not use a general calculation to reverse-engineer electrical work. Electrical design depends on the exact appliance documentation and local conditions. Ask a licensed electrician to review the selected model manual and the proposed route before purchase, and ask the local permitting authority which approvals are expected in your jurisdiction.
Measure finished interior dimensions. Do not estimate them from exterior kit dimensions.
Draw every non-insulated or high-mass surface and record its area.
State which manufacturer’s published adjustment method you are using.
Shortlist actual models only after checking their manuals for room range, clearances, controls and supply requirements.
Send the exact model manual and room plan to a licensed electrician, and ask the authority having jurisdiction about permit expectations.
Revisit the worksheet whenever glass area, log exposure, ceiling height or layout changes.
Questions worth asking a dealer
Does this exact model’s manual give a room-volume range or a selection chart?
Does its published range assume a conventional insulated room, and how does it direct the owner to account for glass or log construction?
Which controls, sensors, guards, stones and mounting parts are included versus separate?
What clearance drawing and ventilation instructions apply to this revision of the manual?
Who should review the final electrical and permitting requirements in this location?
Where a manufacturer’s own sources answer one of these two different ways, we record it rather than choosing the more convenient figure — see every documented specification conflict.