Home Spa Calc

Sauna heater size calculator

Set your room dimensions — by typing, or by dragging the corner of the drawing — and add any glass or stone. The recommendation updates live.

Not sure of your numbers? Start from an example:

Inside face to inside face. Type 7' 6" or 7ft 6in if you like.

Inside width, not the outer frame.

Floor to ceiling. Lower ceilings need smaller heaters.

Door and windows. A standard glass sauna door is about 16 sq ft.

Any uninsulated hard surface. Leave at zero for an all-wood room.

kW per m³

The common convention is 1 kW per cubic metre. Your heater brand may differ.

What this assumes
  • 1 kW per cubic metre of effective volume is the general industry convention, adjustable above.
  • Glass adds 1.2 m³ of effective volume per m², stone and concrete 1.5 m³ per m². These surfaces soak up and lose far more heat than an insulated panelled wall, so the room behaves as if it were bigger than it measures.
  • The result is a range bracketed to ratings that are actually sold. Sizing is a convention, not a physical constant, and a single decimal figure would imply precision nobody has.
  • Manufacturers differ. Check the chart for the brand you are buying — and its minimum room volume, which matters as much as the maximum.
Drag the corner to resize, or type dimensions above. Drawn to scale, with two seated adults for reference.

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Fill in the fields to see your answer.

  • Room volume—
  • Glass adds—
  • Stone adds—
  • Effective volume—
Now you know the size — what will it cost to use? Work out the running cost →

How this calculation works

  1. Room volume
  2. Add for glass and stone
  3. Effective volume
  4. Heater kW range

kW = (L × W × H + glass m² × 1.2 + masonry m² × 1.5) × 1.0

Measure the room inside, multiply for volume, then add extra volume for any surface that loses heat faster than an insulated timber wall. Multiply the total by the sizing rule — conventionally 1 kW per cubic metre — and round out to heater sizes you can actually buy.

Effective volume to heater size, at 1 kW per m³
Effective volumeRoom roughlyTypical heater
3–4.5 m³ (105–160 ft³)2-person corner4.5 kW
4.5–6 m³ (160–210 ft³)Small 2–3 person6 kW
6–8 m³ (210–280 ft³)Typical 4-person8 kW
8–9 m³ (280–320 ft³)4–5 person9 kW
9–10.5 m³ (320–370 ft³)Large family10.5 kW
10.5–12 m³ (370–425 ft³)Large, or lots of glass12 kW

Manufacturers differ. Check the chart for the brand you are buying, including its minimum room volume.

Where the adjustments come from

SurfaceAdds per m²Per ft²
Glass (door, window)1.2 m³0.11 m³
Stone, tile, concrete1.5 m³0.14 m³
Insulated timbernothingnothing

Why glass changes the answer so much

A sauna's heater is sized against how fast the room loses heat, not just how much air it holds. An insulated, foil-backed, cedar-panelled wall holds heat well. A glass door does not — it conducts heat straight out and has no insulation value worth counting. The convention of adding equivalent volume for glazing is a way of saying "this room behaves like a bigger one".

The practical version: a full glass front on a small sauna can push you up a whole heater size. It is worth deciding on the glazing before you buy the heater, not after.

Ceiling height is the cheapest fix

Volume scales directly with ceiling height, and heat stacks at the top of the room where nobody is sitting. Dropping a 2.4 m ceiling to 2.1 m cuts roughly an eighth off the required heater — often a whole size — and makes the room heat faster too. If you are building from scratch, this is the single easiest saving available.

Before you buy

Sauna stones glowing orange inside a heat-storage stove
A heat-storage stove. Too large a heater for the room will scorch the timber and cycle badly, so bigger is not safer.

For the reasoning behind these numbers, read how sauna heater sizing actually works. Once you have picked a heater, the sauna running cost calculator shows what it will cost to use.