How to Size a Sauna Heater: A Practical Guide

The heater (the kiuas) is the most important part of any sauna. If it's too small, the room takes over an hour to warm up and never gives a good burst of steam when water hits the stones. If it's too large, it switches on and off constantly, the heat feels harsh and dry, and the timber near it can scorch. Getting the size right is the difference between a sauna you use every day and one you put up with.

Here is the method we use on every Aqua Spazio project, from compact home cabins to commercial thermal suites.

Step 1: Work out the volume of the cabin

Measure the internal length, width and height in metres and multiply them together.

Length × width × height = volume (m³)

Measure height to the underside of the ceiling. This is one reason good saunas have low ceilings, usually 2.1–2.3 m. Every extra centimetre adds air the heater has to warm, and the heat ends up above the bathers' heads instead of around them.

Step 2: Add extra volume for surfaces that soak up heat

Insulated timber walls reflect heat back into the room. Glass, stone, tile and concrete absorb it. To allow for this, add about 1.2 m³ to your calculated volume for every 1 m² of uninsulated surface.

Common examples:

• Glass doors: a typical full-glass door is about 1.5–1.8 m².

• Glass front walls or panoramic windows: these increase the figure quickly.

• Feature walls in stone or tile: often specified for looks, but they take a lot of energy to heat.

• Solid log walls: log saunas generally need a noticeably larger heater than framed and insulated cabins. Follow the manufacturer's guidance for log construction.

  
  

Step 3: Match the adjusted volume to the heater's rated range

Every reputable manufacturer publishes a minimum and maximum room volume for each heater model. As a rough guide, you need about 1 kW of heater output per 1 m³ of adjusted volume. Always check the specific model's table, though, because stone capacity and design vary.

Choose a heater whose rated range puts your adjusted volume comfortably in the middle, not at the very top.

Worked example 1: Home sauna with a glass door

• Cabin: 2.0 m × 1.8 m × 2.1 m = 7.6 m³

• Glass door: 1.6 m² × 1.2 = +1.9 m³

• Adjusted volume: about 9.5 m³. Look for a heater rated around 9 kW whose range covers 9.5 m³.

Worked example 2: The same sauna with a glass front

• Cabin: 7.6 m³

• Glass front and door: 4.0 m² × 1.2 = +4.8 m³

• Adjusted volume: about 12.4 m³. This now needs a heater of 10.5–12 kW.

The only difference is the glass, yet the heater has gone up a size, and possibly so has the electrical supply (see below). This is why we size the heater at the design stage, not after the joinery has been ordered.

Step 4: Check your electrical supply

This step catches out many UK projects:

• Single-phase (most UK homes): heaters up to about 9 kW can usually run on a dedicated single-phase circuit, as long as the property's incoming supply has the capacity.

• Above about 9 kW: larger heaters are often designed for three-phase power. A three-phase upgrade from your network operator can take time and add significant cost.

• Every installation: the heater needs a dedicated circuit installed by a qualified electrician to BS 7671, which includes specific requirements for locations containing sauna heaters. A heater of this size is a large load, so check what else shares the supply, such as EV chargers, heat pumps and pool plant.

If three-phase power isn't available, reducing the glazing or the ceiling height is often cheaper than upgrading the supply.

Step 5: Consider stones, ventilation and how the sauna will be used

The kW figure is only part of the picture.

• Stone mass: heaters with a large stone capacity give softer, longer-lasting steam. If you like plenty of steam, choose a model with more stones over one with more kW.

• Ventilation: fresh air should come in low, near the heater, and exhaust air should leave on the far side. Poor ventilation makes a correctly sized heater feel stuffy and inefficient.

• Commercial and high-use saunas: gyms, spas, hotels and contrast therapy suites have frequent door openings and long running hours. Size for that, and choose heaters built for continuous commercial use.

• Wood-fired heaters: these are sized by room volume in the same way, but flue routing, hearth protection and clearances matter as much as the output.

Quick checklist

1. Measure the internal volume (L × W × H) in m³.

2. Add about 1.2 m³ per m² of glass, stone, tile or concrete.

3. Pick a heater whose rated range puts that figure in the middle.

4. Confirm single-phase or three-phase supply capacity early.

5. Plan stone capacity, ventilation and usage before ordering.

   
   

Getting it right first time

Heater sizing is linked to the cabin's construction, glazing, electrics and ventilation, so it works best as part of the whole design rather than an afterthought. At Aqua Spazio we design and install bespoke saunas, steam rooms and contrast therapy suites across London and the UK, with the heater, timber and airflow specified together from the start.

Planning a sauna? Get in touch with the dimensions and a few photos of the space, and we'll advise on the right heater and layout.

Frequently asked questions

What size heater do I need for a 2-person sauna?A typical 2-person cabin is around 4–6 m³, so a 4.5–6 kW heater usually covers it. Add about 1.2 m³ to the volume for each m² of glass, and check the heater's rated range before you buy.

Can I run a sauna heater on a normal UK household supply?Usually yes, up to about 9 kW on a dedicated single-phase circuit, as long as the incoming supply has enough spare capacity. Larger heaters often need three-phase power. A qualified electrician must install the circuit to BS 7671.

What happens if my sauna heater is too small?The room takes a long time to heat up, may never reach a comfortable temperature, and the stones won't hold enough heat to give a good burst of steam. The heater also runs flat out all the time, which shortens its life.

Can a sauna heater be too powerful?Yes. An oversized heater switches on and off constantly, the heat feels harsh and dry, and the timber near the heater can scorch. Aim for your room volume to sit in the middle of the heater's rated range.

Does a glass door or glass wall affect heater size?Significantly. Glass loses heat much faster than insulated timber, so add about 1.2 m³ to your volume for every m² of glass. A full glass front can push you up a heater size, and sometimes onto three-phase power.

How long should a sauna take to heat up?A correctly sized electric heater in a well-insulated cabin typically reaches 80°C in about 45–60 minutes. If yours takes much longer, the heater may be undersized or the room may be losing heat through glass or poor insulation.

Do commercial saunas need a bigger heater?Often, yes. Gyms, spas, hotels and contrast therapy suites have frequent door openings and long running hours, so we size them with extra capacity and use heaters built for continuous commercial use.