What size sauna heater do I need?

Choosing the right sauna heater is not simply a matter of looking at the size of the cabin and picking the nearest kilowatt rating.

As a starting point, sauna manufacturers commonly work on roughly 1kW of heater power for every cubic metre of sauna volume. But the calculation changes when the sauna contains large areas of glass, stone or other poorly insulated surfaces.

That means a cabin with an internal volume of 7m³ may need a heater sized for considerably more than 7m³.

The safest approach is:

1. Calculate the sauna's internal volume.
2. Account for glass and other uninsulated surfaces.
3. Check the heater manufacturer's published volume range.
4. Check the electrical requirements before you order.

Here's how it works.

How is sauna heater size calculated?

The basic calculation is straightforward:

Length × width × height = sauna volume

For example, a sauna with internal dimensions of:

2.0m × 1.8m × 2.0m = 7.2m³

The commonly used starting point is approximately 1kW of heater power per cubic metre.

So, at first glance, you might expect a heater around 7kW to be appropriate.

But there's an important complication.

The volume of the room isn't necessarily the same as the heating load.

This is particularly important for modern garden saunas, where large areas of glass are common.

Why does glass change the calculation?

Glass looks great in a sauna. It brings in natural light, creates a connection with the garden and can make a relatively small cabin feel considerably more open.

Thermally, however, glass behaves very differently from an insulated wall.

Harvia's technical guidance states that each square metre of glass, stone or another non-insulated surface can increase the heater requirement by the equivalent of adding 1.2m³ to the sauna volume. It uses a higher factor for uninsulated log walls. HUUM uses a different coefficient, which is why the heater manufacturer's own calculation should always take precedence.

The important point isn't which manufacturer's factor you memorise.

It's this:

More glass means more heat loss, and therefore changes the heater calculation.

A practical example

Take our 2.0m × 1.8m × 2.0m sauna.

The internal volume is:

7.2m³

Now imagine it has a full glass front measuring 1.8m × 2.0m.

That's:

3.6m² of glass

Using Harvia's 1.2m³ adjustment:

3.6 × 1.2 = 4.3m³

So the effective calculated volume becomes approximately:

7.2 + 4.3 = 11.5m³

The cabin hasn't physically become bigger.

But from the heater's point of view, the heating requirement has increased significantly.

That's why simply saying "it's a 7.2m³ sauna, so it needs a 7kW heater" can lead you to the wrong answer.

Three examples of how the calculation changes

Using Harvia's 1.2m³ adjustment for glass:

The middle example is particularly useful.

A 7.2m³ cabin might initially look as though it belongs in the 7kW class.

Once the 3.6m² glass front is included, the calculated volume is approximately 11.5m³.

That's a substantial difference.

And it demonstrates why heater sizing should be done from the complete cabin specification, rather than the headline dimensions alone.

What if my calculation falls between heater sizes?

This is where the manufacturer's published specifications become important.

Heaters aren't manufactured in every possible 1kW increment, and each model has a specified minimum and maximum room volume.

For example, Harvia's Vega range includes:

Harvia advises selecting a heater within the manufacturer's specified volume range rather than simply applying the 1kW-per-m³ rule mechanically. Its guidance also recommends avoiding the very bottom and very top of a heater's specified range where possible.

So, if your calculated volume is close to the upper limit of one heater, the next model may be the more appropriate choice.

For example:

7.4m³ calculated volume

A 6kW Vega is specified for 5–8m³, so it technically falls within the published range.

But it is relatively close to the upper limit.

By contrast:

11.5m³ calculated volume

falls comfortably within the 8–14m³ range of the 9kW Vega.

The lesson is not "always buy the bigger heater."

It's:

Don't choose a heater simply because its nominal kW rating looks right. Look at the manufacturer's actual operating range.

Is a bigger sauna heater always better?

No.

It might seem logical that a more powerful heater would always be an improvement. But sauna heaters aren't simply about producing as much heat as possible as quickly as possible.

The stones are an important part of the system. They absorb and retain heat and produce steam when water is added.

Harvia's energy guidance warns that an oversized heater can heat the room quickly without allowing the stones to absorb heat in the same way. HUUM similarly notes that a more powerful heater does not necessarily produce better steam.

Undersizing isn't desirable either.

A heater operating at the edge of its capacity can take longer to bring the sauna up to temperature and spend longer fighting the heat being lost through the cabin.

So the objective isn't:

The biggest heater you can afford.

Or:

The smallest heater that technically fits.

It's:

The correctly sized heater for the cabin.

What does heater size mean for your electrics?

This is something worth considering before buying the sauna, rather than after it arrives.

The electrical requirement increases with heater output.

For example, the Harvia Vega specifications used in this guide give the following single-phase requirements:

• 6kW: 35A fuse
• 9kW: 50A fuse

The exact supply, cabling and protection requirements must always be confirmed against the heater's installation manual and by your electrician.

This matters because moving from a smaller heater to a larger one isn't necessarily just a £100–£200 change in the heater itself.

It can affect the electrical installation required to operate the sauna.

That's why we recommend discussing the electrical supply before committing to the cabin and heater combination.

Your electrician can establish what your property can accommodate and what installation will be required.

Does ceiling height affect heater size?

Yes.

The heater calculation uses internal volume, so ceiling height is part of the equation.

A sauna measuring 2.0m × 1.8m × 2.0m has a volume of 7.2m³.

Increase the height to 2.3m and the volume becomes:

2.0 × 1.8 × 2.3 = 8.28m³

That's an additional 1.08m³ before taking any glazing into account.

Ceiling height also matters for the physical experience of the sauna. Bench height, head position, heater position and the distribution of hot air all influence how the cabin feels when you're sitting inside it.

So when comparing saunas, don't look only at the external footprint.

Look at the internal geometry as well.

What about wood-fired sauna heaters?

Wood-fired heaters are different.

You shouldn't assume that the kilowatt rating of a wood-burning heater can be compared directly with the kW rating of an electric heater.

For example, Harvia's M3 wood-burning heater is specified for a 6–13m³ sauna and has a stated output of 16.5kW. That doesn't mean it is equivalent to a 16.5kW electric sauna heater.

For a wood-fired sauna, use the manufacturer's published sauna-volume range when selecting the heater.

If you're deciding between electric and wood-fired heating, the choice involves considerably more than heater output, including installation, ventilation, chimney requirements, fuel and how you intend to use the sauna.

What about running costs?

Heater size alone doesn't tell you what a sauna will cost to run.

A thermostatically controlled electric heater doesn't necessarily draw its full rated power for the entire session. Once the sauna reaches temperature, the heater cycles to maintain it.

Harvia gives an example of approximately 7–9kWh for a two-hour session using a 6kW family sauna, although actual consumption will vary with cabin construction, starting temperature, outdoor conditions, target temperature, glazing and usage.

So if you're comparing saunas, don't look at the heater's kW rating and assume:

9kW = 50% more expensive to run than 6kW.

The real-world picture is more complicated.

If running costs are an important part of your decision, they deserve to be considered alongside insulation, glazing, heater sizing and warm-up time.

Five things to check before buying a sauna

Before you commit to a cabin, work through these five questions.

1. What is the internal volume?

Calculate:

length × width × height

Use the internal dimensions rather than the external footprint.

2. How much glass does it have?

Measure the glass area and apply the correction factor specified by the heater manufacturer.

Don't assume that a 7m³ cabin needs a 7kW heater simply because the room volume says 7m³.

3. What is the heater's published volume range?

Check the actual manufacturer's specification.

Look at the minimum and maximum room volume and avoid choosing a heater simply by rounding your calculated figure to the nearest kW.

4. Can your electrical supply support it?

Check the heater's supply requirements before ordering.

Your electrician should confirm the appropriate circuit, cabling, protection and installation requirements for the specific heater.

5. Are you comparing the whole heating system?

The heater isn't the only consideration.

Think about:

• Heater
• Stones
• Controls
• Electrical installation
• Cable run
• Ventilation
• Cabin insulation
• Glazing
• Installation requirements

A sauna is a system, not simply a wooden cabin with a heater bolted inside it.

The short answer

So, what size sauna heater do you need?

There isn't one answer based purely on the cabin's footprint.

As a starting point, allow roughly 1kW per cubic metre of sauna volume, then account for glass and other uninsulated surfaces. After that, choose a heater whose manufacturer's published volume range comfortably accommodates the calculated figure. Finally, check the electrical requirements before you commit to the cabin.

The important calculation is therefore not:

"How big is my sauna?"

It's:

"How much volume does the heater actually have to heat?"

Get that calculation right and the rest of the heater specification becomes much easier to understand.

If you are still choosing the cabin rather than the heater inside it, our outdoor sauna buying guide takes the five decisions in the order they have to be made, and the cabins are under outdoor saunas. The URSA is a worked example of everything on this page: a full glass front, a 10.5kW heater and a single phase circuit of around 50 amps.