Enter the inside size of your sauna room. You'll get the heater output you need and the heaters on sale in the UK that are rated for that room.
Calculated volume = room volume + glass m² + uninsulated wall m² (k = 1). Then about 1 kW per calculated m³. Rule of thumb from HUUM; the heater maker's stated room range and installation instructions decide, and an electric heater must be connected by a registered electrician.
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Calculator by outdoorsauna.uk, using UK prices checked 30 September 2026. A guide only, not professional advice: important information.
The calculator starts with a typical four-seat room: 2 m long, 1.8 m wide and 2 m high, with no glass. That’s 7.2 m³, so about a 7 kW heater. Change the numbers to match your sauna, and the list underneath shows the heaters on sale in the UK whose makers say they suit a room that size, cheapest first.
How to work out what size sauna heater you need
Heater makers rate each heater for a range of room sizes in cubic metres. The quick way to find yours is HUUM’s rule of thumb: about 1 kW of heater for every “calculated” cubic metre of room.
- Measure the hot room inside. Use the inside length, width and height of the room you’ll sit in, not the outside of the building and not the changing area.
- Work out the volume. Length × width × height. A 2 m × 1.8 m × 2 m room is 7.2 m³.
- Add the surfaces that lose heat. For every square metre of glass, or of bare log wall that isn’t insulated, add 1 m³. A typical glass door (about 1.8 m × 0.7 m) adds about 1.2 m³; a full glass front can add 4 m³ or more.
- Round to a heater. The total is roughly the kW you need. Then check it against the room range the heater’s maker states.
HUUM’s own example: a 2 m × 1.8 m × 2.2 m room is 7.92 m³. With a 1.8 m × 2.2 m glass wall (3.96 m²), the calculated volume is 11.88 m³, so roughly a 12 kW heater. The same room without the glass would need about 8 kW. Glass makes that much difference.
Rule of thumb and worked example: HUUM: Sauna Heater Size Calculator. Checked 30 September 2026.
Sizing a heater for a barrel sauna
A barrel is a cylinder, so its volume is π × radius² × length, using the inside diameter. Choose “Barrel (round)” in the calculator and enter the inside length and diameter. A barrel 2 m long inside with a 1.8 m inside diameter holds about 5.1 m³, noticeably less than a square room of the same length, which is why barrels often need a smaller heater. Add the glass if it has a glass end.
Most barrels in our data come with, or are designed for, a 6 kW to 9 kW electric heater, or a wood stove.
What size heater for 2, 4 and 6-seat saunas?
These are the room sizes heater makers state for each output, from the 39 electric heater listings in our data that give one:
| Heater output | Room size stated for heaters of this output |
|---|---|
| 4.5 kW | HUUM 3–7 m³; Harvia 3–6 m³ |
| 6 kW | HUUM 5–10 m³; HUUM 5–9 m³; Harvia 5–8 m³ |
| 6.8 kW | Harvia 6–10 m³ |
| 7 kW | Harvia 5–10 m³ |
| 8 kW | Harvia 7–12 m³ |
| 8.5 kW | HUUM 8–13 m³ |
| 9 kW | HUUM 8–13 m³; Harvia 8–14 m³ |
| 10.5 kW | HUUM 10–16 m³ |
| 11 kW | Harvia 9–18 m³ |
| 12 kW | HUUM 12–18 m³ |
| 15 kW | HUUM 15–23 m³ |
| 18 kW | HUUM 18–28 m³ |
As a rough guide for garden saunas:
- Two-seat barrels and small cabins (3 m³ to 6 m³): 4.5 kW.
- Four-seat barrels and cubes (5 m³ to 9 m³): 6 kW to 8 kW.
- Six-seat saunas (8 m³ to 14 m³): 9 kW, more with a glass front.
- Big cabins and glass-fronted rooms: 10.5 kW and up.
Can a sauna heater be too big?
Yes. HUUM’s own advice is that a more powerful heater doesn’t necessarily give better steam. An oversized heater tends to heat the air quickly and then switch off often, and it costs more to buy and to wire in: anything drawing more than a 13 A socket supplies (about 3 kW) needs its own circuit, and the biggest heaters are three-phase only. Use our electrical supply calculator to see the current your heater would draw.
A heater that’s too small is the worse mistake: it runs flat out, takes longer to heat the room and may never reach a proper sauna temperature. If you’re between two sizes, the maker’s stated range decides.
Sizing a wood-burning stove
Wood stoves are rated by room size too. Choose “Wood-burning” in the calculator to list the stoves whose stated range includes your room. Wood stoves usually cover a wider range than electric heaters, and the smoke control rules matter more than the size: check them before you buy.
What a bigger heater costs to run
Each extra kilowatt adds to the bill while the heater is on. At the current price cap, a 6 kW heater costs about £1.97 a session and a 9 kW about £2.96, on the assumptions in our running cost calculator.
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