What size sauna heater do I need? The kW sizing guide

Sauna heaters are sized in kilowatts against cubic metres, and getting it right matters more than brand: an undersized heater never quite gets there on a January evening, and an oversized one gives you harsh air and scorched stones instead of proper löyly. The maths takes two minutes.

The rule of thumb — then the corrections

Start at 1 kW per m³ of internal volume for a well-insulated cabin. Then correct for the ways real saunas leak heat:

• Glass: for every 1 m² of glass (door, panoramic front), size as if the sauna were ~1.5 m³ bigger. Glass-fronted pods climb a full heater size quickly.
• Uninsulated or thin-walled cabins — barrels above all: treat as one class worse; pick from the top of the maker’s volume band. Why insulation earns its money twice is covered in sauna insulation.
• Log or solid-timber walls: timber soaks up heat before the air warms — same upward correction.
• Outdoor siting in the UK: exposed, windy positions in winter justify rounding up — where you put it is part of the thermal spec.

What that gives you in practice

Sauna	Internal volume	Heater
Compact 1–2 person cabin	3–4.5 m³	3.5–4.5 kW
Typical 4-person insulated cabin	6–8 m³	6–8 kW
4-person barrel (42mm staves)	5–7 m³	8–9 kW
6-person / glass-fronted	9–12 m³	9 kW+ — check three-phase

Always finish against the manufacturer’s stated volume range for the specific model — the stone mass and thermostat are matched to it. Within that range, choose the larger heater; it cruises where a smaller one strains, heats faster, and recovers instantly when you throw water. (Temperature targets explains what you’re sizing for.)

The supply question — where sizing meets the electrician

Every real sauna heater needs a dedicated circuit; none of this plugs in. Approximate draws at UK 230V single-phase:

Heater	Current	Meaning
4.5 kW	~20 A	modest dedicated radial
6 kW	~26 A	the common 4-person spec
8 kW	~35 A	heavier cable, check consumer-unit headroom
9 kW	~39 A	practical single-phase ceiling
10.5 kW+	—	usually three-phase — rare in UK homes

If the arithmetic points past 9 kW, the cheap fix is almost always more insulation, less glass or a lower ceiling, not a supply upgrade. The full wiring picture — cable runs, RCDs, Part P — is in electrical requirements, and what the electrician charges is in sauna installation cost.

Wood-fired? Stove ratings follow the same volume logic with the same barrel corrections — manufacturers state m³ ranges — plus flue and clearance rules: wood-fired vs electric.

Sizing errors we keep seeing

• Sizing to the room you wish you had — measure internal volume, not the footprint.
• Ignoring the door — a glass door alone is ~1.5 m³ of phantom volume.
• “Bigger is faster” beyond the band — past the maker’s range you get hostile heat, not speed.
• Forgetting the stones — more stone mass means softer heat and better steam; cheap heaters skimp exactly there.

Heater size drives both the purchase price and the running cost — the sauna cost calculator folds your size, insulation and usage into one realistic number, and running costs shows what each kW actually costs per session.