Buying
Sauna heater size guide: what kW do you need?
Roughly 1kW per cubic metre, then adjustments for glass, log walls and outdoor placement. Undersizing is the mistake you pay for every session.
Most indoor cabins and self-build kits are sold without a heater, which leaves buyers picking a kW rating from a dropdown with no idea what it means. Get it wrong low and the sauna never quite reaches temperature and costs more to run. Get it wrong high and you trip a breaker or pay for a bigger circuit than you needed.
The rule: 1kW per cubic metre
Heaters are rated against the cabin's volume in cubic metres, not its floor area. Multiply length by width by height, then allow roughly 1kW per cubic metre as a starting point. A 2m × 2m cabin at 2.1m high is 8.4 cubic metres, which points at an 8kW heater or the nearest size up.
| Cabin volume | Typical heater | Circuit | Suits |
|---|---|---|---|
| Up to 4.5 m³ | 3-4.5kW | 16-20A | 1-2 person indoor |
| 4.5-6 m³ | 6kW | 25-32A | 2-3 person indoor |
| 6-9 m³ | 8kW | 32A | 3-5 person, small outdoor |
| 9-13 m³ | 9kW | 32A+ | Large outdoor cabins |
| 13 m³ and above | 10.5kW+ | Per design | Commercial or very large |
Every manufacturer publishes a volume range for each model, and that range is the number that actually governs. Harvia, which makes more sauna heaters than anyone else, lists a minimum and maximum cubic metre figure against every heater it sells. Use the maker's range for the model you are buying rather than the rule of thumb, and treat the rule as a sanity check.
The adjustments that matter
The base figure assumes an insulated timber cabin indoors. Four things push the requirement up, and each one is worth roughly the same correction.
- Glass: add about 1.5 cubic metres of equivalent volume for every square metre of glass, since glass loses heat far faster than clad timber
- Uninsulated log walls: add roughly 1.2 cubic metres per square metre of log wall on older or budget kit builds
- Outdoors: an exposed cabin in a British winter behaves like a larger room, so size up rather than down
- Stone or tile inside: masonry surfaces absorb heat during warm-up and need extra output to bring up with the air
That is why a glass-fronted cabin with the same floor plan as a solid one takes a bigger heater. It is also why the same model needs more output in a garden than in a spare bedroom, which is one of the running-cost differences behind what a sauna costs per session.
Why undersizing is the expensive mistake
An undersized heater does not simply run cooler. It runs at full power for far longer trying to reach a setpoint it may never hit, so it uses more electricity per session than a correctly sized heater that reaches temperature and then cycles. At Ofgem's current cap rate that difference compounds over years. You pay twice: once in the session you did not enjoy, and again on the bill.
Oversizing has smaller consequences but real ones. A heater beyond the cabin's range heats the air fast while leaving the stones cold, which produces a dry, harsh room and weak löyly, and it may demand a heavier circuit than your consumer unit can spare.
Stones are part of the sizing
Heater capacity is quoted with a stone load, and that load is what stores the heat you throw water onto. A heater filled to its rated stone capacity gives a fuller, softer heat and better löyly than the same unit half filled. Underfilling to save money on stones defeats the point of buying a traditional heater at all.
Stones are consumables. They crack and crumble with thermal cycling, and a set lasts a few years before it needs replacing at £40 to £80. Repack them loosely rather than tightly, because airflow through the stone bed is what stops the elements overheating.
Check the circuit before you order
Heater output decides the circuit, and the circuit decides whether the job is straightforward or expensive. Anything from 3kW upward wants its own dedicated supply, and by 6kW that is typically a 32A circuit run from the consumer unit by a registered electrician. If the board has no spare way, that adds several hundred pounds before the heater is even connected.
Have the electrician look at the consumer unit before you order the heater, because the answer occasionally changes which heater you should buy. A new circuit is notifiable under Approved Document P, so the circuit has to be certified whichever output you land on.
If the cabin itself is not settled yet, size the room first and the heater second: how many people a sauna actually seats is the decision that sets the volume, and whether you burn wood or run a circuit changes what sizing even means. Both are steps in choosing a home sauna, and the shells on our indoor sauna rankings all publish the volume you need to size against.
Frequently asked questions
What size sauna heater do I need?
Allow roughly 1kW per cubic metre of cabin volume, then check that figure against the manufacturer's published range for the model. A 2m × 2m × 2.1m cabin is 8.4 cubic metres, which points at an 8kW heater.
How do I calculate sauna volume?
Multiply internal length by width by height in metres. Use internal dimensions rather than external ones, because the walls are not the room. Then add equivalent volume for glass and uninsulated log walls.
What happens if the heater is too small?
It runs at full power continuously and may never reach the setpoint, which costs more electricity per session than a correctly sized heater that reaches temperature and cycles. Undersizing is the more expensive mistake.
Can a heater be too big?
Yes. A heater beyond the cabin's rated range heats the air quickly while leaving the stones cold, giving a harsh dry room and weak steam. It may also need a heavier circuit than your consumer unit can supply.
Does glass change the heater size?
Significantly. Allow about 1.5 cubic metres of equivalent volume per square metre of glass, because glass loses heat much faster than clad timber. A glass-fronted cabin needs more output than a solid one of the same size.
What size heater does a 2-person sauna need?
Usually 4.5kW to 6kW for a typical indoor two-person cabin of around 4.5 to 6 cubic metres. That sits on a 20A to 32A dedicated circuit depending on the exact output.
How many stones does a sauna heater need?
Fill it to the manufacturer's rated capacity, packed loosely rather than tightly so air can move through the bed. Underfilling gives thin heat and poor löyly, and tight packing makes the elements run hot.
How often should sauna stones be replaced?
Every few years for regular use, at £40 to £80 a set. Thermal cycling cracks them, and crumbled stones restrict airflow through the heater, which shortens element life.
Do outdoor saunas need bigger heaters?
Yes, size up rather than down. An exposed cabin loses heat to wind and ground in a way an indoor one does not, so the same volume behaves like a larger room in winter.
Is a bigger heater faster?
Within the cabin's rated range, yes, a higher output reaches temperature sooner. Beyond that range the gain disappears because the stones cannot absorb heat fast enough, and you get hot air rather than a properly heated room.