Sizing a sauna heater: kW, amps and when you need three-phase

The question that stalls most sauna purchases is not which model. It is whether the house can run it.

The answer comes from one number, the volume of air in the room, and everything else follows. Here is the calculation, what changes it, and what your licensed electrician needs from you.

Start with volume, not floor area

A heater is sized on the cubic metres of air it has to warm, not the floor area. A 2 × 2 m room with a 2.1 m ceiling holds 8.4 m³. The same footprint with a 2.4 m ceiling holds 9.6 m³ and wants a bigger heater.

The industry states the base rule plainly, and the major heater makers agree on it: one kilowatt of heater power for each cubic metre of sauna volume. An 8 m³ room wants roughly an 8 kW heater.

The measurement is taken inside the lining, floor to ceiling, not from the external footprint on the spec sheet.

Add for glass

The rule assumes a timber sauna room. Glass is the surface that changes the number, and almost every sauna has at least a glazed door.

Surface	Add to your volume
Glass	1.2 m³ per m²

A full-glass door of 1.8 m² adds about 2.2 m³ to the calculation. On a small sauna that is a quarter of the total volume, from one door. A glazed front wall can add more than the room itself.

A published room volume is geometry. It is the air in the room, and nothing about it tells you whether the glazing was allowed for. So do not assume it was. Whoever pairs a heater with a sauna should be accounting for the glass doors and glass walls that sauna carries, and that is a fair question to put to whoever is selling you the pair. We size ours on the room including its glazing.

Where a room is being calculated from scratch, or more glass is being specified than the model normally carries, the correction above should be applied.

A barrel is not a box

Width x depth x height gives you the box around a barrel, not the barrel. A cylinder fills 78.5 per cent of that box whatever its diameter, so the box method over-states a barrel by a little over a quarter, every time, and sends you up a heater size for nothing.

So use π r² times the steam room length, on inner dimensions. Or measure the box and multiply by 0.785. Our own range:

Model	Inner diameter and room length	Sauna volume
Barrel Sauna Anita S	1.9 m, 1.4 m long	about 4 m³
Barrel Sauna Anita M	2.1 m, 1.45 m long	about 5 m³
Barrel Sauna Anita L	2.1 m, 1.88 m long	about 6.5 m³

Rounding to the nearest cubic metre is enough. Heaters are sold in bands three to six cubic metres wide, so half a cubic metre either way lands in the same band and selects the same heater. Precision past that changes nothing that can be acted on.

Two things to watch. Listings quote outer dimensions, so take the wall thickness off before you calculate. And the figure you want is the steam room, not the whole barrel: a model with a changing room or a terrace has a shorter sauna room than its overall length suggests, and the heater is sized on the steam room alone.

How heaters are actually rated

Here is the part that resolves the confusion, and most sizing guides never mention it.

Manufacturers do not rate a heater at a single cubic-metre figure. They rate it for a band of room volumes, and matching your room to the band is the actual selection method. Our own range:

Heater	Output	Rated for	Supply	Stones
Narvi Minex	3.6 kW	2 to 4 m³	230 V single phase	15 kg, included
Narvi NM 600	6 kW	5 to 8 m³	400 V three-phase or 230 V single phase	30 kg, included
HUUM Drop	6 kW	5 to 9 m³	400 V three-phase or 230 V single phase	55 kg, included
Narvi NM 900	9 kW	8 to 14 m³	400 V three-phase	30 kg, included
Harvia M3	16.5 kW	6 to 13 m³	Wood-fired, no electrical supply	30 kg, not included

A 6 kW heater covers up to 8 or 9 m³. That works out at around 0.7 kW per cubic metre at the top of the band and 1.2 at the bottom, which is why the 1 kW/m³ rule of thumb sits in the middle and reads as conservative at the top end.

So the 6.5 m³ Anita L takes a 6 kW heater, comfortably inside the band. The box method would have put that barrel at 8.3 m³, pushed you to 9 kW, and taken you to a much heavier circuit for no benefit.

Where the manufacturer's band is available it is the figure to work from. The 1 kW/m³ rule is the fallback, for a custom build or a conversion where no band has been published.

Every heater in our sauna heaters range lists the band it is rated for, so once you have the volume the shortlist is short.

One check before you act on your own arithmetic. If the number you calculate lands somewhere the market does not sell, the calculation is usually wrong rather than the market. Sauna makers size thousands of these rooms a year.

Wood and electric are not comparable on kW

A wood-burning stove's kilowatt rating is not the same measure as an electric heater's, and comparing them directly is meaningless.

Our Harvia M3 is rated 16.5 kW and suits rooms of 6 to 13 m³. Our Narvi NM 900 is rated 9 kW and suits 8 to 14 m³. The wood stove carries nearly twice the number for roughly the same room.

A wood fire's output swings through the burn, so the stove is rated at its peak while the room sees an average. Between fuels, the room-volume band is the comparison that holds. The kW figure is not.

Single phase or three phase, before you choose a sauna

Most Australian houses are single-phase, and buyers routinely assume that rules out a proper sauna heater. It does not.

The installation manuals for the heaters we stock publish both connections model by model, and the pattern is worth knowing before you settle on a sauna size.

Heater	Room volume	Single phase, 230 V	Three phase, 400 V
3.6 kW	2 to 4 m³	1 x 16 A	Not offered
4.5 kW	3 to 6 m³	1 x 20 A	3 x 10 A
6 kW	5 to 8 m³	1 x 35 A	3 x 10 A
9 kW	8 to 14 m³	Not specified	3 x 16 A

The 3.6 kW is the Narvi Minex, the 6 kW is the Narvi NM 600, and the 9 kW is the Narvi NM 900.

Three things follow.

The small heater asks for an ordinary circuit. The Minex runs on 230 V single phase behind a 16 A fuse. That is a normal dedicated domestic circuit, the same order as an oven point. For a room of 2 to 4 m³, which covers a compact cabin sauna or a short barrel, the electrical question is close to a non-question.

Single phase works up to 6 kW, which covers rooms to 8 m³. That takes in every barrel we sell and most cabin saunas, so a single-phase house is not shut out of a proper heater.

The 9 kW is a three-phase heater. No single-phase circuit is published for it. If the room you want is over 8 m³, the supply question is settled for you.

The step worth planning around is between our own two single-phase heaters. Sixteen amps for the Minex is ordinary. Thirty-five amps for the NM 600 is a substantial dedicated circuit, and whether a domestic board has room for it is a question about the board rather than about the heater, and it is worth answering before an order goes in rather than after.

Two routes stay open if the board is full, or if the room is over 8 m³.

Adding three-phase. The electricity distributor and a licensed electrician quote it between them. Worth asking about anyway if an EV charger, a workshop or a heat pump is on the list, and it puts the Narvi NM 900 within reach at a comfortable 16 A per phase.

The wall-mounted option. The HUUM Drop covers 5 to 9 m³ on either supply and carries 55 kg of stones, which is nearly twice what the floor-standing units in the same band hold. More stone means a softer heat and a longer hold after the element cuts out.

Wood-fired. The Harvia M3 takes no electrical supply at all. It needs a flue, a chimney and clearances to combustibles instead, which is a different set of questions and a different article.

Which supply the house has, and what spare capacity is left on the board, are worth establishing before the sauna is chosen. One look at the meter box and one call to a licensed electrician settles both.

What that means at the switchboard

Manufacturers publish cable sizes alongside the fuse ratings in the table above. Those are stated for European installation conditions, and a cable that satisfies a Finnish manual is not automatically the right cable here. Size depends on the length of the run, how the cable sits, ambient temperature and voltage drop.

So the fuse ratings are useful as the shape of the job, and no more than that. The circuit is designed and installed by a licensed electrician, working to AS/NZS 3000, and certified afterwards by whoever the state requires.

One warning about cable figures found online. Most sauna material written in English comes from the United States, where cable is specified in American wire gauge and written as "10/2" or "8/2". Australian cable is specified in mm². A figure carried over from a US guide is not a figure to hand a licensed electrician here.

What applies at every size

Four things hold for every electric sauna heater in Australia.

A licensed electrician does the work. Not optional, not DIY, in any state. Connecting a sauna heater yourself is an offence in every Australian jurisdiction, and it voids both the product warranty and, in most cases, the home insurance.

The heater gets a dedicated circuit. It does not share, and it never runs on an extension lead.

RCD protection is required, per AS/NZS 3000:2018.

A compliance certificate is issued when the work is done. Every state requires one, under its own name, and the electrician arranges it. It is worth keeping, for insurance and at sale.

Saunas sit under Section 7 of AS/NZS 3000, the part covering special installations.

Before you get a quote

With these to hand, the conversation takes ten minutes instead of three visits.

• Heater model and kW rating, plus the manufacturer's installation manual
• Whether the supply is single or three-phase, and what spare capacity the board has
• Distance from the switchboard to the sauna, and how the cable would run: through a wall, under the house, buried to a separate building
• Whether the sauna is a room inside the house or a separate building in the garden
• That the sauna arrives as a kit with no wiring in it, so the whole run is new work

That last point changes the job. A detached sauna at the back of the block needs a submain rather than a circuit, and possibly its own sub-board, so ask about both. We cover that separately in our guide to running power to a garden building.

Everything above is the manufacturer's guidance and the standard's general requirements, so you know what you are asking for. The sizing of your circuit, its protection and its cable is your licensed electrician's, for your specific run.