The kilowatts are counted on the equivalent volume

Anyone choosing a sauna heater almost always starts from the right rule — one kilowatt per cubic metre — and applies it to the wrong number. Because the cabin is not made of air alone: the full-height glass door, the panoramic wall, bare log and stone steal heat as if the sauna were larger than it is. The Finnish rule has said it forever: every square metre of glass counts as one cubic metre and a fifth more to heat, and uninsulated surfaces add their percentage. This calculator builds the equivalent volume, applies the kilowatt rule to it and picks the size from the commercial series — with the dimensionless metric that compares cabins: the loss index.

The formula: from the true volume to the volume that counts

The model is the manufacturers' rule made explicit: Veq = V·(1 + (f+w)/100) + 1.2·G. The true volume V is increased for uninsulated walls (f) and for the cold around an outdoor sauna (w), and every square metre of glass G adds 1.2 equivalent cubic metres. The required power is P = Veq — one kilowatt per equivalent cubic metre — and the heater is chosen as the first size of the commercial series that covers P: 4.5 / 6 / 8 / 9 / 10.5 / 12 / 15 / 18 kilowatts. The index ι = Veq/V says how much the cabin loses against its volume: the number that compares two saunas better than any price list.

The volume scale: from the two-seater to the family sauna

Glass 0.5 square metres, walls at 10 percent, indoor sauna; only the volume grows:

The family row is a perfect draw: 10.5 required, 10.5 on the nameplate — one hundred percent utilisation, the exact heater with no margin. And from 9 cubic metres upward every row lights the same signal: the power exceeds 9 kilowatts and the heater asks for three-phase — the electrical count enters the project before the cabin does.

A complete numerical example

The typical home sauna: 6 cubic metres — two metres by two, with benches and breathing room — a glass door of 0.5 square metres of light and 10 percent of uninsulated surfaces. The equivalent volume is 6 × 1.10 + 1.2 × 0.5 = 6.6 + 0.6 = 7.2 cubic metres; the required power 7.2 kilowatts; the right size is the 8 kW of the series, used at 90 percent — the healthy margin that heats in three quarters of an hour without ever starving the stones. The loss index is 1.20: the cabin asks a fifth more than its volume — the physiology of real glass and wood.

How to read the loss index

The four bands of the index tell which cabin you are heating. Below 1.10 is the thermal cabin: insulated, little glass, the equivalent volume nearly matches the true one. Between 1.10 and 1.30 the well-insulated sauna: the physiology of the home cabin with its glass door. Between 1.30 and 1.55 the glass asks for kilowatts: glazed walls and bare surfaces make the heater work for a cabin that is not there. From 1.55 upward it is more showcase than sauna: the landscape is paid in kilowatts — legitimate, but it must be known beforehand.

The glass: the square metre worth a cubic metre and a fifth

The variable G is the most underrated toll of the project. The tempered glass of doors and panoramic walls does not hold radiant heat: every square metre loses like 1.2 extra cubic metres of air, and a three-square-metre glazed wall adds to the heater the load of a whole small cabin. The countermoves exist — low-emissivity glass, a narrower door, glazing stopped at half height — and each one removes equivalent volume before adding kilowatts. The practical rule: first the glass is drawn, then the heater is chosen — never the other way round.

The stones: where the löyly is decided

Correct sizing serves one purpose: scorching stones, not just hot air. The undersized heater never reaches temperature; but the oversized heater is the most common and most misunderstood mistake: it brings the air to ninety degrees quickly, the thermostat cuts out, and the stones — the mass that must make the steam — stay lukewarm. The result is the sauna that "heats but does not embrace": poor löyly, harsh steam, early fatigue. That is why the calculator recommends the first size that covers the need, not one more: utilisation between 85 and 100 percent is the territory where the stones work and the steam is soft.

International perspective: the same rule from Helsinki to Denver

The rule of one kilowatt per equivalent cubic metre is the lingua franca of heater manufacturers: it holds for the Finnish sauna of Helsinki, the German garden cabin and the American barrel sauna. What changes are the grids: Europe switches to three-phase 400 V above 9 kilowatts, North America reasons in 240 V with dedicated circuits and amps to count; everywhere the big heater calls its electrician before its carpenter. And the loss index is dimensionless: 1.20 tells the same cabin in Helsinki, Munich or Denver — the metric that travels without a grid.

Model limits and correct use

This is the pre-sizing of the electric heater for a domestic dry sauna: it does not cover wood-burning stoves — chosen by stone mass more than kilowatts — nor steam baths, which are another system, nor commercial saunas beyond 18 kilowatts with their dedicated designs. The volume is measured on the real inner cabin and the air change remains mandatory: a sauna breathes, and the air change calculator says how much air must move in the room hosting it too. And before ordering, the home electrical load calculator checks that meter and wiring carry the heater's kilowatts — because the perfect sauna with the wrong connection is just a very expensive wooden room.

The glass scale: the toll metre by metre

A 6-cubic-metre sauna, walls at 10 percent; only the glass area grows:

It is the table with three exact figures in a row: the wooden box touches the 1.10 border — the doorway of physiology — the full door touches 1.30, the frontier where glass starts asking for kilowatts, and at two square metres the power is exactly 9.0: the first millimetre of three-phase territory, with the 9 kW heater used at one hundred percent. From there on, every square metre of glass is a size step up: the panoramic wall is paid in heater steps, not in metres of price list.

The wall scale: the insulation that works for free

The same 6-cubic-metre sauna with its half-square-metre door; only the wall surcharge grows:

The reading is in the size column: the first three rows mount the same 8 kW heater — insulation absorbs the first twenty points without changing machine — then full log asks for the 9 and stone for the 10.5 with three-phase. It is the silent lever of the project: insulated panels behind the cladding cost once, the kilowatts of a bare wall cost at every switch-on — and unlike glass, nobody is watching the view here.

The exact borders of the bands

The model's frontiers can be touched by hand. The 6-cubic-metre cabin with a full door, a quarter of cold surfaces and the outdoor toll: equivalent volume 9.3 — index exactly 1.55, the doorway of the showcase, with three-phase already lit and the 10.5 at 88.6 percent. The same all-wood cabin but with half its surfaces uninsulated: power of exactly 9.0 kilowatts — the first millimetre of three-phase, index 1.50 and the 9 kW heater at a round hundred percent. And five points less of surcharge are enough to step back: 8.7 kilowatts, index 1.45, the same 9 kW heater at 96.7 — half a day of laying mineral wool is worth the border between single and three-phase.

Three-phase: the electrical border of the sauna

Nine kilowatts are the border dividing two different projects. Below, the heater lives on single-phase with its dedicated circuit — right section, its own breaker, no surprises. Above, in Europe, it takes 400 V three-phase: a dedicated line from the panel, sized cable, and a meter that often needs upgrading — paperwork and lead times that people love discovering after ordering the cabin, when it is late. The right sequence is the reverse: first the count, then the electrician, then the order. And in North America the same threshold speaks another language — 240 volts, amps and dedicated breakers — but tells the same story: a big heater is an electrical project before it is a wooden furniture piece.

Electric or wood-fired: two trades, one requirement

The equivalent volume holds for both, but from there the roads part. The electric is bought in series sizes, regulates to the degree, starts on a timer and lives on its utilisation bands: the everyday sauna, switched on like a lamp. The wood stove is chosen by stone mass and firebox rating, asks for a compliant flue and its flame times: the ritual sauna, where heating is part of the rite. The requirement computed here remains the compass for both — but wood adds the local flue rules, and no calculator replaces the chimney sweep.

The mistakes of the DIY project

The first mistake is applying the kilowatt rule to the bare volume: the glass door and the log do not forgive, and the sauna stays lukewarm forever. The second is the bigger heater "for safety": scorching air, lukewarm stones, poor löyly — oversizing is a defect, not a reserve. The third is drawing the glass after buying the heater: every added square metre moves the requirement by more than a kilowatt, and yesterday's size does not cover today's wall. The fourth is ignoring the air change: a sealed sauna makes heavy steam and suffering wood. And the fifth is discovering three-phase with the cabin already ordered: the 9-kilowatt border is read on the calculator, not on the bill of the delay.

Frequently asked questions

How many kilowatts does a sauna need?

One kilowatt per equivalent cubic metre: the true volume increased for uninsulated walls and outdoors, plus 1.2 cubic metres for every square metre of glass. The 6-cubic-metre home sauna with a glass door asks 7.2 kilowatts: the right size is the 8 kW heater.

What is the equivalent volume?

The volume the heater "feels": the true one plus the tolls of the surfaces that lose heat. Veq = V·(1+(f+w)/100) + 1.2·G. It is the number the kilowatt rule applies to — using the bare volume is the mistake behind half of all lukewarm saunas.

What is the loss index?

The ratio between equivalent and true volume: a dimensionless metric that compares cabins. Below 1.10 the thermal cabin; between 1.10 and 1.30 the physiology; between 1.30 and 1.55 the glass asks for kilowatts; above 1.55 more showcase than sauna — the landscape is paid in current.

Why does glass weigh so much?

Because it does not hold radiant heat: every square metre loses like 1.2 extra cubic metres of air. A three-square-metre panoramic wall loads the heater like a whole small cabin: low-emissivity glass and a narrower door are kilowatts saved forever.

Is a bigger heater not safer?

No: it is the most common mistake. The oversized heater brings the air to temperature quickly, the thermostat cuts out and the stones stay lukewarm: poor löyly and harsh steam. The right size is the first that covers the need, with utilisation between 85 and 100 percent.

When is three-phase needed?

From 9 kilowatts upward, in Europe, the heater asks for 400 V three-phase with a dedicated line; below, single-phase models with their own circuit suffice. North America reasons in 240 V and amps. The rule holds everywhere: the electrician is called before the cabin is ordered.

What wall surcharge should I use?

Zero for a well-insulated prefabricated cabin; 10-20 percent for solid wood or some bare surfaces; 30-50 for full log, stone or uninsulated outer walls. It is the f input of the calculator — and added insulation pays back every winter.

And for a garden sauna?

The outdoor surcharge (10-30 percent) counts the cold around the cabin: a barrel or garden house loses towards winter air, not towards a room. Together with the walls it can move the size by one or two steps: an outdoor sauna is sized for winter.

How long does the sauna take to reach temperature?

With the right size and healthy utilisation, 30-50 minutes from cold, stones included. Much longer times tell of an undersized heater or underrated losses; very short times with poor steam tell of a heater too big, warming the air and not the stones.

Does it work for wood-burning stoves?

The equivalent volume yes, the series no: wood stoves are chosen by stone mass and firebox rating, with the local flue rules. The calculator covers domestic electric 4.5-18 kilowatts: for wood, use the requirement as a reference and the maker's specification.

Does a sauna need an air change?

Yes, always: air enters under the heater and leaves under the opposite bench, several times an hour. Without it the steam turns heavy and the cabin unhealthy. The air change is part of the thermal design: the heater warms the incoming air too.

What does the calculation NOT cover?

Wood and gas stoves, steam baths, commercial saunas beyond 18 kilowatts, controls and accessories, local rules on flues and wiring. It is the pre-sizing of the domestic electric heater: confirmation belongs to the installer on the real cabin.

Updated July 2026