Why sizing is done on volume rather than floor area
A sauna is a room heated to a large temperature difference with a heater that also has to bring a mass of stones up with it. What the heater fights is the heat leaving through every surface, and for a room of fixed proportions that scales with surface area, which scales with volume to the two thirds power. Over the narrow range of sizes that domestic saunas actually come in, volume is close enough to linear and is far easier to measure, which is why the industry rule of thumb has been stated in kilowatts per unit volume for decades.
The common starting figure is roughly one kilowatt for every 45 cubic feet of insulated, wood-lined room, which is about 22 watts per cubic foot. This calculator takes that as an input because it is a convention rather than a law, and because different heater manufacturers publish charts that differ by ten or fifteen percent either side of it. If you have a chart from the manufacturer of the heater you are considering, work backwards from it: divide its recommended kilowatts by the volume of one of its listed rooms and use that as your watts per cubic foot.
Glass, and why it is treated as extra volume
A glazed panel is a hole in the insulation. An insulated wood-clad sauna wall might be R-13 or better; a sheet of tempered single glazing is under R-1, and even insulated glass rarely clears R-2. At the temperature difference a sauna runs at, that is a very large local loss concentrated in a small area.
Rather than turn the calculation into a full envelope model, the trade handles glass by adding notional volume: for each square foot of glazing, add somewhere between one and two cubic feet to the room. That keeps the arithmetic in the same units and produces a sensible answer, and the multiplier is exposed here as an input so you can be honest about what you have. A small insulated-glass window in a door is at the low end. A full glass wall on a modern glass-fronted sauna is at the high end, and it can easily add a third to the effective volume of a small room.
| Surface | Cu ft added per sq ft | Reason |
|---|---|---|
| Small insulated-glass window | 1.0-1.4 | Modest area, two panes, sealed unit |
| Glass door | 1.5 | Larger area plus perimeter losses at the frame |
| Full single-glazed wall | 2.0+ | Very low R over a very large fraction of the envelope |
| Solid timber or log wall | 1.2 | Wood insulates, but a solid wall has no insulation layer |
| Tile, stone, concrete, masonry | 1.5-2.0 | Poor insulation plus a large thermal mass to warm |
Rounding up, and why not to round far
Heaters come in steps, so the calculated figure has to become a purchasable one. Going up one step is normal. Going up two or three is a mistake people make on the theory that more is safer, and it is not, for two reasons.
An oversized heater in a small room cycles hard against its thermostat, which means the stones and the air swing rather than settling, and the room ends up hot at the ceiling and cool at the bench without the even field a correctly matched heater produces. It also draws more current for the same result, which changes what the supply has to be built for and therefore what the installation costs. The convention in the trade is to match the chart and go up one step at most, and to treat a room that seems to need much more than the chart says as a room with an envelope problem rather than a heater problem.
What this calculator does not decide
It gives an output in kilowatts. It does not choose a heater, and it says nothing at all about the electrical installation that would feed one. Sauna heaters are permanently connected appliances on a dedicated supply, and the conductors, the protective device, the disconnecting means and every clearance and distance around the heater itself come from the instructions supplied with that specific model, from the local code, and from a licensed electrician whose work gets inspected. There is a reason the industry treats it that way, and it is not paperwork. Take the model and its nameplate to an electrician and let them determine the installation. Where you want to add up nameplate figures on an existing circuit for your own understanding, the circuit load calculator does that arithmetic in general terms.
Questions people ask
What size sauna heater do I need for a 6 by 7 room?
A 6 by 7 room with a 7 foot ceiling is 294 cubic feet, which at the common 22 watts per cubic foot comes to about 6.5 kW before any additions. Add a 12 square foot glass door at 1.5 cubic feet per square foot and the effective volume rises to 312, giving about 6.9 kW, which rounds to a 7 kW heater. Change nothing but the glazing to a full glass wall and the answer moves up a step or two. That sensitivity is the point of the calculator: the room dimensions on their own do not settle it.
Why does glass make such a large difference?
Because the temperature difference is enormous and glass has almost no thermal resistance. A sauna at 175 degrees with 40 degree air outside is a 135 degree difference across the envelope. An insulated wall at R-13 passes about 10 BTU per hour per square foot at that difference; single glazing at under R-1 passes more than ten times that. A square foot of glass therefore behaves like ten or more square feet of wall, and on a small room a glass front can be a third of the total loss. Treating each square foot as one and a half cubic feet of extra volume is a shortcut that gets close to the same answer without a full envelope calculation.
Do log or solid timber walls need a bigger heater?
Yes, and the effect is in two parts. A solid timber wall has no insulation layer, so its steady heat loss is higher than a clad wall with insulation behind it even though wood itself insulates reasonably. Separately, solid timber is a large thermal mass that has to be warmed before the room settles, which lengthens the heat-up rather than raising the steady demand. The volume addition here covers the first effect. For the second, the heat-up time calculator handles thermal mass directly.
Is a bigger heater always better?
No. An oversized heater short-cycles against its thermostat, which gives an uneven room rather than a hotter one, and it draws more current for the same result, which raises the cost of the installation feeding it. The usual convention is to match the manufacturer chart and go up at most one step. If the number that comes out looks far larger than the heater sizes available for the room, that is normally a signal about the envelope, most often glazing or missing insulation, rather than an argument for a much larger heater.
Can this tell me what circuit the heater needs?
No, and it will not, because that is licensed work with real consequences. A sauna heater is a permanently connected appliance on a dedicated supply, and the conductors, the protective device, the disconnecting means and the way it is all arranged are determined by the local code, by the instructions supplied with that specific heater, and by an electrician whose installation gets inspected. The current figure on this page is nothing more than kilowatts divided by volts, shown so you can talk to an electrician with a number in hand. Every clearance around the heater comes from the manufacturer instructions in the same way.