Sauna Heater Size Calculator

Sauna heater size calculator

Enter the inside size of your sauna and what it is built from. You'll get the heater power that suits the room, the electrical supply it needs, and a quick check for wood-burning stoves.

Measure in

Shape and inside size

Measure inside the finished walls, from floor to ceiling.

Walls

Solid timber stores heat and takes longer to warm up, so it needs more power.

Door

Glass doesn't insulate, so a glass door lets more heat out than a wooden one.

Windows

Enter the glass size of each window.

Stone and tile

Bare stone, tile or concrete on the walls, for example behind the heater. Leave at 0 for an all-wood interior.

Where it stands

A sauna outdoors in a cold winter loses more heat through its walls and roof.

Heater type

8kW

Heater sizes in kW. The shaded band shows every size that suits your room.

Volume to heat

Plan the whole saunaOur sauna plans come with full drawings, a technical description and a bill of materials, heater included.

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How the calculation works

Heater makers size their heaters by room volume, roughly 1 kW for every cubic metre (about 35 ft³). A bare volume figure isn't the whole story though. The materials around the room decide how fast heat leaves it, so the calculator turns your room into a heating volume first and sizes the heater from that.

Glass, stone and tile

These surfaces don't insulate and soak up heat. Following the rule used by Harvia, HUUM and other heater makers, each square metre of them adds 1.2 m³ to the heating volume. A standard glass door adds about 1.6 m³, which often means one heater size up.

Solid timber walls

Log and barrel walls have no insulation and a lot of thermal mass. They take longer to warm up, so the room volume is multiplied by 1.5.

Choosing between sizes

Each heater suits a range of rooms. We recommend the smallest size that keeps your room out of the top of its range. A slightly larger heater warms up faster and runs less hard. A heater that's too big for a small room overheats it and makes the steam harsh. Always check the room volume range in the manual of the model you pick.

Electrical supply

In North America the breaker is sized at 125% of the heater's current, because a sauna heater counts as a continuous load. Wire gauges shown are for copper at the 60 °C rating, which is the conservative default. Your electrician confirms the final sizing against the heater manual and local code.

Questions

What size heater do I need for a 6 × 7 ft sauna?

A 6 × 7 ft sauna with a 7 ft ceiling holds 294 ft³. With insulated walls and a glass door, a 9 kW heater suits it best. With a wooden door, 8 kW is enough. Solid log walls with a glass door push it to 12 kW.

Is a bigger heater better?

Up to a point. One size above the minimum warms the room faster and lasts longer. Going well above the range makes the room overheat before the stones are hot, and the steam turns sharp and short.

Can I run a sauna heater on 120 V?

Only very small heaters, usually under 2 kW, made for one-person rooms. Almost every home sauna heater needs a dedicated 240 V circuit.

How do I size a wood-burning stove?

Wood stoves are rated by room volume rather than kW, because their output varies with how hard you fire them. Use the heating volume from this calculator and choose a stove where it sits in the middle of the rated range.

Does ceiling height matter?

Yes. Every extra inch of ceiling is volume the heater has to warm, and the heat collects above the top bench where nobody sits. Around 7 ft (2.1 m) is a good inside height for most home saunas.