Sauna Tools
Sauna Heater Size Calculator
Not sure what size sauna heater you need? Enter your room dimensions below and we'll calculate the required power in kilowatts (kW), then show you the heaters from our range that fit your sauna — best match first. It takes about ten seconds.
Heater Calculator
Find the heater that fits your sauna
Enter your sauna size and we'll show the heaters that fit — best match first.
Heater Calculator
Find the heater that fits your sauna
Enter your sauna size and we'll show the heaters that fit — best match first.
Enter your room size to see the heaters that fit.
Heaters sized for your sauna
Best fit firstSizing is a guide for an insulated indoor room; final selection should confirm the manufacturer's room range and your electrical supply. For glass-heavy, outdoor or commercial builds we recommend an expert review.
How sauna heater sizing works
Choosing the right heater comes down to one number: the volume of your sauna room. A heater that's too small will run flat-out and never quite get there; one that's too large wastes energy and money. The goal is a heater rated to comfortably heat your room's cubic volume.
1. Room volume
Measure the interior length, width and ceiling height and multiply them together. For a barrel sauna, switch the shape selector to Barrel and enter diameter and length — we compute the true cylinder volume (a barrel is much smaller than the box it fits inside). Following the industry standard used by Harvia, a sauna needs roughly 1 kW of power for every 1 m³ (about 35 ft³) of well-insulated space. A typical 6 × 5 × 7 ft room is about 6 m³ (210 ft³) and needs around 6 kW.
2. Glass and uninsulated surfaces
Glass doors, windows and any uninsulated or heat-storing surfaces (stone, brick, concrete, massive log) lose heat far faster than insulated timber — each square metre behaves like roughly an extra 1.2 m³ of volume. If your build has a large glass front or masonry surfaces, step up to the next heater size, or ask our team.
3. Indoor vs outdoor
This calculator sizes for an insulated indoor room. An outdoor sauna sits in colder ambient air and loses more heat through its shell, so give it roughly 25% more power than the figure shown — or talk to us and we'll size it for your climate.
This tool is a guide for an insulated indoor room. Always confirm the manufacturer's stated room range and your available electrical supply before you buy — and for glass-heavy, outdoor or commercial builds, talk to our team.
Frequently asked questions
How many kW do I need for my sauna?
Multiply your room's length, width and height to get its volume, then allow about 1 kW per 1 m³ (≈35 ft³) — the standard Harvia uses. For example, a 6 × 5 × 7 ft room (≈6 m³ / 210 ft³) needs roughly 6 kW. Use the calculator above to get an exact figure for your room and the heaters that match it.
I have a barrel sauna — how do I size it?
Choose the Barrel shape and enter the diameter and length. A barrel is a cylinder, so its real volume is much smaller than the rectangular box it would fit inside — entering it as a box would oversize the heater. The calculator handles the cylinder maths for you.
Does a glass door change the size of heater I need?
Yes. Glass loses heat much faster than insulated wood, so a large glass door or window raises the power you need — each square metre behaves like about 1.2 m³ of extra volume. For a glass-heavy build, step up to the next heater size or contact us for a tailored recommendation.
Indoor vs outdoor sauna heaters — what's the difference?
An outdoor sauna is surrounded by colder air and loses more heat through its walls, so it needs a more powerful heater than an indoor sauna of the same size — typically around 25% more. This calculator sizes for an insulated indoor room, so add roughly a quarter more power for an outdoor build, or ask us.
How do I measure my sauna's volume?
Measure the interior length, width and ceiling height of the room (not the exterior), then multiply the three figures. You can enter your measurements in feet or metres — use the Feet/Metres switch at the top of the calculator and it converts and sizes accordingly.
Is it better to choose a slightly larger heater?
If you're between two sizes, the larger heater is usually the safer choice: it reaches temperature comfortably without running at maximum all the time, which extends its life. The calculator already shows every heater that fits your room so you can compare options.
Browse heaters by brand
Explore our full range, or jump straight to a brand: