Key facts
- Harvia support rule of thumb: about 1 kW per cubic metre (about 35 cubic feet) of room. Its North American heater tables allow larger rooms per kW for well insulated, paneled rooms, so use the table for your model.
- Glass, stone, tile, concrete or other uninsulated surface: add 1.2 cubic metres per square metre (Harvia). That works out to about 3.9 cubic feet per square foot.
- Log saunas: multiply the volume by 1.5 (Harvia North American manual, section 2.3).
- Choose a heater that puts your room near the middle of its range. The calculated figure is usually the minimum (Harvia support).
- Harvia counts walls as well insulated when they have at least 4 inches of insulation, a foil vapor barrier and 1/2 to 5/8 inch wood paneling (section 2.1).
Step 1: measure the room in cubic feet
Measure inside the finished walls: length times width times height, in feet. A 5 ft by 7 ft room with a 7 ft ceiling is 245 cubic feet. Measure to the finished ceiling, not the framing. Harvia's North American manual sets a 75 inch minimum ceiling height and says a lower ceiling cuts the volume you have to heat. It also recommends keeping about 47 inches or less between the top bench and the ceiling and keeping the ceiling no higher than 7 ft 6 in. Heat rises, so a tall ceiling mostly heats air above your head.
Step 2: add volume for glass and other cold surfaces
Glass and masonry soak up heat that paneled, insulated walls hold in. Harvia's support article handles this by treating each square metre of glass, stone or other uninsulated surface as 1.2 cubic metres of extra volume. Converted to US units, every square foot of glass adds about 3.9 cubic feet.
Harvia's own wood heater example uses a 0.9 by 2.1 m glass door: 1.89 square metres times 1.2 adds 2.27 cubic metres, turning an 11.5 cubic metre room into a 13.77 cubic metre room. That is about a 20 percent jump from one door.
| Glass area | Square feet | Cubic feet to add (about) |
|---|---|---|
| Glass door panel, 20 by 60 in | 8.3 | 33 |
| All-glass door, 24 by 80 in | 13.3 | 52 |
| Glass front, 5 ft wide by 7 ft | 35 | 138 |
| Glass front, 6 ft wide by 7 ft | 42 | 165 |
Our conversion: 1 square foot is 0.0929 square metres, times 1.2, is 0.111 cubic metres, which is 3.94 cubic feet. Count a tiled or stone accent wall the same way.
Step 3: log walls get a 1.5 multiplier
Solid log walls store a lot of heat before the room warms up. Harvia's North American manual says to multiply the room volume by 1.5 for log saunas, and its support article applies the same factor to uninsulated log walls. Barrel saunas are built from solid staves, so read the sizing note from the barrel maker rather than assuming the insulated-room tables apply.
Step 4: compare against the heater's published range
Every heater line publishes a minimum and maximum room size. Here are two we checked, with the electrical draw that comes with each size.
| Harvia Virta (North America) | kW | Room (cu ft) | Amps at 240 V, 1 phase | Copper wire (90 °C) |
|---|---|---|---|---|
| HL6U1 / HLS6U1 | 6.0 | 177 to 294 | 25 | #12 |
| HL7U1 / HLS7U1 | 6.8 | 177 to 354 | 28.25 | #10 |
| HL8U1 / HLS8U1 | 8.0 | 177 to 431 | 33.4 | #8 |
| HL9U1 / HLS9U1 | 9.0 | 177 to 494 | 37.5 | #8 |
| HL11U1 / HLS11U1 | 10.5 | 177 to 635 | 43.75 | #6 |
Source: Harvia Virta North American manual, tables 2 and 3. The manual also lists a 75 inch minimum height and a 28 square foot minimum floor for these models.
Finnleo publishes ranges by heater line: Designer 100 to 425 cubic feet, Himalaya 175 to 500, Laava 390 to 950 and Magma 720 to 2,000. Within the Himalaya line, the installation manual lists the 6.8 kW size for 175 to 350 cubic feet (28.3 A at 240 V, #8 AWG) and the 9.0 kW size for 310 to 500 cubic feet (37.5 A, #6 AWG). Wire sizes differ between brands for similar loads, so follow the manual for the heater you install.
Step 5: aim for the middle of the range
Harvia's support article says to pick a heater that puts your room near the middle of its range and to avoid both ends. Its example: a 9.5 cubic metre room is too close to the top of a 6 to 10 cubic metre heater, so it points to the next size up (8 to 14). It adds that when you replace a wall-mounted heater with a pillar style heater, you should go up about 1 kW and check the fuses and cables. An undersized heater runs longer to reach temperature and wears out sooner. Going bigger than you need mainly costs more in wire and breaker size.
Worked examples
These are illustrations using the Harvia Virta table above. They are not a quote or an installation plan.
- 5 by 7 by 7 ft, paneled walls, solid wood door. 245 cubic feet. The 6.0 kW (177 to 294) fits, with the room close to the middle of its range. Electrical: 25 A, #12 copper per the table.
- The same room with a 24 by 80 in all-glass door. 245 plus about 52 is about 297 cubic feet. That is just past the 6.0 kW maximum of 294, so step up. The 6.8 kW (177 to 354) covers it, and the 8.0 kW (177 to 431, middle about 304) puts it nearest the middle. Electrical rises to 28.25 A on #10 or 33.4 A on #8.
- 6 by 6 by 7 ft with a full 6 ft wide glass front. 252 plus about 165 is about 417 cubic feet. The 8.0 kW tops out at 431, too close to the edge. The 9.0 kW (to 494) is the smallest with headroom, and the 10.5 kW (to 635, middle about 406) is the closest to Harvia's middle-of-range advice. That 10.5 kW size draws 43.75 A on #6 copper.
- 6 by 7 by 7 ft log sauna. 294 cubic feet times 1.5 is 441. Size it as a 441 cubic foot room: the 9.0 kW or the 10.5 kW, not the 6.0 kW the raw volume suggests.
Before you order
- Use the sizing table in the manual for your exact heater. Ranges are model specific and assume the insulation and paneling the manual describes.
- Confirm the panel can carry the larger heater. Going from 6 kW to 10.5 kW moves you from about 25 A to about 44 A in the Harvia table. See what size breaker and wire a sauna heater needs and, if you also run a chiller, running a sauna and a cold plunge on one panel.
- Plan the vents with the heater. See where the intake and exhaust vents go.
- Pull the electrical permit. See our guides for Washington and Oregon and Portland.
Sources
- Harvia Virta heater owner's manual for Canada and the USA (sections 2.1 and 2.3, tables 2 and 3)
- Harvia support: How do I select the correct heater power? (updated May 11, 2026)
- Harvia support: Choosing a wood-burning heater (glass door worked example)
- Finnleo: Sauna heaters and controls (room size ranges by heater line)
- Finnleo Himalaya SL2 heater installation manual (72-0132)
Figures as published by each maker and checked on October 23, 2026. The glass conversion to cubic feet and the worked examples are our own arithmetic. Manuals change, so use the version that ships with your heater. This guide does not replace the heater manual, a licensed electrician or local code.
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- What size breaker and wire does a sauna heater need?
- Where do sauna intake and exhaust vents go?
- Outdoor saunas in rainy climates
- What does a home sauna cost to run?
- The best home saunas of 2026, ranked by use case
Frequently asked questions
How many cubic feet does a 6 kW sauna heater heat?
It depends on the model. In the Harvia Virta manual for North America, the 6.0 kW HL6U1 is rated for 177 to 294 cubic feet in a well insulated, wood paneled room. Finnleo's Himalaya line starts at 175 cubic feet for its smallest size. Always use the table in the manual for the exact heater you are buying.
How much does a glass door change the heater size?
Harvia's sizing guidance treats each square metre of glass, stone or other uninsulated surface as 1.2 cubic metres of extra room volume. In US units that is about 3.9 cubic feet added for every square foot of glass. A 24 by 80 inch all-glass door is about 13.3 square feet, so it adds roughly 52 cubic feet.
Do log saunas need a bigger heater?
Yes. Harvia's North American manual says to multiply the room volume by 1.5 for log saunas, and its support article applies the same 1.5 factor to uninsulated log walls. A 294 cubic foot log room is sized like a 441 cubic foot insulated room.
Should I pick the heater at the top or the bottom of its range?
Neither. Harvia's support article advises choosing a heater whose range puts your room near the middle and avoiding the ends. It also says the calculated figure is usually the minimum. An undersized heater runs longer to reach temperature and wears out sooner.
Does a lower ceiling mean a smaller heater?
It can. The Harvia North American manual notes that lowering the ceiling reduces the volume to heat, down to its 75 inch minimum height. It recommends no more than about 47 inches from the top bench to the ceiling and a ceiling no higher than 7 ft 6 in.
Does a bigger heater need bigger wire?
Yes. In the Harvia Virta North American table, the 6.0 kW model draws 25 A at 240 V and uses #12 copper, while the 10.5 kW model draws 43.75 A and uses #6. See our breaker and wire guide before you order.
Keep reading
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