Sauna heater size calculator
Updated: 21 October 2026
You have a tape measure and a glass door, and the heater title only offers a kilowatt number. A sauna heater size calculator is the sum that turns the finished inside of the cabin into a figure you can hold against that listing’s own band. This page uses Harvia’s published method as the rule of thumb, then checks it only against the bands printed on the listings in our [sauna heater comparison](/hub/steam-baths/sauna-heater). The Products Compared editorial team did not survey your cabin. A qualifying purchase through our links can earn a commission at no extra cost to you. If the listing prints no band, the sum cannot rescue it.
You have a tape measure and a glass door, and the heater title only offers a kilowatt number.
You have a tape measure and a glass door, and the heater title only offers a kilowatt number. A sauna heater size calculator is the sum that turns the finished inside of the cabin into a figure you can hold against that listing’s own band. This page uses Harvia’s published method as the rule of thumb, then checks it only against the bands printed on the listings in our sauna heater comparison. The Products Compared editorial team did not survey your cabin. A qualifying purchase through our links can earn a commission at no extra cost to you. If the listing prints no band, the sum cannot rescue it.
Our top picks
| Badge | Product | Claimed space | Stated output | Heater type | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Mxmoonant Mxmoonant 6 kW digital sauna heater | 5–9 m³ (170–300 ft³) | 6 kW | Electric stone stove | 8.2 | about £297.00 | View offer |
| Alternative |  CGOLDENWALL CGOLDENWALL 4.5 kW steam generator | 3–4.5 m³ | 4.5 kW | Steam generator | 6.7 | about £426.54 | View offer |
| Best value |  Mxmoonant Mxmoonant 6 kW sauna heater | 5–9 m³ (170–300 ft³) | 6 kW | Electric stone stove | 7.6 | about £269.00 | View offer |
On this page
Measure the finished room
Use the interior after the lining, not the garden shed’s outer shell and not the timber kit’s external brochure size. Length times width times height, in metres, is the cubic metres. A cabin that is 2 m long, 2 m wide, and 2 m high inside the lining is 8 m³. A lower ceiling changes the sum quickly: 2 m by 2 m by 1.9 m is 7.6 m³. Write the three measurements down before you look at a 6 kW title.
Benches, the heater itself, and a duckboard do not get subtracted in Harvia’s method. Infrared cabins, steam rooms, and wood stoves are outside this sum. If you only have external measurements, stop and measure inside. A 10 cm lining on each wall is enough to invent a room the heater was never sized for.
Add the glass, then stop at the listing’s band
Harvia’s guide adds about 1.2 m³ for each square metre of glass or bare masonry, on top of the 1 kW per cubic metre starting point. A smaller light, 1 m² of glass, adds 1.2 m³ and turns 8 m³ into 9.2 m³. Finnmark’s buyer guide uses about 1.5 m³ per square metre of single glazing. That is a different maker’s factor. Do not correct Harvia’s 1.2 with Finnmark’s 1.5.
Harvia then asks you to sit in the middle of that heater’s own range. Their worked case is a different model: a calculated volume around 8.5 m³ still points at a heater whose comfortable band is about 7–9 m³, not at the top of a model sold as 6–10 m³. The printed range on the listing you are buying wins. If the listing has no band, you cannot apply the test. The Anjur, the Caiqinlen, the Alvinlite, and the WILLOWYBE print no electric room band in the file.
What 5–9 m³ means on these pages
Two listings print a dry-cabin band. The Mxmoonant WS-37-WJY at £297.00 and the WS-15-WJY at £269.00 both state 6 kW for 5–9 m³, also written as 170–300 ft³. The check is whether your adjusted volume sits inside 5–9 m³ and not on the 9 m³ ceiling.
Take the 2 m by 2 m by 2 m cabin, 8 m³, plus a 1 m² glass door. Harvia’s add-on makes 9.2 m³. Both of these listings stop at 9 m³. Harvia’s own middle-of-range rule would not park 9.2 m³ on the top of a 5–9 m³ heater. A cabin of 1.8 m by 1.8 m by 2.0 m is 6.48 m³ before glass. One square metre of glass makes about 7.7 m³, which sits inside 5–9 m³ with room above and below. That is the shape of a pass. It is not a recommendation to buy either Mxmoonant, because the supply, the missing plug, and the stones are still open. Who each stove is for, once the band fits, is on the best sauna heater page.
After the band, the current is a different question. SaunaShop’s shop method puts 6 kW at about 26 A at 230 V. That figure is not part of the volume sum. It is the reason to read how to install a sauna heater before you assume a socket will do.
Steam, wood, and log walls are different sums
The CGOLDENWALL CNA 643D states 3–4.5 m³ at 25–55°C. That band belongs to its steam room, not to Harvia’s dry-cabin arithmetic. Do not add 1.2 m³ for a glass sauna door and then ask whether 9 m³ fits a 4.5 kW generator sold for a small wet room. The Caiqinlen steam listing prints 35–55°C and no cubic metres at all, so this calculator cannot size it.
A log wall or a barrel that is not insulated is Harvia’s other factor: multiply the volume by about 1.5. An 8 m³ uninsulated log interior becomes about 12 m³ before you even add glass. Nothing in this set prints a band that wide. The WILLOWYBE wood stove has no kilowatt figure, so you cannot size it with this sum at all. If the band is missing, or you are sitting on 9 m³ of a 5–9 m³ stove with a glass door still to add, stop.
Common questions
How much do I add for a glass door?
Harvia’s method adds about 1.2 m³ for each square metre of glass. A 1 m² door on an 8 m³ room makes 9.2 m³. Finnmark’s 1.5 factor is a different guide. Do not add both.
Is 1 kW per cubic metre enough?
It is Harvia’s starting point for an insulated cabin, before glass and before you check the heater’s own range. It is not enough on its own when the door is glass, and it is not the rule for a steam generator or a wood stove.
What if my result sits on the top of a 5–9 m³ listing?
Treat that listing as too small for the way you will use it. Harvia’s guide avoids the top edge, especially with glass. Both Mxmoonant listings stop at 9 m³, so 9.2 m³ after a glass door does not sit comfortably inside them.
Can I use the same sum for a steam generator?
No. Sauna Helper excludes steam rooms from the Harvia-style sum. The CGOLDENWALL’s 3–4.5 m³ band is its own steam figure. The Caiqinlen page does not print a volume you can test.
What about a log cabin or a barrel?
Harvia multiplies an uninsulated log interior by about 1.5, then you still have to land inside a listing’s own band. An 8 m³ shell becomes about 12 m³, which is outside every electric band in this set.
Which listings in this set even print a room band?
The two Mxmoonant stoves state 5–9 m³. The CGOLDENWALL steam generator states 3–4.5 m³, which is a different sum. The Anjur, Caiqinlen, Alvinlite, and WILLOWYBE do not print an electric room band in the file.
Related pages
Sources
Public UK sources used for this comparison. Listing prices, stars and review counts are the amazon.co.uk figures saved for this set on 20 October 2026.
- Harvia, how to select heater power (retrieved 20 October 2026)
- SaunaShop FAQ (retrieved 20 October 2026)
- Installer Online, electrical requirements for sauna cabins (retrieved 20 October 2026)
- Approved Document P (GOV.UK) (retrieved 20 October 2026)
- IET, Part P frequently asked questions (retrieved 20 October 2026)
- NBS, BS EN 60335-2-53:2011+A11:2023 (retrieved 20 October 2026)
- NHS Collingwood Surgery, sauna or jacuzzi in pregnancy (retrieved 20 October 2026)
- Defra, smoke-exempt appliances (retrieved 20 October 2026)