Sauna Heater Size Calculator
By Caleb Whitaker Prices checked
Enter the inside size of your sauna. The calculator returns the heater output to look for, the running current at 240V and the most a session will cost at your electricity rate.
For a barrel, use the diameter for both width and height. The result will be a little high, which is the safe direction.
How the calculator works
- Volume is length times width times height, in cubic feet.
- Heater size. Insulated rooms use the common rule of thumb of about 1 kW per 47 cubic feet. Outdoor barrels use 1 kW per 25 cubic feet, which matches how sellers equip them: a 6 kW heater in a 140 cubic foot Redwood Outdoors barrel and 6 to 6.9 kW in a 170 cubic foot SaunaLife barrel.
- Glass loses heat faster than wood, so a large glass door or window adds 1 kW.
- Amps is watts divided by 240.
- Session cost is kW times hours times your rate. It assumes full power the whole time, so your real cost will be lower.
The default rate of 18.2 cents is the U.S. average home price for 2026 from the Energy Information Administration.
Before you buy
This is an estimate. The heater maker's volume chart and installation manual are the final word, and your electrician sizes the breaker and wire. Then compare prices on the sauna heaters page.
Worked examples
| Room | Volume | Rule | Heater to look for |
|---|---|---|---|
| Barrel, 6 ft long, 6 ft diameter | 216 cu ft | 1 kW per 25 cu ft | 8.6 kW, so 9 kW |
| Barrel, 140 cu ft (Redwood) | 140 cu ft | 1 kW per 25 cu ft | 5.6 kW, so 6 kW |
| Barrel, 170 cu ft (SaunaLife E7) | 170 cu ft | 1 kW per 25 cu ft | 6.8 kW, so 6.9 to 8 kW |
| Insulated room, 6 x 6 x 7 ft | 252 cu ft | 1 kW per 47 cu ft | 5.4 kW, so 6 kW |
| Insulated room, 6 x 8 x 7 ft | 336 cu ft | 1 kW per 47 cu ft | 7.1 kW, so 8 kW |
| Insulated room with glass door, 6 x 8 x 7 ft | 336 cu ft | Plus 1 kW for glass | 8.1 kW, so 9 kW |
The barrel example uses diameter for width and height, so it runs high on purpose. Your heater's own volume chart overrides these figures.
Amps by heater size
| Heater | Amps |
|---|---|
| 4.5 kW | 18.8 A |
| 6 kW | 25.0 A |
| 8 kW | 33.3 A |
| 9 kW | 37.5 A |
| 10.5 kW | 43.8 A |
Your electrician sizes the breaker and wire from the heater's installation manual, which may call for a larger breaker than the running current.
Gas and wood heaters
Gas heaters are rated in BTU and by room volume range. Wood stoves carry higher kW figures for the same room because output swings with the fire. Use the maker's chart for both. See gas sauna heaters and wood-burning stoves.
Common questions
Is a bigger sauna heater always better?
No. An oversized heater cycles off quickly and the stones may not get hot enough to make good steam. Stay inside the maker’s volume range for your room.
Why do outdoor barrels need a bigger heater?
The solid wood walls are not insulated and the sauna sits in outdoor air, so more heat escapes than from an insulated indoor room.
What size heater do I need for a 6x6 sauna?
For an insulated 6 x 6 x 7 ft room, about 252 cu ft, the rule of thumb gives 5.4 kW, so a 6 kW heater. Check the heater maker's chart.
How many amps does a 6 kW sauna heater draw?
About 25 amps at 240V. The electrician sizes the breaker from the heater manual.