Size the heater, then see what it costs in January.
Heater charts assume a sauna in a warm house. A backyard sauna starts every session at the outdoor temperature, behind walls of solid wood. This calculator sizes the heater the way the manufacturers do, then estimates how long it takes to heat up and what each session costs at your outdoor temperature and your electricity rate.
Example: a 6′ × 6′ barrel
6 kW · 25 A
Ready at 176°F in about 20–30 minutes on a 68°F day and about 55–85 minutes at 0°F. About $1.10 of MG&E electricity per session in summer, and about $2.20 in a cold snap.
Manufacturer ranges · EIA July 2026 rates
Your sauna, by the numbers.
Start with a preset or set your own dimensions. Everything updates as you go. The chart shows how the wait grows as the weather gets colder.
Rated for this room: Harvia KIP-60 (170–300) · HUUM Drop 6 (177–353)
Circuit for a 6 kW heaterFrom the manuals: Harvia KIP-60: 10 AWG · HUUM Drop 6: 30 A breaker, 10 AWG. Your electrician sets the final breaker and wire to code.
Physics estimate, shown as ±20%. It hasn't been checked against measured heat-up times yet. Send yours and help fix that.
How each number is worked out.
The same calculator runs on each of these pages, next to the method and the tables behind it.
Heater size
Room volume, adjusted for solid-wood walls and glass, matched to Harvia's and HUUM's rated ranges. Why a 170 cu ft barrel needs a heater rated for nearly 300.
Heat-up time
A two-part physics model: air and stones heat fast, the wood walls lag behind, and both lose heat to the cold outside. Tables from 80°F down to −20°F.
Electricity cost
kWh to heat up plus kWh to hold temperature, at MG&E's rate or any state's average. Why a January session uses about twice the power of a July one.
Send us your heat-up time.
Every heat-up time published online comes from a seller. We'd like real ones. Next time you fire your sauna, note the outdoor temperature and how long the thermometer took to reach temperature. We'll publish the reports together, so anyone can check the estimates against them.
Sizing, wiring and running a sauna.
What size heater does a 6 × 6 barrel sauna need?
About 6 kW. A 6-foot-diameter, 6-foot-long barrel holds about 170 cubic feet. Solid wood walls count as 1.5 times the volume, and a glass door adds about 32 cubic feet, for a sizing volume of about 286. That sits inside the rated ranges of the Harvia KIP-60 (170–300) and HUUM Drop 6 (177–353).
What circuit does a 6 kW sauna heater need?
A 6 kW heater draws 25 amps at 240 volts. HUUM's spec for its 6 kW Drop is a 30 A breaker and 10 AWG wire, and Harvia lists 10 AWG for the KIP-60. Your electrician sets the final breaker and wire under the electrical code and the heater's manual.
How long does an outdoor sauna take to heat up in winter?
Longer than sellers usually say. Our physics estimate for a 6-foot barrel with a 6 kW heater is about 20–30 minutes on a 68°F day, 50–75 minutes at 10°F, and 65–100 minutes at −10°F. Most of the extra time goes to heat lost through the single-layer wood walls.
How much does it cost to run a sauna?
For a 6-foot barrel with a 6 kW heater and an hour of use, about 6 kWh on a 68°F day, 12 kWh at 0°F and 13 kWh at −10°F. At MG&E's 18.4¢ per kWh that's roughly $1.10 to $2.40 a session.
How accurate is the heat-up estimate?
It's a physics estimate built from published material properties, shown as a range of ±20%. It hasn't been checked against measured times yet. Wind, a frosted door seal or a heater set below its maximum will slow things down. That's why we're collecting owners' heat-up times.
- Harvia KIP heaters, 240V single-phase: rated room volumes, amps and supply wire (KIP-30, -45, -60, -80). Harvia KIP-60, Harvia KIP-80
- HUUM Drop heaters, 240V: rated room volumes, amps, breaker and wire (Drop 4.5, 6, 7.5, 9), from published spec sheets. Spec sheet
- Sizing adjustments: HUUM's method multiplies the volume by 1.5 for walls without at least R-13 insulation and adds 3.2 cu ft per sq ft of glass (4.8 if the walls are insulated). Harvia's equivalent: 1.2 m³ per m² of glass or uninsulated surface, and a factor of 1.5 for uninsulated log walls. HUUM sizing, Harvia support
- Electricity rates: U.S. Energy Information Administration, Electric Power Monthly Table 5.6.A, residential average by state, July 2026 (preliminary). MG&E residential rate as in our cost guide. EIA
- Heat-up model assumptions (handbook values): 42 mm solid wood walls with conductivity 0.11 W/m·K; framed walls at about R-13; single tempered glass at 5.8 W/m²·K; 6 air changes an hour through the vents; about 3.5 kg of stones per kW of heater; a 10 mm layer of wood (450 kg/m³, 1.7 kJ/kg·K) warming across walls and benches, starting at the outdoor temperature. Shown as ±20%. Not yet checked against measured heat-up times.