What size sauna heater do I need?

The right sauna heater comes down to one number: your room’s volume, adjusted for glass and stone, sitting comfortably inside the heater maker’s rated range. Get it right and the room heats evenly and the stones get hot enough for good steam. Get it wrong and it costs you. HUUM’s DROP manual says the manufacturer’s warranty is not valid if the heater’s output doesn’t match the calculated room volume.

Here are the three steps we use, with worked examples.

Step 1: Measure the inside of the room

Measure inside the finished walls, not the outside of the sauna.

• Box-shaped rooms: length × width × height. A room 6 × 5 × 7 ft is 210 cu ft (about 5.9 m³).
• Barrel saunas: 3.14 × radius² × length, using the inside diameter and inside length.
• Sloped or peaked ceilings: split the room into simple shapes and add them up, or send us a sketch.

Ceiling height matters more than most people expect. HUUM recommends a room height of 79–91 in (2,000–2,300 mm) for its DROP heater and suggests lowering a taller ceiling to cut the volume. Homecraft notes that in rooms with 8 ft ceilings, the hottest air pools above the bathers’ heads.

Step 2: Add volume for glass, stone and log walls

Glass, brick, tile and concrete soak up heat, so they count as extra volume. Each maker publishes its own rule. Use the rule for the brand you’re buying.

A full-glass sauna door is often around 12 sq ft, but measure yours. By HUUM’s rule that door adds about 40 cu ft; by Harvia’s, about 47 cu ft.

Step 3: Choose a heater whose range fits

Every heater has a minimum and maximum room size set by its maker. The HUUM DROP 6 kW is rated for 177–318 cu ft (5–9 m³). The Harvia KIP 6 kW (240 V) is rated for 169–300 cu ft.

• Aim for the middle. Harvia recommends a model that puts your room near the midpoint of its range, not at either end.
• Don’t undersize. Harvia notes that a heater that’s too small has to run longer and harder, which shortens its life.
• Between two sizes? Send us your measurements. We’ll check both options against the manufacturer’s current tables before you pay anything.

Worked examples

Inside dimensions. “Look at” is the heater size whose published range fits the adjusted volume, using HUUM’s and Harvia’s own tables.

Two lessons. A single glass door pushed the first room past the top of the HUUM DROP 4.5 kW range (248 cu ft). And a glass front took the 294 cu ft room from a 7.5–8 kW heater with solid walls to 9–10.5 kW, with 10.5 kW closer to the middle of its range.

Outdoor and barrel saunas

Outdoor saunas follow the same steps, with three differences.

1. Barrels have curved walls. A barrel measuring 6.6 ft across and 5.6 ft long inside holds about 192 cu ft before you add the glass door.
2. No maker publishes a separate barrel rule. Applying the log-wall adjustment to every stave gives numbers far above what barrel builders actually fit. Start from the barrel maker’s recommended heater and check it against the heater’s published range. Ask us to check it when you request a quote.
3. Winter is harder work. Our advice, not a manufacturer rule: if you’ll use the sauna in January, don’t pick a heater at the bottom of its range, and plan where the control and contactor will live in the cold. Our controls guide explains the options. Some owners choose a wood-burning stove instead.

Amps at 240 V: what your electrician needs

Amps = watts ÷ volts. At 240 V single-phase:

Breaker and wire sizes come from the manual for your exact model, not from the amps alone. HUUM lists a 50 A breaker and 8 AWG minimum wire for the DROP 9 kW, for example, and Homecraft lists a 50 A breaker for its Revive 9 kW. Your electrician confirms both. Our Ontario electrical checklist covers the rest.

Five common sizing mistakes

• Measuring the outside. Exterior dimensions include the wall thickness, so they overstate the volume.
• Forgetting the glass. Doors, windows and glass fronts all count. So do bare brick or concrete walls in a basement.
• Mixing units. 1 m³ is about 35.3 cu ft, and 1 m² is about 10.8 sq ft. Convert before you compare.
• Choosing at the edge of a range. A room that barely fits the top of a range is a sign to look at the next size up.
• Using one brand’s rule for another brand’s heater. The adjustments differ, so use the rule that matches the heater you’re buying.

Next steps

• Try the Heater Finder. Enter your room and glass area and it shows only heaters whose maker-rated range fits.
• Browse electric sauna heaters. Each product page lists its room range in cu ft and m³.
• Not sure? Send us your measurements. We’ll double-check the size and list what else the heater needs, such as a control, contactor box or stones. You pay nothing until you approve a written invoice.