How to Size a Commercial Sauna for a Villa, Gym or Hotel

Sizing runs in one direction: people → benches → cabin → volume → heater → electrical. Skip a step and the error lands in the last one - usually as a 6 kW heater gasping at a glass-walled 14 m³ room at 7 pm in the rain.

Step 1: people, honestly counted

Count the peak hour, not the brochure: a villa couple is 2; a padel club's post-match wave is 4-6 at once; a hotel spa cycling guided groups wants 8-10. Each seated adult needs ~60 cm of bench; lounging doubles it. Our standard tiers - and their cabins - follow:

Step 2: the 1 kW per m³ rule - and its tropical asterisks

Base rule: ~1 kW of heater per cubic metre of cabin. Then the asterisks that matter here: every square metre of glass behaves like extra volume (add ~1.2 m³ worth per m² of glazing); an uninsulated stone or concrete wall steals heat all evening; and commercial duty wants the next heater up, not the exact match - stone mass is what recovers temperature between guest waves. This is why our cabins pair fixed size-to-heater combinations: the guesswork is pre-made.

Step 3: geometry guests feel

• Ceiling at 2.1-2.2 m: higher wastes heat above heads; lower cramps steam room ergonomics.
• Two bench tiers from 6 people up: heat stratifies, and tiers sell the choice (novices low, veterans high).
• Door swings out, hinge away from the heater - safety code and common sense in one.
• Heater guard rail and lockable wall controller for commercial rooms - staff set 85-95 °C; guests get heat, not switches.
• Ventilation: intake under the heater, exhaust behind the top bench - the tropical anti-mould detail most builds miss.

Electrical, before the electrician asks

6 kW runs on robust single-phase; from 9 kW plan 3-phase. A 15 kW resort heater is a 3×32 A conversation your building engineer wants early - which is why every free layout ships with the electrical reservation sheet. Pair the cabin with cold and the numbers combine: the configurator matches heater and chiller in one pass.

Worked example: an 8-person hotel spa sauna

A hotel spa that cycles guided groups of eight walks through the method like this. People: 8 at peak, seated, so about 4.8 m of bench (8 × 60 cm) split across two tiers. Cabin: 3.0 × 2.4 m with a 2.1-2.2 m ceiling, about 15 m³. Heater: the standard pairing is a 13.5 kW Harvia. Tropical asterisks: the spa wants a 2 m² glass front, which behaves like roughly 2.4 m³ of extra volume and takes the effective room to about 17.4 m³ - so the heater steps up to the 15 kW model we use in 10-person cabins. Electrical: 3-phase, reserved in the layout before the spa fit-out closes.

Mistakes that make tropical saunas lukewarm

• Sizing from brochure capacity instead of the peak hour.
• Panoramic glazing without a heater step-up.
• Building against an uninsulated concrete or stone wall that soaks up heat all evening.
• Choosing the exact-match heater for commercial duty, so the room never recovers between guest waves.
• Skipping the air path - no low intake under the heater, no exhaust behind the top bench - which also invites mould.
• Leaving electrical to the end and discovering that a 9 kW+ heater needs a supply the building does not have.

Match the sauna to the cold side

Guests alternate between hot and cold, so the cold side has to absorb roughly the sauna's population every 10-15 minutes. If the sauna outgrows the cold, the queue forms at the tub; if the cold outgrows the sauna, capex sits idle.

Hotel-specific planning - ventilation in 80-90% humidity, cost bands and timelines - is in the hotel sauna guide; cabin options are on commercial saunas.