In short
Calculate the internal room volume in cubic metres, add 0.5–1.0 m³ for a glazed door, 1.5–2.5 m³ for a full glass wall and 1.2 m³ for every square metre of uninsulated stone or concrete, then choose a heater whose stated range covers the adjusted figure. Check the electrical supply before ordering: above 4.5 kW usually means three-phase.
The rule of thumb is one kilowatt per cubic metre of room volume. It is a reasonable starting point and it is wrong often enough to be worth checking, because it assumes a fully insulated timber room with no glass and no cold surfaces.
Step one: calculate the real volume
Length × width × height, in metres. Use the internal dimensions of the finished cabin, not the shell — a 70 mm insulated build-up on all sides takes more space than people expect.
Step two: add for the surfaces that lose heat
- Glazed door
- Add 0.5–1.0 m³
- Full glass wall
- Add 1.5–2.5 m³
- Uninsulated stone or concrete wall
- Add 1.2 m³ per m² of exposed surface
- External wall, unheated side
- Add 0.5 m³ per m²
- Ceiling above 2300 mm
- Recalculate — heat stratifies and the bench gets cold
Volume additions by surface
Want this answered for your room rather than in general?
Talk to a specialistAdd the corrections to your calculated volume, then choose a heater whose stated range covers the adjusted figure. A 6 kW heater rated for 5–8 m³ suits a 6 m³ room with a glazed door; the same room with a full glass wall is closer to 8 m³ and sits at the top of that heater's range.
Step three: check the supply before you order
- —Above 4.5 kW, most residential installations in Kathmandu need three-phase.
- —The circuit must be dedicated, with a 30 mA RCD — never shared with lighting or a plunge chiller.
- —Cable to the control unit is heat-resistant silicone, sized to the heater, not to the run length alone.
Oversizing is not a safe default. A heater too large for the room reaches setpoint fast, cycles off, and gives you sharp dry heat with a cold bench — the opposite of what a Finnish sauna is supposed to feel like. Size it correctly and the room does the work.
