Heater size calculator

Measure the room’s length, width and height in metres, then multiply them together. The result is its volume in cubic metres, before any allowance for construction or surfaces that affect heating.

Length × width × height = room volume in cubic metres.

Heavenly Saunas uses a room measuring 2 metres long, 1.75 metres wide and 2 metres high as an example. That gives a volume of 7 cubic metres. At its starting guideline of around one kW per cubic metre, the estimated power has the same numerical value in kW, before adjustments.

Keep this measured volume separate from the adjusted volume used in some sizing methods. Glass or uninsulated walls do not make the room physically larger; the adjustment represents their additional heating requirement. Room volume is a starting point, not a complete heater selection.

There is no single conversion from room volume to heater power across these sources. Alongside Heavenly Saunas’ guideline, Luxway gives one kW for every 1.3 cubic metres. Its method divides volume by that figure to estimate the power requirement, and it states that its calculations apply to private-use saunas.

The allowances differ too. Heavenly Saunas suggests adding one to 2 kW for factors such as windows or poor insulation. Luxway adjusts the room volume before converting it to power.

Harvia’s calculator considers dimensions and log construction, along with a glass door and the area of non-insulated walls. HUUM also separates actual room volume from a calculated volume that accounts for non-insulated surfaces.

These are separate approaches. Mixing a starting ratio from one source with allowances from another would not reproduce either published method. For heater selection, keep to the relevant manufacturer’s calculation from start to finish.

Harvia and HUUM both identify glass and heat-storing materials as reasons why dimensions alone are not enough. Their examples include brick and concrete, as well as log surfaces. A glass door or window can matter even though the measured room volume stays unchanged.

HUUM adds an allowance for non-insulated surfaces to the actual volume. Its table gives glass, log walls and other non-insulated surfaces a coefficient of one. Multiplying the surface area in square metres by that coefficient gives the additional volume.

Luxway takes a different approach. It describes adding half a cubic metre per square metre of tiles, glass and similar surfaces. For glass or uninsulated walls facing outside, its allowance is one cubic metre per square metre. It also describes a volume multiplier of 1.2 for relevant surface materials.

Record the material as well as its area. Those details determine how the chosen method accounts for the surface.

Harvia asks for width, height and depth alongside construction information. HUUM likewise stresses that the room’s construction matters as much as its size when selecting a heater.

As well as your measurements, you will need to know about glass doors or windows and the area of non-insulated surfaces in square metres. Note any log surfaces, brick or concrete. Insulation and ventilation information also matters: Heavenly Saunas says poor insulation and additional ventilation can increase the power requirement.

Luxway highlights insulation too, but its guidance for uninsulated rooms is internally inconsistent. The text says to add 1.5 cubic metres; the formula instead multiplies the volume by that amount. Adding a fixed volume and applying a multiplier are different operations, so this passage should not be treated as an unambiguous calculation.

HUUM explicitly warns that more power does not necessarily produce better steam. Heavenly Saunas also says a heater can perform poorly when it is too small or too large for the room.

Luxway explains why oversizing can cause problems with stone heating. A powerful heater may bring the room to temperature quickly and then run for shorter periods, leaving the stones insufficiently heated for water to evaporate effectively. The highest available kW rating is not a substitute for a heater sized to the room.

Let the calculated requirement guide your choice of power and model, rather than choosing by appearance alone. HUUM’s calculator is intended to help with both. Electrical work is separate from this sizing exercise. Use a qualified electrician.