How this is calculated
Inspect the versioned inputs.
| Constant | Value | Scope & source |
|---|---|---|
| harviaCalculatorKwPerM3 | 0.7 kW/m³ | Live-calculator implementation; distinct from support prose's approximate1kW/m³ rule. Not a universal physical law. harvia-live-calculator ↗ Verified 2026-11-08 |
| harviaCalculatorMinimumKw | 2.3 kW | Live-calculator lower clamp for nonzero volume. harvia-live-calculator ↗ Verified 2026-11-08 |
| harviaCalculatorMaximumKw | 30 kW | Live-calculator upper clamp; do not imply applicability beyond the tool's range. harvia-live-calculator ↗ Verified 2026-11-08 |
| harviaBareVolume | 1.2 m³/m² | Add for non-insulated glass, stone or equivalent surfaces; avoid counting the same area twice. harvia-heater-sizing ↗ Verified 2026-11-08 |
| huumBareVolume | 1 m³/m² | CORE US profile only. Do not reuse Harvia's1.2 allowance for HUUM model matching. huum-core-us-manual ↗ Verified 2026-11-08 |
| harviaLogFactor | 1.5 multiplier | Non-insulated log interiors. Live calculator confirms application to physical volume before other allowances. harvia-heater-sizing ↗harvia-live-calculator ↗ Verified 2026-11-08 |
| harviaKWPerM3 | 1 kW/m³ | Approximate manufacturer starting rule; model chart controls final selection. harvia-heater-sizing ↗ Verified 2026-11-08 |
Input defaults that are not sourced measurements are illustrative planning assumptions. Exact unit definitions, the 52-week convention and algorithm details are in the shared methodology.
Worked example
An example you can inspect.
A 10 m³ room plus 2 m² of bare surface is 12.4 adjusted m³ under Harvia and 12 adjusted m³ under HUUM. These methods are not interchangeable.
Separate the room measurement from the sizing allowance.
Measure the finished internal width, length and height first. Their product is physical volume. Then list the exposed glass, masonry, tile and other surfaces covered by the selected manufacturer's rule. Enter their area once in the non-insulated-surface input. The adjusted volume is a sizing convention, not a claim that the room physically grew. Keep your area worksheet so a supplier can see what was included, especially where a glass door and adjacent panels might otherwise be counted twice.
Take a model-specific question to the supplier.
A room falling inside a chart interval is a candidate for that exact example, not a guarantee of acceptable performance. Review construction, available supply, placement, ventilation and the latest instructions together. The bounded chart deliberately returns no match where the source data does not establish one. Do not bridge an empty result by selecting the next larger number or mixing another manufacturer's surface allowance with this chart. Outdoor location and climate remain useful project context, but this tool has no verified coefficient that converts those labels into additional kilowatts.
Questions, considered
Questions about this tool
How should I interpret the heater size calculator result?
No universal outdoor, climate or wall-material multiplier is supplied. Confirm against your heater manufacturer’s sizing chart; no matching sample does not mean no suitable product exists. Compare manufacturer-specific adjusted volumes, heater chart examples and electrical load without mixing different sizing methods.
Which assumptions matter in the heater size calculator?
Harvia: apply the log factor to physical volume, then add 1.2 m³ per m² of non-insulated surfaces. Include a glass door in area once. HUUM CORE: add 1.0 m³ per m²; solid-log combinations require manufacturer review. Match only the HUUM profile to CORE chart ranges. Electrical load uses actual model kW at the selected supply. Warm-up is a cited example, not a climate prediction.
Primary sources
Follow the source.
- Harvia — live sauna calculator and calculation module ↗
Public JS chunk9800-96989cff6cbf203f.js, module80743, read by HTTP
Observed implementation: multiply physical volume for log first, then add fixed glass-door and measured-area allowances. Power uses a different multiplier from support prose. Version-specific implementation, subject to future change. No source code copied into the project.
Checked 2026-11-08 - Harvia — How do I select the correct heater power? ↗
What to consider when choosing the correct power; Choosing a heater based on calculator results; updated May 11, 2026
Harvia's rough kW/m³ starting rule is not identical to selecting a model by its declared room-volume range. Prefer a model's volume-range midpoint; avoid treating the rough number as an exact universal requirement.
Checked 2026-11-08 - HUUM CORE US installation and operation manual, v9 ↗
Printed pp.4–6: sizing, Tables1–3 and installation; pp.9–11: room and ventilation
CORE9/11 rows are consistent with P/V. CORE6 is excluded: its 240V table pairs6.5kW with25A, which does not match P/V; the208V row also conflicts. Preserve this discrepancy rather than silently correct a manufacturer's table.
Checked 2026-11-08 - Harvia — FAQ about the energy consumption of sauna bathing ↗
Section2: How much energy does the heater consume?
A manufacturer example for a properly sized heater in an insulated room. Thermostat cycling means nameplate kW multiplied by clock time is an upper-bound scenario, not guaranteed consumption.
Checked 2026-11-08