Sauna Heater Sizing, Simplified (with calculator)
Why Sizing Matters
Note, a TDLR calculator is included after the article.
Undersized heaters take forever to warm up and yield weak löyly. Oversized units short-cycle and create harsh, uneven heat. The sweet spot comes from a simple calculation: room volume plus adjustments for cold surfaces (glass, stone, brick) and insulation quality.The Core Rule (Volume → kW)
Most manufacturers size by room volume. A widely used rule of thumb is roughly 1 kW per 45–50 ft³ (about 1 kW per 1–1.5 m³) in a well-insulated, all-wood sauna. Always confirm with a brand calculator or manual for your model.
- Brand calculator: Harvia Sauna Calculator
- General guidance: Nordica: 1 kW per 45–50 ft³, OLI: 1–1.5 m³ per kW
Adjust for Glass, Stone & Masonry (Cold Surfaces)
Cold, dense surfaces act like extra volume because they absorb heat. Many manuals recommend adding a “volume equivalent” for each square meter of glass/stone.
- +1.2 m³ per 1 m² of uninsulated wall/ceiling (e.g., plaster, stone) — per Helo manuals (source)
- +1.5 m³ per 1 m² of glass/stone for certain models — per Helo Saunatonttu manual (source)
Tip: If you have a big glass door or a masonry wall, size up or improve insulation to stay within the heater’s rated volume range.
Step-by-Step Sizing (with Examples)
- Calculate room volume (L × W × H). Convert ft³ to m³ by dividing by 35.3.
- Add cold-surface equivalents (glass/stone/masonry) using the manual’s factor (e.g., +1.2–1.5 m³ per 1 m²).
- Map to kW using 1 kW per 1–1.5 m³ (or 1 kW per 45–50 ft³), then verify against a manufacturer chart/calculator.
Worked Example A (All Wood, Well-Insulated)
Room: 6′ × 8′ × 7′ = 336 ft³ = 9.5 m³
Rule: ~1 kW per 45–50 ft³ → 336/50 ≈ 6.7 kW (or 336/45 ≈ 7.5 kW)
Pick: 7–8 kW class (confirm with brand calculator).
Check on Harvia’s calculator
Worked Example B (Same Room, Glass Door + Masonry)
Room: 9.5 m³ base.
Cold surfaces: glass door 0.8 m² + masonry wall 2.0 m².
Add: (0.8 + 2.0) × 1.2 m³ ≈ +3.36 m³ → Adjusted volume ≈ 12.9 m³.
Rule: 12.9 m³ → ~9–13 kW depending on the 1–1.5 m³ per kW rule.
Decision: Either choose a higher-output heater within rated limits, or reduce glass/masonry exposure to bring the adjusted volume down. Verify in the specific manual.
Insulation, Benches & Stones Matter Too
- Insulation & air-sealing: Better insulation shrinks warm-up time and keeps you in range. (Use the calculator above and your model’s manual.)
- Bench geometry & ventilation: Good airflow and “feet above the stones” design make a correctly sized heater feel hotter at the same setpoint. See Sauna Ventilation & Bench Design.
- Stone mass & stacking: The right stones and a breathable stack keep steam soft and powerful. See Sauna Stones 101.
Wood, Pellet, or Electric: Sizing Notes
- Electric: Follow the manufacturer’s chart strictly; add volume for glass/stone per the manual; confirm breaker/wire sizing.
- Wood/Pellet: Thermal mass and flue design affect warm-up. You still size to room volume, then ensure stone capacity and airflow match. See How Stove Design Shapes the Experience.
Quick Reference Table
| Room Volume (m³) | Typical Heater Class (kW) | Notes |
|---|---|---|
| 5–7 | 4.5–6 | Small indoor rooms; minimal glass |
| 8–10 | 6–8 | Common backyard saunas |
| 11–14 | 8–10 | Watch glass/masonry adjustments |
Always verify against your heater’s official volume chart or calculator.
Authoritative Resources
- Harvia – Sauna Calculator
- Helo Vienna STS Manual (adds 1.2 m³ per 1 m² uninsulated surface)
- Helo Saunatonttu Manual (adds 1.5 m³ per 1 m² glass/stone)
Related Reading
- The Physics of Löyly
- How Stove Design Shapes the Experience
- Sauna Ventilation & Bench Design
- Sauna Stones 101
Bottom Line
Start with volume, add equivalents for glass/stone, and confirm with a manufacturer calculator. Dial in benches, stones, and ventilation, and your heater will deliver the smooth, powerful löyly you’re after.