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How the sizing formula works

Heater sizing follows the manufacturers' published rule: room volume (m³) divided by an insulation factor gives the required kW. Harvia's sizing guidance uses an average of 1 kW per cubic meter of sauna volume, with more power required for glass surfaces and heat-storing structures; HUUM's calculator adds the uninsulated-surface area to the room volume before applying 1 kW per calculated cubic meter. This calculator applies the same method:

• Volume: width × depth × height
• Calculatory volume: volume + glass area (m² × 1 m³) — Harvia/HUUM glass method
• Insulation factor: good ×1.0, medium ×1.25, poor ×1.5
• Cold rooms: +1.5% power per °C below 15 °C ambient (capped +25%)
• Result: calculatory volume × 0.7 kW/m³, rounded to standard 0.5 kW heater steps

Sources: Harvia — How do I select the correct heater power · HUUM sauna heater size calculator

Circuit requirements: 120V vs 240V vs 3-phase

• 120V plug-in: only small heaters up to ~2.2kW. Standard 15–20A household circuit.
• 240V single phase: the standard residential sauna circuit for 2.5–9.6kW heaters. Dedicated breaker, typically 30–50A. 9kW is the practical residential ceiling.
• 3-phase: above ~9.6kW — commercial service or splitting the load across multiple single-phase heaters. NEC continuous-load rule: breaker sized at 125% of heater amps.

Warm-up time estimate

A correctly sized electric heater brings a traditional sauna to 80–90 °C in roughly 30–45 minutes; undersizing stretches this past an hour and the heater never fully recovers between sessions. The estimate starts at 30 minutes and adds time for medium/poor insulation, glass exposure and cold ambient temperature (clamped 20–75 min).

Matching heaters from our verified dataset

The calculator filters our verified sauna heater dataset to the kW band your room needs. Each match links to the full spec sheet and current price.

Safety

• Always use a dedicated circuit for your sauna heater when possible
• NEC continuous-load rule: heater draw ≤ 80% of breaker rating
• Consult a licensed electrician for permanent installations — most jurisdictions require a permit for 240V sauna circuits
• Never use extension cords or power strips with a sauna

Disclaimer: This calculator provides estimates based on manufacturer-stated sizing rules (Harvia, HUUM). Always consult a licensed electrician before installation.