Heat Transfer Coefficient Converter
Convert between W/(m²·K), Btu/(h·ft²·°F), kcal/(h·m²·°C), and more heat transfer coefficient units — for HVAC, thermal engineering, and building science.
Quick Reference — 1 unit = ? Kilowatt per m² per Kelvin (kW/m²·K)
| Unit | Value |
|---|---|
| Watt per m² per Kelvin (W/m²·K) | 0.001 |
| Kilowatt per m² per Kelvin (kW/m²·K) | 1 |
| Watt per cm² per Kelvin (W/cm²·K) | 10 |
| Btu/(h·ft²·°F) | 0.005678263 |
| kcal/(h·m²·°C) | 0.001163 |
| cal/(s·cm²·°C) | 41.84 |
Formula
Multiply by (from factor / to factor) | h = Q / (A × ΔT)
Frequently Asked Questions
What is a heat transfer coefficient, and why does it matter?
The heat transfer coefficient (h) measures how effectively heat moves between a surface and a fluid (air, water, etc.) per unit area per degree of temperature difference: h = Q / (A × ΔT), where Q is heat flow (watts), A is surface area, and ΔT is the temperature difference. It is central to HVAC design, radiator and heat exchanger sizing, building insulation (U-values), and electronics cooling — a higher coefficient means heat transfers more efficiently for the same surface area and temperature gap.
How do you convert Btu/(h·ft²·°F) to W/(m²·K)?
1 Btu/(h·ft²·°F) = 5.678263 W/(m²·K). This conversion combines three separate unit changes: Btu→J (×1055.06), hour→second (÷3600), ft²→m² (÷0.092903), and °F→K interval (×9/5 inverted, i.e. ÷(5/9)). US engineering specs (e.g. for radiators, insulation, and heat exchangers) often use Btu/(h·ft²·°F), while most of the world uses W/(m²·K) — this converter bridges the two directly.
What are typical heat transfer coefficient values for common situations?
Reference ranges: free convection in air: 5–25 W/(m²·K). Forced convection in air (fans, wind): 10–200 W/(m²·K). Free convection in water: 50–1,000 W/(m²·K). Forced convection in water (pumped systems): 50–10,000 W/(m²·K). Boiling water: 2,500–100,000 W/(m²·K). Condensing steam: 4,000–100,000 W/(m²·K). This is why liquid cooling vastly outperforms air cooling in electronics and engines — water moves heat away far more efficiently per unit area.
How is the heat transfer coefficient used in building U-values?
A building's overall U-value (thermal transmittance, W/(m²·K)) combines the surface heat transfer coefficients on both sides of a wall or window with the material's own thermal resistance: 1/U = 1/h_inside + R_wall + 1/h_outside. Lower U-values mean better insulation. Modern UK/EU building regulations typically require wall U-values below 0.18–0.30 W/(m²·K) and window U-values below 1.2–1.6 W/(m²·K), compared to older single-glazed windows around 5.0–5.8 W/(m²·K).
What is the difference between heat transfer coefficient and thermal conductivity?
Thermal conductivity (k, in W/(m·K)) is a property of a material itself — how well heat moves through its bulk (e.g. copper: ~400 W/(m·K); still air: ~0.025 W/(m·K)). The heat transfer coefficient (h, in W/(m²·K)) describes the boundary process at a surface where a solid meets a moving fluid, and depends on fluid velocity, viscosity, and flow pattern, not just material properties. Both are needed together to fully model heat flow through a wall and into the surrounding air or water.