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.
Each step along the axis is ten times the one before — the heat-transfer-coefficient units span 5 orders of magnitude, so a straight scale would bunch almost all of them at one end.
Instant insight
The "per" part is where these go wrong
Heat flux, heat density and heat transfer coefficients are all rates per unit area, volume or degree. Two figures in different units are only comparable once the per-what matches.
Write the full unit out — W/m²·K, not just W — before comparing two sources.
Check the direction of the conversion
Going from w_m2k to kw_m2k, the number should get bigger if the new unit is smaller, and smaller if it is bigger.
If it moved the wrong way, the factor has been applied upside down — the most common conversion error there is.
Worked out in your browser — nothing is sent anywhere. Comparison figures are approximate.
Common conversions
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)
Everyday Uses
Window and wall U-values
European U-values in W/m²·K, US specs in BTU/hr·ft²·°F — convert to compare insulation performance across catalogs.
Heat exchanger specs
Datasheets from different countries use different units — convert coefficients before sizing equipment.
Thermal engineering courses
Homework switches unit systems constantly — convert h-values without factor errors.
Energy audits
Compare building envelope performance when reports mix metric and imperial conventions.
Insulation performance
Compare U-values quoted in W/m²K against imperial figures when specifying glazing or wall build-ups.
HVAC sizing
Convert coefficients between unit systems when working from equipment data sheets of different origin.
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.