Convert heat transfer coefficient between W/(m²·K), BTU/(h·ft²·°F) and kcal units.
Heat • 5 units
All 5 units on the Heat Transfer Coefficient Converter are defined against the Watt/(m²·K) (W/(m²·K)), so each result is one conversion factor away from a single reference rather than the end of a chain of roundings.
The conversion this page is most often opened for is ready before you type anything: 1 W/(m²·K) = 0.176110194 BTU/(h·ft²·°F). Change either side and every row in the table recalculates with it.
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The default step on this page, watts per square metre per kelvin into BTU per hour per square foot per degree Fahrenheit, divides by about 5.678 — so 1 W/(m²·K) is 0.1761 in imperial units. The calorie row is the outlier in the table at 41,840, because calories per second per square centimetre per degree compresses three separate unit changes into one entry, and a value in it is four orders of magnitude away from the SI figure it represents.
Individual coefficients combine into an overall U-value by adding resistances rather than by averaging. The reciprocal of the U-value is the sum of the inside film resistance, every layer's thickness divided by its conductivity, and the outside film resistance. A wall with an inside film of 8 W/(m²·K), 100 mm of insulation at 0.035 W/(m·K) and an outside film of 25 works out as 0.125 + 2.857 + 0.04, giving a U-value of about 0.33 W/(m²·K). The insulation contributes 95% of the resistance in that stack, which is why the film coefficients can often be approximated without much affecting the answer — and why they cannot be for a bare metal surface, where they are the only resistance present.
The reason a single coefficient can be quoted at all is that it bundles a great deal of physics into one number. Correlations derive it from dimensionless groups — the Nusselt number in terms of Reynolds and Prandtl numbers for forced convection, Rayleigh number for natural convection — each valid only over the geometry and flow range it was fitted to. A coefficient taken from a table for a horizontal cylinder does not transfer to a vertical plate, and a value measured in clean conditions will not survive fouling. Converting the units is exact; the number being converted rarely is.
Quick reference — 1 Watt/(m²·K) (W/(m²·K)) is equal to:
| Kilowatt/(m²·K) | kW/(m²·K) | 0.001 |
| Calorie/(s·cm²·°C) | cal/(s·cm²·°C) | 0.0000239006 |
| BTU/(h·ft²·°F) | BTU/(h·ft²·°F) | 0.176110194 |
| Kilocalorie/(h·m²·°C) | kcal/(h·m²·°C) | 0.859845228 |
The heat transfer coefficient describes how readily heat crosses a boundary between a solid surface and a moving fluid, measured in watts per square metre per kelvin. It is a property of the situation rather than of a material — it depends on fluid velocity, viscosity, geometry and whether flow is laminar or turbulent — which is what distinguishes it from thermal conductivity, a fixed property of a substance. The range is enormous: still air manages around 5 W/(m²·K), forced air 25–250, and boiling water several thousand. That span is why a fan transforms a heat sink's performance while changing nothing about the metal.
q = h × A × ΔT (Newton's law of cooling)h = q ÷ (A × ΔT)1 W/(m²·K) = 0.17611 BTU/(h·ft²·°F)Overall U-value: 1/U = 1/h₁ + t/k + 1/h₂where:
Assumptions: Depends on flow conditions, not on the material alone — the same surface has a different h in still and moving air. Published values are typical ranges for a flow regime, not constants.
Find the heat removed by natural convection, then by forced air, from the same part.
Resulth = 80 W/(m²·K) removes 64 W — eight times natural convection
Note what did not change: the heat sink's thermal conductivity is a fixed material property, while h is set by airflow. That is the fundamental difference between the two quantities, and why data sheets quote conductivity for materials and coefficients for conditions.
The heat transfer coefficient describes how readily heat crosses a boundary between a surface and a fluid, in watts per square metre per kelvin. Unlike conductivity, it is not a property of any single substance: it depends on the fluid, how fast it is moving, whether it is changing phase, and the geometry it is flowing over. The same wall has a different coefficient on a still day and in a gale.
The range across mechanisms is enormous, which is why the choice of cooling method dominates any thermal design. Natural convection in air manages only single figures to low tens of W/(m²·K); forced air reaches the tens to low hundreds; flowing water reaches thousands; and boiling or condensing fluids reach tens of thousands, because latent heat moves far more energy than a temperature change in the same fluid. Nothing about the material being cooled shifts those figures — the fluid and its motion do.
Building work uses the same units for the U-value of an assembly, and the imperial equivalent, BTU per hour per square foot per degree Fahrenheit, is larger by a factor of about 5.678. A U-value already includes the surface films on both sides along with every layer between them, so it is the reciprocal of the total R-value rather than a conductivity — which is why lower is better for a U-value and higher is better for an R-value, a sign convention that reverses halfway through many specifications.