Heat Transfer Coefficient Converter
The convective heat transfer coefficient relates heat flux to a temperature difference, with the SI unit of watts per square meter per kelvin (W/(m2.K)). Engineers in different traditions use kilocalories per hour per square meter per degree Celsius or the US customary BTU per hour per square foot per degree Fahrenheit. This converter reduces any of these to W/(m2.K) as a common base. The key factors are exact: 1 kcal/(h.m2.C) equals 1.163 W/(m2.K) and 1 BTU/(h.ft2.F) equals about 5.678263 W/(m2.K).
Heat transfer coefficient conversion
Base value (W/m2.K) = input * factor(from unit)
Output = base value / factor(to unit)
1 kcal/(h.m2.C) = 1.163 W/(m2.K)
1 cal/(s.cm2.C) = 41,868 W/(m2.K)
1 BTU/(h.ft2.F) = 5.678263 W/(m2.K)
A kelvin and a degree Celsius are the same size, so W/(m2.K) and W/(m2.C) are identical. The BTU factor uses the international table BTU.
Heat transfer coefficient context
- The SI unit is the watt per square meter per kelvin (W/m2.K).
- Because a kelvin equals a degree Celsius in size, per-K and per-C forms are numerically equal.
- Free convection in air is roughly 5 to 25 W/(m2.K); forced convection is higher.
- 1 kcal/(h.m2.C) equals 1.163 W/(m2.K) using the thermochemical calorie of 4.1868 kJ/kcal.
- The BTU here is the international table BTU, giving about 5.678 W/(m2.K) per BTU/(h.ft2.F).
Heat transfer coefficient: frequently asked questions
Why are W/(m2.K) and W/(m2.C) the same?
A temperature difference of one kelvin equals a difference of one degree Celsius, because the two scales share the same unit size and differ only in zero point. So a coefficient expressed per kelvin is numerically identical per degree Celsius.
How is BTU/(h.ft2.F) converted?
It combines the international table BTU (1,055.05585262 J), the hour (3,600 s), the square foot (0.09290304 m2), and the Fahrenheit degree (5/9 K). The net factor is about 5.678263 W/(m2.K).
What is a typical heat transfer coefficient?
Free convection in air is around 5 to 25 W/(m2.K), forced air convection 25 to 250, and boiling or condensing water can exceed several thousand W/(m2.K). Values depend strongly on geometry and flow.
What calorie is used for the kcal factor?
The factor 1.163 W/(m2.K) per kcal/(h.m2.C) uses the international table kilocalorie (4.1868 kJ), divided by 3,600 s, which gives exactly 1.163 watts per kcal per hour.
Is this the same as a U-value?
The overall heat transfer coefficient (U-value) used in building science has the same units, W/(m2.K), so this converter applies to U-values as well as convective film coefficients.
Official sources
- NIST Special Publication 811: Guide for the Use of the International System of Units.
- BIPM: SI Units.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.