Heat Transfer Coefficient Converter

The heat transfer coefficient measures how effectively heat moves between a solid surface and a surrounding fluid — air, water, oil, or any other gas or liquid — for each degree of temperature difference between them. It's the key quantity that determines how quickly a hot object cools in moving air versus in still air, or how efficiently a heat exchanger transfers energy from one fluid stream to another.

Our Heat Transfer Coefficient converter supports the SI unit, watt/square meter/K, along with watt/square meter/°C and joule/second/square meter/K (both numerically identical to the SI unit, since a kelvin and a Celsius degree are the same size, and a joule/second is a watt by definition). Calorie-based units — calorie (IT)/second/square centimeter/°C and kilocalorie (IT)/hour/square meter or square foot/°C — appear in older thermodynamic references, alongside CHU/hour/square foot/°C (Centigrade Heat Unit). For US HVAC and building science, the converter includes Btu (IT and th)/second or hour/square foot/°F, the standard units on US heat exchanger and radiator specifications.

To use the converter, select your starting and target units from the dropdown lists and enter a value — the result is calculated instantly to up to 12 significant digits. For example, converting 25 W/(m²·K) — a typical forced-air convection value — to Btu/(hr·ft²·°F) gives about 4.40, letting you compare a European heat transfer coefficient directly against a US HVAC equipment datasheet.

Heat transfer coefficients vary enormously depending on the mechanism and fluid involved: natural (free) convection in still air typically gives values around 5-25 W/(m²·K), forced convection with fans or pumped flow raises this to roughly 25-250 W/(m²·K) for gases, and phase-change processes like boiling and condensation can exceed 2,500-100,000 W/(m²·K) — which is why liquid cooling, heat pipes, and refrigeration systems are so much more effective at removing heat than simple air cooling.

These conversions are essential in heat exchanger design (sizing the surface area needed to transfer a target amount of heat), HVAC equipment engineering (radiators, coils, and convector performance ratings), electronics thermal management (heat sink and fan-cooling performance), and building envelope modeling (surface film coefficients account for convective heat exchange at interior and exterior wall and window surfaces). Because heat transfer coefficient specifications mix SI, imperial, and calorie-based units depending on the industry and region, a reliable converter prevents costly design errors when combining data from multiple sources.

All conversion factors used in this tool are drawn from standard, internationally recognized unit definitions, so results are dependable for heat exchanger design, HVAC engineering, and thermal management calculations. The converter is free, requires no account, and works equally well on desktop and mobile devices.