Heat Transfer Coefficient Converter
About Heat Transfer Coefficient Converter
The heat transfer coefficient (commonly denoted h) measures how effectively heat moves between a solid surface and an adjacent fluid, or between two fluids separated by a wall, through convection or conduction. It is defined by Newton's law of cooling, Q = h × A × ΔT, where Q is the rate of heat transfer in watts, A is the contact surface area, and ΔT is the temperature difference driving the heat flow. The SI unit is the watt per square meter per kelvin, W/(m²·K), and because a kelvin and a degree Celsius represent the same size temperature step, W/(m²·K) and W/(m²·°C) are numerically identical.
Watt/square meter/K [W/(m²·K)] is the base SI unit used throughout scientific literature, CFD simulations, and modern engineering datasheets. It is the natural reference point for this converter and the unit every other supported unit is ultimately expressed against. Joule/second/square meter/K is dimensionally and numerically identical to W/(m²·K), since a watt is defined as exactly one joule per second — the two labels are simply different ways of writing the same physical unit, occasionally used interchangeably in physics texts.
Calorie (IT)/second/square centimeter/°C is a CGS-based unit found in older thermodynamics and heat transfer textbooks, particularly from European and Japanese sources. One calorie (IT) per second per square centimeter per degree Celsius equals 41,868.00000482 W/(m²·K) — an enormous multiplier that reflects the very small area (cm² instead of m²) in the denominator. This unit still appears in legacy calorimetry and materials-testing literature.
Kilocalorie (IT)/hour/square meter/°C is a metric-but-non-SI unit historically common in European and Asian HVAC and boiler engineering. 1 kcal(IT)/(h·m²·°C) = 1.163 W/(m²·K) exactly. A radiator or heat exchanger surface once specified as 400 kcal/(h·m²·°C) corresponds to 400 × 1.163 = 465.2 W/(m²·K) in modern SI terms — a conversion still needed when working with equipment manuals from before SI unification. Kilocalorie (IT)/hour/square foot/°C combines this same energy unit with an imperial area, equal to 12.5184278205 W/(m²·K), and turns up in mixed-unit industrial specifications.
Btu (IT)/hour/square foot/°F and its thermochemical counterpart Btu (th)/hour/square foot/°F are the standard units for heat transfer coefficients throughout US mechanical and HVAC engineering. 1 Btu(IT)/(h·ft²·°F) = 5.6782633411 W/(m²·K), while 1 Btu(th)/(h·ft²·°F) = 5.6744658974 W/(m²·K) — the tiny difference stems from the IT calorie (4.1868 J) versus the thermochemical calorie (4.184 J) that underlie the respective Btu definitions. A typical forced-air HVAC coil rated at 25 Btu/(h·ft²·°F) corresponds to roughly 142 W/(m²·K).
Btu (IT)/second/square foot/°F and Btu (th)/second/square foot/°F use the same imperial units but express the rate per second instead of per hour, making the numeric values 3,600 times larger: 20,441.748028012 W/(m²·K) and 20,428.077230618 W/(m²·K) respectively. These per-second variants appear in high-heat-flux applications such as rocket nozzle cooling and combustion research, where hourly rates would be inconveniently large.
CHU/hour/square foot/°C uses the Centigrade Heat Unit, an older hybrid unit equal to the heat needed to raise one pound of water by one degree Celsius. Because the pound-mass and Celsius-degree scaling happen to cancel the Fahrenheit-degree scaling used in the Btu unit, 1 CHU/(h·ft²·°C) is numerically identical to 1 Btu(IT)/(h·ft²·°F): both equal 5.6782633411 W/(m²·K). This unit occasionally appears in older British engineering references.
This heat transfer coefficient converter supports watt/square meter/K, watt/square meter/°C, joule/second/square meter/K, calorie (IT)/second/square centimeter/°C, kilocalorie (IT)/hour/square meter/°C, kilocalorie (IT)/hour/square foot/°C, Btu (IT)/second/square foot/°F, Btu (th)/second/square foot/°F, Btu (IT)/hour/square foot/°F, Btu (th)/hour/square foot/°F, and CHU/hour/square foot/°C. All conversions are instant, free, and precise to 12 significant digits — suitable for heat exchanger sizing, HVAC coil selection, radiator design, and electronics thermal management.
Frequently Asked Questions — Heat Transfer Coefficient
Question: What is the heat transfer coefficient?
Answer: The heat transfer coefficient (h) quantifies the rate of heat exchange between a solid surface and a surrounding fluid (or between two fluids across a wall) by convection or conduction. It is defined by Newton's law of cooling: Q = h × A × ΔT, where Q is the heat transfer rate (watts), A is the surface area, and ΔT is the temperature difference between the surface and the fluid. The SI unit is watt per square meter per kelvin, W/(m²·K).
Question: How do I convert kcal(IT)/(h·m²·°C) to W/(m²·K)?
Answer: Multiply by 1.163. 1 kcal(IT)/(h·m²·°C) = 1.163 W/(m²·K). Example: a heat exchanger surface rated at 500 kcal/(h·m²·°C) equals 500 × 1.163 = 581.5 W/(m²·K). This conversion is common when comparing older European/Asian equipment specs (given in kcal/h units) with modern SI-based datasheets.
Question: How do I convert Btu(IT)/(h·ft²·°F) to W/(m²·K)?
Answer: Multiply by approximately 5.6783. 1 Btu(IT)/(h·ft²·°F) = 5.6782633411 W/(m²·K). Example: an HVAC coil with a film coefficient of 50 Btu/(h·ft²·°F) equals 50 × 5.6783 ≈ 283.9 W/(m²·K). US mechanical engineering datasheets almost always quote h in Btu/(h·ft²·°F), so this conversion is essential for international HVAC and heat exchanger design.
Question: What is a typical range of values for the heat transfer coefficient?
Answer: Typical values vary hugely by mechanism: natural convection in air ≈ 5-25 W/(m²·K); forced convection in air ≈ 10-500 W/(m²·K); forced convection in water ≈ 500-10,000 W/(m²·K); and boiling or condensing water ≈ 3,000-100,000 W/(m²·K). This wide range is why selecting the right cooling medium and flow regime is critical in heat exchanger and electronics cooling design.
Question: Why does the Btu (IT) value differ from the Btu (th) value?
Answer: The International Table (IT) Btu and thermochemical (th) Btu are defined from slightly different calorie standards (4.1868 J vs 4.184 J respectively), so units built from them differ by about 0.07%. For example, 1 Btu(IT)/(s·ft²·°F) = 20,441.748 W/(m²·K) while 1 Btu(th)/(s·ft²·°F) = 20,428.077 W/(m²·K). The difference is small but matters in precision thermal engineering and metrology.
Question: What is the CHU/(h·ft²·°C) unit?
Answer: The Centigrade Heat Unit (CHU) is an older imperial-metric hybrid unit of energy equal to the heat needed to raise 1 pound of water by 1°C. The CHU/(h·ft²·°C) heat transfer coefficient unit numerically equals the Btu(IT)/(h·ft²·°F) coefficient (both equal 5.6782633411 W/(m²·K)), because the pound-mass and degree-scale ratios cancel out. It appears mainly in older British engineering literature.
Question: How is the heat transfer coefficient used in real engineering design?
Answer: Engineers use h to size heat exchangers, radiators, condensers, and HVAC coils via Q = h·A·ΔT, and to combine it with wall conduction resistance into an overall heat transfer coefficient (U-value) for a full assembly. In electronics cooling, h determines whether natural convection, a fan, or liquid cooling is required to keep a chip below its maximum junction temperature.
Question: What units does this heat transfer coefficient converter support?
Answer: This converter supports watt/square meter/K, watt/square meter/°C, joule/second/square meter/K, calorie (IT)/second/square centimeter/°C, kilocalorie (IT)/hour/square meter/°C, kilocalorie (IT)/hour/square foot/°C, Btu (IT)/second/square foot/°F, Btu (th)/second/square foot/°F, Btu (IT)/hour/square foot/°F, Btu (th)/hour/square foot/°F, and CHU/hour/square foot/°C. All conversions are instant and accurate to 12 significant digits.