Heat Transfer Coefficient Converter

The heat transfer coefficient (h, or U for a whole wall or window) is how much heat passes through each unit of area for each degree of temperature difference across it: Q = U * A * ΔT. Its SI unit is the watt per square meter per kelvin, W/(m²·K). The lower the U-value of a wall or window, the better it insulates.

A difference of 1 K is the same as one of 1 °C, so W/(m²·K) and W/(m²·°C) are the same number. For example, a double-glazed window is about 1.2–2.8 W/(m²·K), and a modern insulated wall about 0.15–0.3.

How to convert heat transfer coefficients

Type a value into any field, and every other unit is calculated at once. US U-values are in Btu/(h·ft²·°F). For example, a window of 1.4 W/(m²·K):

1. In Btu/(h·ft²·°F): divide by 5.67826.

   1.4 / 5.67826 = 0.247 Btu/(h·ft²·°F)

2. In kcal/(h·m²·°C): multiply by 0.859845.

   1.4 × 0.859845 = 1.2 kcal/(h·m²·°C)

Common conversions

The Btu and calorie values are the International Table (IT) units.

Heat transfer coefficient units

• Watt / meter² / K (W/m²/K): The amount of heat in watts transferred through one square meter of material for a temperature difference of one Kelvin.
  Conversion Factor: 1 W/m²/K = 1 W/m²/K (Base Unit)

• Watt / meter² / °C (W/m²/°C): Similar to W/m²/K, but for a temperature difference of one degree Celsius. Because a temperature difference of one Kelvin is equal to a temperature difference of one Celsius, both units are equal.
  Conversion Factor: 1 W/m²/°C = 1 W/m²/K

• Kilocalorie (IT) / hour / meter² / °C (kcal/h/m²/°C): The amount of heat in kilocalories transferred through one square meter of material for a temperature difference of one degree Celsius over an hour.
  Conversion Factor: 1 kcal/h/m²/°C = 1.163 W/m²/K

• Kilocalorie (IT) / hour / foot² / °C (kcal/h/ft²/°C): Similar to kcal/h/m²/°C, but for square feet in the imperial system.
  Conversion Factor: 1 kcal/h/ft²/°C ≈ 12.5184 W/m²/K

• Btu / second / foot² / °F (Btu/s/ft²/°F): The amount of heat in British Thermal Units transferred through one square foot for a temperature difference of one degree Fahrenheit over a second.
  Conversion Factor: 1 Btu(IT)/s/ft²/°F ≈ 20,441.748 W/m²/K; 1 Btu(th)/s/ft²/°F ≈ 20,428.0772 W/m²/K

• Btu / hour / foot² / °F (Btu/h/ft²/°F): The amount of heat in British Thermal Units transferred through one square foot for a temperature difference of one degree Fahrenheit over an hour.
  Conversion Factor: 1 Btu(IT)/h/ft²/°F ≈ 5.6783 W/m²/°C; 1 Btu(th)/h/ft²/°F ≈ 5.6745 W/m²/°C

• CHU / hour / foot² / °C (CHU/h/ft²): The amount of heat in Centigrade Heat Units transferred through one square foot for a temperature difference of one degree Celsius over an hour.
  Conversion Factor: 1 CHU/h/ft² ≈ 5.6783 W/m²/°C

Frequently asked questions

How do you convert a U-value to an R-value?

The R-value is the inverse of the U-value: R = 1 / U. A U-value of 0.25 W/(m²·K) is an R-value of 4 m²·K/W. In US units, R-values are in h·ft²·°F/Btu, and 1 m²·K/W = 5.67826 h·ft²·°F/Btu, so the same wall is about R-22.7.

Is W/(m²·K) the same as W/(m·K)?

No. W/(m·K) is a material's thermal conductivity, and W/(m²·K) belongs to a layer of a given thickness: U = k / thickness. Conductivities are converted in the Thermal Conductivity Converter.