W/m2K To U Value Calculator

Convert W/m²K U-values and thermal resistance between metric and imperial units, or solve U = 1/R and R = 1/U for whole building assemblies.

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Calculator Functions

Function	What it does	What it requires
Convert U-value	Converts a U-value between W/(m²·K) and Btu/(h·ft²·°F)	Requires the existing U-value and its unit
U-value from R	Finds the U-value from a total thermal resistance	Requires the total R of the whole assembly in m²·K/W or h·ft²·°F/Btu
R-value from U	Finds the total thermal resistance from a U-value	Requires the U-value of the whole assembly in W/(m²·K) or Btu/(h·ft²·°F)
U-value from conductivity	Estimates the U-value of a flat element built from solid layers	Requires the thickness and thermal conductivity of 1 to 6 layers

The first three functions list the U-value and the R-value in both metric and imperial units with every result. U-value from conductivity takes each thickness in mm, cm, m or in and each conductivity in W/(m·K). Its optional surface resistances start at 0.13 m²·K/W inside and 0.04 m²·K/W outside, and either can be changed or set to 0.

U-Value Formulas

W/(m²·K), also written W/m²K, is the unit of U-value itself [1]. A wall quoted at 0.25 W/m²K has a U-value of 0.25, and nothing needs converting unless the figure is wanted in imperial units or as an R-value. m²·K/W is the unit of thermal resistance, the reciprocal.

Convert U-value

One Btu/(h·ft²·°F) equals 5.678263 W/(m²·K) [2], so a metric U-value is divided by that factor and an imperial one is multiplied by it. 0.25 W/(m²·K) is 0.0440 Btu/(h·ft²·°F). No reciprocal is taken when only the unit changes.

U-value from R

The main formula applies directly: a total R of 2.5 m²·K/W gives U = 0.4 W/(m²·K). An imperial R-value is converted to metric first:

R-15 in h·ft²·°F/Btu is 2.642 m²·K/W, which gives U = 0.379 W/(m²·K).

R (m²·K/W)	U (W/(m²·K))	R (h·ft²·°F/Btu)	U (Btu/(h·ft²·°F))
0.1	10.000	0.57	1.761
0.2	5.000	1.14	0.881
0.25	4.000	1.42	0.704
0.5	2.000	2.84	0.352
1	1.000	5.68	0.176
1.5	0.667	8.52	0.117
2	0.500	11.36	0.088
2.5	0.400	14.20	0.070
3	0.333	17.03	0.059
4	0.250	22.71	0.044
5	0.200	28.39	0.035
6	0.167	34.07	0.029
8	0.125	45.43	0.022
10	0.100	56.78	0.018

R-value from U

A U-value of 0.25 W/(m²·K) is a total R of 4 m²·K/W, or 22.71 h·ft²·°F/Btu. This is the resistance of the same whole assembly that the U-value describes.

U-value from conductivity

Each solid layer has a resistance of its thickness d in metres divided by its thermal conductivity λ in W/(m·K). The layers are added together with the inside and outside surface resistances, and U = 1 ÷ R [1]. The starting values of 0.13 and 0.04 m²·K/W are those for a wall, where heat flows horizontally. For a roof the inside value is 0.10, and for a floor it is 0.17 [1].

A single 100 mm layer with λ = 0.04 W/(m·K) has R = 0.1 ÷ 0.04 = 2.5 m²·K/W. With the wall surface resistances the total is 2.67 m²·K/W and U = 0.375 W/(m²·K).

Limits of This Estimate

• A U-value describes a whole assembly. The R-value of one insulation layer is not the total R of a wall, roof or window.
• U-value from conductivity covers flat elements made of uniform solid layers. Layers bridged by studs, joists or metal fixings need the combined method of BS EN ISO 6946, ground floors and basements are covered by BS EN ISO 13370, and windows and doors by BS EN ISO 10077-1 [1].
• An air gap is not a solid layer and should not be entered as one. Thermal conductivity in W/(m·K) is a property of a material, not a U-value.
• The result does not show compliance with a building code.

Sources