Thermal conductivity is a characteristic of a material that represents the amount of heat (in Watts) transmitted through the material with a thickness of one meter due to the temperature change of one degree Celsius. The U value is a related quantity that represents the amount of heat lost per square meter of a material. Thus, the U value characterises the quality of the thermal insulation of materials used, for example, for walls or windows. The lower U-value corresponds to a material's greater resistance to the heat flow.

Retrieve the thermal conductivity for the material of interest from a table such as the one at the Engineering Toolbox website. For example, the thermal conductivity of glass is 0.9.

Measure the material thickness with a ruler or get this value from the material specifications; for example, the glass thickness is one-quarter (0.25) inch.

Multiply the thickness in inches by 0.0254 to convert it to meters. In this example, the thickness is 0.25 x 0.0254 = 0.00635 meters.

Divide the thermal conductivity by the material thickness (in meters) to calculate the U value. In this example, the U value is 0.9 / 0.00635 = 141.73.