Used in heat transfer calculations, it is a measure of the overall transfer of heat through convective and conductive barriers. It is expressed as the heat flux divided by the difference between the temperature in the bulk of two fluids. Since many types of heat transfer equipment use pipes and tubes, it is necessary to specify the area of heat transfer, where:

where h_i and h_o are the inside and outside surface heat transfer coefficients, x is the tube wall thickness, and k is the thermal conductivity of the wall. If the outside area is specified, the overall heat transfer coefficient is based on the outside area and is written as U_o, while if the inside area is specified, then the coefficient is denoted by U_i. The SI units are W m⁻² K⁻¹.