In the calculation of the overall heat transfer coefficient (or the U-value), both the convective and the conductive resistance that exists between two different fluids that are divided by a wall are considered. The total of the two separate resistances is the reciprocal of that coefficient. The coefficient may be utilized to determine the total transfer of heat through a heat exchanger construction or through a wall. The U-value may give an indication of which heat exchanger is giving the most inefficient heat transfer. The formula can be adjusted to determine which heat exchanger requires attention.

The amount of fouling that occurs in a heat transfer process is factored in when calculating the coefficient. The rate at which heat is transferred will be reduced by the degree to which scale or film is deposited on the surface through which heat will be transferred. Other thermal resistances to the heat transfer may occur because of the formation on rust, impurities in the fluids, and reactions that may occur between the wall and the fluid.

The factors that affect the sizes of the different coefficients include, the flow rates of the fluids, physical structure of a heat transfer region, the fluids' properties, and the type of heat transfer involved.

After the area of heat transfer has been decided on, and then the layout can be determined. Because of that, designing the heat exchanger becomes an interactive process. At the beginning of this process, the selection of typical values of the coefficient is chosen for a number of different types of heat exchangers.

It is not a simple process to accurately calculate the different coefficients, and quite often this is because various important factors are unknown. Because of this difficulty, typical and established values of the overall coefficient are appropriate for most practical purposes.