A method used to determine the pressure drop across pipe fittings such as valves, bends, elbows, and T-pieces. The equivalent length of a fitting is that length of pipe that would give the same pressure drop as the fitting. Since each size of pipe or fitting requires a different equivalent length for any particular type of fitting, it is usual to express equivalent length as so many pipe diameters, and this number is independent of pipe. For example, if a valve in a pipe of diameter, d, is said to have an equivalent length, n, pipe diameters, then the pressure drop due to the valve is the same as that offered by a length, nd, of the pipe. Values are determined experimentally.