Equivalent length in fluid mechanics refers to the concept of representing the combined effect of multiple fittings, bends, valves, or other obstructions in a piping system as an equivalent length of straight pipe. In some cases, minor losses are expressed in terms of an equivalent length of straight pipe that would cause the same head loss. The equivalent length Le​ can be calculated using the following formula,

Where:

• Le​ is the equivalent length of pipe,
• $D$ is the diameter of the pipe,
• $K$ is the loss coefficient,
• $f$ is the Darcy-Weisbach friction factor, which depends on the Reynolds number and the roughness of the pipe wall.