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Engineering Toolbox/Equivalent Length / K Conversion

Pressure, Level & Process

Equivalent Length / K Conversion

Convert a Darcy-based fitting loss coefficient into the length of straight pipe that produces the same friction loss, or convert an equivalent length back to K.

Equivalent-length calculation

Use a Darcy friction factor appropriate to the same flow and pipe basis as the K-value.

Use the Darcy/Moody factor, not the Fanning factor.

Field use

Translate local losses into a straight-pipe equivalent.

Useful when a hydraulic worksheet or line-loss method uses equivalent pipe length rather than summing individual K coefficients.

Calculation basis

K = f · Le / D
Le = K · D / f

This is the Darcy-Weisbach equivalence between local-loss coefficient and straight-pipe friction loss.

Equivalent length is not an intrinsic fixed property of every fitting: the result depends on the K-value and Darcy friction factor used. Keep the coefficient source, pipe basis and flow regime consistent. Do not mix Darcy and Fanning friction factors.

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