Equivalent Length of a Fitting
Enter your known values, leave one input blank, and solves for the missing one. Try different units for next level excitement!
Learning zone
Set Darcy–Weisbach equal to the minor-loss equation — f(L/D)(v²/2g) = K(v²/2g) — and the velocity heads cancel, leaving L_eq = KD/f. The point is bookkeeping: rather than tracking two different loss equations, a designer converts every valve and fitting into a phantom length of pipe, adds it to the measured run, and computes one friction loss for the whole circuit. A 90° elbow (K = 0.9) on 4 in pipe with f = 0.02 is worth 0.9 × 0.333/0.02 = 15 ft of straight pipe.
Because f cancels out of nothing here, the equivalent length depends on the pipe's friction factor — which is why the old rule "an elbow equals 30 diameters" is only true near f = 0.03. On smooth plastic with f = 0.018 the same elbow is worth 50 diameters. Fire-protection and refrigeration codes finesse this by publishing fixed equivalent-length tables for a nominated schedule and material; those tables are fine inside their intended context and misleading outside it.
- = Equivalent length
- = Resistance coefficient
- = Inside diameter
- = Darcy friction factor
- Equivalent length — Hazen–Williams Head Loss, Pipe Internal Volume
- Resistance coefficient — Minor Loss from K Factor, Valve Flow Coefficient (Cv)
- Inside diameter — Hazen–Williams Head Loss, Pipe Internal Volume
- Darcy friction factor — Darcy–Weisbach Head Loss, Laminar Friction Factor (f = 64/Re)
Pipe friction losses
9 formulasDarcy–Weisbach, Hazen–Williams, friction factors, minor losses and equivalent length — every way the trade computes head loss in pipe.
Valve sizing coefficients
4 formulasCv, Kv, K factors and equivalent length — the four coefficients that turn a valve or fitting into a pressure drop at a given flow.