Equivalent Length of a Fitting

L_{eq} = \frac{K D}{f}

Worked example: 6 m equivalent on 100 mm pipe at f 0.025 → K = 1.5 — press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Try different units for next level excitement!

Here the solver did the work — could you?

Minor losses →

UniversityFluid Mechanics, HVAC & Refrigeration

Test your skills in the Exam Room: new numbers every attempt — free lessons for students, no sign-up, just pure learning.

See your Report Card

Compete with your friends
share your results

Learning zone

Equivalent Length of a Fitting explained

Set Darcy–Weisbach equal to the minor-loss equation — f(L/D)(v²/2g) = K(v²/2g) — and the velocity heads cancel, leaving $L_{\mathrm{eq}} = KD/f$ . 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 of a Fitting formula

L_{eq} = \frac{K D}{f}

Where

$L_{eq}$ = Equivalent length (m)
$K$ = Resistance coefficient
$D$ = Inside diameter (mm)
$f$ = Darcy friction factor

Missing one of these? Work it out first, then come back

Equivalent length — Rectangle Perimeter, Rectangle Diagonal
Resistance coefficient — Minor Loss from K Factor, Mole Ratio from a Balanced Equation
Inside diameter — Hazen–Williams Head Loss, Pipe Internal Volume
Darcy friction factor — Darcy–Weisbach Head Loss, Laminar Friction Factor (f = 64/Re)

Part of these formula sets

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.