What this calculator does
Fittings and valves cost more head than most engineers expect. This calculator takes the K-value of a fitting, the velocity in the pipe and the pipe diameter, and returns the head loss, the pressure drop and the equivalent length of straight pipe that would cause the same loss.
How to use it
- Enter the flow velocity in the pipe — for water, 1.5–2.5 m/s is a sensible design target.
- Enter the pipe diameter and the total K-value for all the fittings on that section.
- Typical K-values: 0.3 for a standard elbow, 0.9 for a 90° elbow, 1.5 for a gate valve and 6.0 for a globe valve.
- Add the losses from every fitting in series, then compare the total against the head available.
Governing formula
h_L = K · V²/(2g) ; L_e = K·D/f Where:
-
K= Loss coefficient of the fittings [–] -
V= Mean velocity [m/s] -
L_e= Equivalent length [m]
Worked example — 2 m/s water through a 100 mm pipe with K = 2.4
The figures below were produced by running this page's own engine with the example inputs (Flow velocity (m/s), Pipe diameter (mm), Total K-value of the fittings (–)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Head loss | 0.49 m |
|---|---|
| Pressure drop | 4.8 kPa |
| Velocity head | 0.2 m |
| Equivalent straight length | 12 m |
| Assessment | Through a K = 2.4 fitting the head loss is 0.49 m (≈ 5 kPa water). Equivalent straight length ≈ 12.0 m at f = 0.02. |
- The velocity head at 2 m/s is 0.20 m.
- With K = 2.4 (say two elbows and a gate valve) the loss is about 0.49 m.
- That is roughly 12 metres of equivalent straight pipe at f = 0.02 — fittings add up quickly.
Assumptions & limitations
Engineering calculation. Uses the K-method for local losses and converts the result to an equivalent straight length at a fixed friction factor of 0.02.
- K-values depend on the fitting geometry, size ratio and Reynolds number; tabulated values assume turbulent flow.
- Equivalent length assumes a Darcy friction factor of 0.02 — for very smooth or very rough pipe the conversion will be off.
- Minor losses are added to the major (frictional) loss; neither includes the static head.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- Crane TP-410 — loss coefficients for fittings and valves.
- ASME B31.3 — process piping design, pressure drop criteria.
- Idelchik, Handbook of Hydraulic Resistance.