Minor loss coefficients (K) for pipe fittings
Reference K coefficients for the velocity-head method of computing local losses in fittings, valves, entrances and exits — the values behind the fittings count in the LOGOS pumping calculators.
The velocity-head method
Every fitting, valve, entrance or exit disturbs the flow and dissipates energy beyond the straight-pipe friction. The classical way to account for it is the K coefficient (resistance coefficient): the loss of each accessory is a multiple of the velocity head,
h_loc = K · v²/(2g)
where v is the mean velocity in the pipe the fitting is installed on and g = 9.81 m/s². For a line, sum the coefficients: h_loc,total = ΣK · v²/(2g) — which is exactly how the LOGOS calculators consume the fittings count you enter per segment.
Reading the table
The K values below are representative single values for clean water in fully open condition (valves) — appropriate for preliminary sizing and for the vast majority of utility lines. Three things to keep in mind:
- Short radius vs long radius matters more than the angle. A short 90° elbow (K = 0.9) loses more than twice a long-radius 90° bend (K = 0.4).
- Valve style is a decade-wide choice. Gate 0.2, swing check 2.5, angle 5, globe 10 — choosing the valve type changes the local loss by a factor of 50.
- Entrances and exits are geometry, not hardware. A protruding (re-entrant) entrance doubles the loss of a sharp-edged one (1.0 vs 0.5); the exit always costs one full velocity head.
K, equivalent length and Cv
Manufacturers publish the same physics in three currencies. K applies directly to the velocity head. Equivalent length (Le/D) converts the fitting to fictitious meters of straight pipe — convenient in hand calculations, but it silently couples the fitting loss to the pipe friction factor. Flow coefficient Cv (or Kv) expresses capacity at a reference pressure drop and is the standard for control valves. For sized control valves, skip K entirely and use the Cv method — that is what the LOGOS control-valve calculators do.
Minor loss coefficients K for fittings and valves
| Fitting / valve | K (–) |
|---|---|
| 90° elbow (short radius) | 0.9 |
| 45° elbow | 0.4 |
| 90° bend (long radius) | 0.4 |
| 45° bend (long radius) | 0.2 |
| Sharp-edged entrance | 0.5 |
| Re-entrant (protruding) entrance | 1 |
| Pipe exit | 1 |
| Sudden expansion | 1 |
| Sudden contraction | 0.5 |
| Gate valve (fully open) | 0.2 |
| Globe valve (fully open) | 10 |
| Angle valve (fully open) | 5 |
| Swing check valve | 2.5 |
| Foot valve with strainer | 1.75 |
| Wye junction | 0.4 |
| Tee, flow through run | 0.6 |
| Tee, flow through branch | 1.8 |
| Tee, bilateral flow | 1.8 |
Representative single values for preliminary sizing, as used by the LOGOS hydraulic calculators. For critical fittings, use the manufacturer’s K or Cv.
Standards & methods
- Crane TP-410 (method)
- Idelchik (method)
- Darcy-Weisbach
Frequently asked questions
Why is a globe valve K = 10 while a gate valve is 0.2?
Geometry. A fully open gate valve leaves an almost straight bore, while a globe valve forces the flow through two 90° turns and an annular seat even when fully open. That is the price of good throttling capability: globe valves control well and lose much; gate valves lose little and control poorly.
Do K values depend on pipe diameter?
Rigorously yes — K of geometrically similar fittings drops somewhat with size and with Reynolds number. Single-value tables like this one are the accepted approximation for preliminary design. For large or critical lines, use the manufacturer’s data (K, Cv or equivalent length).
What is the difference between K and equivalent length (Le/D)?
Two encodings of the same loss: K applies directly in h = K·v²/2g, while Le/D converts the fitting into meters of straight pipe via Le = K·D/f. Equivalent length looks convenient but ties the fitting loss to the friction factor of the pipe, which changes with roughness and Reynolds — the K method is cleaner.
When do minor losses dominate the calculation?
In short runs full of fittings — pump suction manifolds, equipment interconnections, pump houses. As a rule of thumb, when total straight length is below ~50 diameters, the ΣK term usually outweighs distributed friction and deserves more attention than the roughness choice.
Is the pipe exit K = 1.0 always?
Discharging into a tank or atmosphere, yes: the entire velocity head is lost, so K = 1.0 regardless of edge shape. It is one of the most commonly forgotten terms in suction-side NPSH checks.