Water hydraulics
Straight-pipe friction loss
- where
- friction pressure loss (Pa; the calculator shows kPa)
- Darcy friction factor
- developed pipe length (m)
- internal pipe diameter (m; any unit you pick is converted)
- fluid density (kg/m³)
- mean velocity (m/s)
Explain this equation
Works out the friction pressure loss along a straight length of pipe. Use it when working out the pump head needed for a circuit.
f is the Darcy friction factor, describing how rough and turbulent the flow is. L / d is the length in pipe diameters. ρ × v² / 2 is the dynamic pressure of the moving water.
Friction loss grows with length and falls with bore, because a bigger pipe has less wall per litre carried. It rises with the square of velocity, since faster flow drags harder. The calculator shows kPa and the matching head in metres.
Sense check: 10 m of 20 mm bore at 1 m/s with f 0.025 loses about 6.3 kPa, about 0.64 m head. Double the velocity and the loss goes up about four times.
Watch out: f must be the Darcy factor, not Fanning, which is a quarter of it. Fittings and static lift are not included. Not for gas pipe sizing.
Enter your values to see the result.
Good to know
Use a justified Darcy factor for Reynolds number and roughness. Fittings and static lift are excluded. This liquid model must not be used to approve gas pipe sizes.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.