Irrigation Flow Rate & Pressure Loss Calculator
Estimate water velocity, friction head loss and pressure loss across irrigation mainlines and sub-mains from pipe inside diameter, length and system flow.
Calculate Pipe Velocity and Friction Loss
Enter the pipe’s actual inside diameter—not its nominal size. Results estimate straight-pipe friction using the Hazen–Williams equation.
How the Estimate Works
The calculator converts every entry to SI units and applies the Hazen–Williams equation for water flow in a full circular pipe.
Equation inputs
- hf = friction head loss in meters
- L = pipe length in meters
- Q = flow in cubic meters per second
- C = Hazen–Williams roughness coefficient
- d = actual inside diameter in meters
What the answer includes
“Straight-pipe” results cover wall friction only. “Adjusted” results add the percentage allowance entered in the calculator.
Pressure loss is calculated from water head using 9.80665 kPa per meter of head.
Hazen–Williams is an empirical water-pipe method. It can understate loss in some very small drip hoses, so use manufacturer friction tables for drip laterals and emitters. This tool is intended mainly for irrigation mains and sub-mains.
Three Details That Change the Result
Use inside diameter
Nominal pipe size is a product label. Wall thickness and pressure class change the actual bore, and the equation is highly sensitive to diameter.
Match pipe condition
The C factor represents hydraulic roughness. Confirm an appropriate design value for material, size, age and expected internal condition.
Add components separately
Valves, filters, meters, fittings and elevation can materially change required inlet pressure. Use supplier data whenever it is available.
Do not treat friction loss as total dynamic head. A complete pump or zone calculation also considers elevation, required downstream pressure, component losses and an appropriate design margin.
Flow and Pressure Loss FAQ
What is friction head loss?
It is the loss of mechanical energy caused by water rubbing against the pipe wall. It increases with flow and length and usually decreases sharply as inside diameter increases.
Should I enter nominal pipe size or inside diameter?
Enter actual inside diameter. Pipes with the same nominal size can have different bores because wall thickness and pressure class differ.
Which Hazen–Williams C factor should I use?
Use a design value appropriate to the pipe material, size, age and condition. The default 150 is only a starting input and should be checked against your project specification or supplier data.
Does this calculator include fittings and elevation?
It calculates straight-pipe friction and can add a simple percentage allowance. It does not directly calculate elevation head or individual valve, filter, meter and fitting losses.
Why does a small diameter change pressure loss so much?
Diameter is raised to the power of 4.87 in this Hazen–Williams form, so even a modest reduction in actual bore can produce a large increase in estimated friction loss.
Need Help Selecting Irrigation Components?
Share your flow, pipe size, pressure and field layout with the Trex Irrigation team.
Technical references: University of Florida IFAS — Hydraulic Considerations; USDA NRCS — Microirrigation Handbook; Washington State University — Pipeline Pressure Loss.
This calculator is for preliminary planning, not a stamped hydraulic design. Verify final inputs, component losses, pressure ratings and operating conditions with qualified project personnel and manufacturer data.