Pipe Flow Calculator
Every pipe has a comfortable flow range. Push too much water through a small pipe and velocity rises, friction loss explodes, and heads mist or fog. This tool checks velocity and pressure loss per 100 ft of pipe.
Inputs
Result
Planning estimate - not a substitute for a pressure-tested engineering design.
Formula
Velocity (ft/s) = 0.4085 × Q ÷ d² (Q in GPM, d in inches). Friction loss per 100 ft from Hazen-Williams: hf = 452 × Q1.85 ÷ (C1.85 × d4.87) feet of head, ÷ 2.31 for PSI.
Example
10 GPM in 1-inch PVC (d = 1.049, C = 150): velocity ≈ 3.7 ft/s (fine), loss ≈ 3.6 PSI per 100 ft. The same flow in 3/4-inch pipe would exceed 6 ft/s - too fast for a main line.
Assumptions
- Full pipe, steady flow, water at normal temperature.
- Sleeves, glue joints and fittings add extra loss - treat this as the straight-pipe baseline.
Limitations
This tool is a planning aid for DIY design and pre-design work. It does not replace an on-site pressure and flow test, a catch-cup uniformity test, or local code and backflow requirements. For steep slopes, long pipe runs, large elevation changes or commercial-scale systems, have a qualified irrigation designer verify the plan before installation.
What This Means
Designers commonly keep lateral velocity under 5 ft/s and main-line loss low so distant heads still get working pressure. Velocity limits also protect against water hammer when valves snap shut.
Next Step
Turn a velocity check into a pressure budget with the GPM to PSI Calculator, and size zones with the Sprinkler Zone Calculator.
Related Guides
Sources
Frequently Asked Questions
What velocity should irrigation pipe run at?
Most irrigation design guides recommend under 5 ft/s in supply lines and laterals to limit friction loss and water hammer.
Does pipe length change velocity?
No - velocity depends on flow and diameter only. Length changes friction loss, not speed.