Pipe Flow Rate Calculator
The water flow a pipe carries for a given pressure drop over its length — from the diameter and the pipe material's roughness coefficient — by the Hazen–Williams formula, with the velocity, the head loss per meter and the flow at a different diameter.
For water in pressure pipes, engineers use the Hazen–Williams relation: the flow depends on the pipe's roughness coefficient C (about 150 for plastic, 130 for new steel, 100 for old iron), the diameter to the power 2.
How the pipe flow rate calculator works
For water in pressure pipes, engineers use the Hazen–Williams relation: the flow depends on the pipe's roughness coefficient C (about 150 for plastic, 130 for new steel, 100 for old iron), the diameter to the power 2.63 and the hydraulic gradient to the power 0.54. Diameter dominates — the same pressure pushes six times the flow through a pipe twice as wide. Velocity above about 2 m/s is noisy and erosive, below 0.5 lets sediment settle.
Formula: Q = 0.2785 × C × D^2.63 × S^0.54 (Q in m³/s, D in m, S = head loss / length); v = Q / A
Worked examples
| Inputs | Flow rate (L/s) | Note |
|---|---|---|
| A 50 mm plastic pipe, 6 m of head over 60 m | 4.258 | about 4 L/s |
| Old iron | 3.042 | a third less |
| A 25 mm pipe | 0.688 | a sixth of the flow |
FAQFrequently asked questions
What is the Hazen–Williams formula?
An empirical relation between flow, pipe size, roughness and head loss for water at ordinary temperatures. It is the workhorse of water-supply and irrigation design because it needs no iteration.
What C value should I use?
About 150 for plastic and new copper, 140 for new ductile iron, 130 for new steel, 100 for old cast iron, 80 or lower for badly tuberculated pipes. Design uses a value for the pipe's aged condition.
Why does diameter matter so much?
Flow goes as diameter to the 2.63 power at a fixed gradient — nearly the square of the area. Going up one pipe size is almost always cheaper than accepting the pressure loss.
What velocity should I aim for?
Between about 0.6 and 2 m/s: slow enough to avoid noize and water hammer, fast enough to keep the pipe clean. Above 3 m/s plastic pipe erodes and fittings are noisy.
When is Hazen–Williams wrong?
For fluids other than water, for very hot or cold water, and outside turbulent flow. Darcy–Weisbach with a friction factor covers those; for water in supply pipes the two agree closely.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.