Hazen-Williams Pipe Flow Calculator
Hazen-Williams pipe flow calculator. Returns friction head loss and velocity in a pressurised water pipe from the C coefficient, diameter, length and flow. Educational.
Hazen-Williams Pipe Flow Calculator
Please tick the acknowledgement above before calculating.
How to use the Hazen-Williams calculator
Choose the pipe's C coefficient
C describes the pipe's smoothness — high for new plastic (≈150), lower for old cast iron (≈100). It captures roughness without you measuring it directly.
Enter diameter, length and flow
Use the internal diameter in millimetres, the pipe run length in metres, and the flow rate in litres per second. Diameter has a huge effect — it's raised to nearly the fifth power.
Acknowledge the disclaimer
Hazen-Williams applies to water near room temperature in full, pressurised pipes. Tick the acknowledgement to reveal the result.
Read head loss and velocity
You get the friction head loss over the length, the flow velocity, and the loss per 100 m. Keep velocities in a sensible range (roughly 0.6–2.5 m/s) and check the head loss against your available pressure.
Hazen-Williams — sizing water pipes since 1905
An empirical shortcut that stuck
Whenever water flows through a pipe, friction against the pipe wall consumes energy, showing up as a drop in pressure along the run — the friction head loss. Getting it right is the heart of designing any water system, from a building's plumbing to a city's mains: too much loss and the flow or pressure at the far end is inadequate; oversize the pipe to avoid it and you waste money. The Hazen-Williams equation, published in 1905, became the workhorse for this because it is simple and tuned specifically for water. In its SI form, head loss equals 10.67 times the length times the flow raised to the power 1.852, divided by the roughness coefficient C raised to 1.852 and the diameter raised to 4.871. The single coefficient C bundles up the pipe's roughness — around 150 for smooth new plastic, 130 for typical PVC in service, 100 for older cast iron, and lower still for tuberculated pipe — so the designer doesn't have to compute a friction factor from first principles the way the more general Darcy-Weisbach method requires.
Two exponents do the heavy lifting. Flow raised to 1.852 means head loss climbs steeply as you push more water through: roughly quadruple the flow and the loss rises about thirteenfold. Diameter raised to 4.871 means the bore is overwhelmingly the most powerful lever: increasing the diameter by half (say 100 mm to 150 mm) cuts the head loss for the same flow to a small fraction. This is why, when a system runs short of pressure, the cure is almost always a bigger pipe rather than a smoother one.
"Diameter is everything: Hazen-Williams raises it to nearly the fifth power, so a modest increase in bore slashes friction loss. When a water system runs short of pressure, the answer is usually a bigger pipe."
Where it applies — and where it doesn't
Hazen-Williams earns its place by being fast and well-calibrated, but its empirical nature sets boundaries. It was derived for water at ordinary temperatures (around 15 °C) flowing turbulently in full, pressurised pipes, and it loses accuracy outside that envelope: it is not valid for other fluids, for very viscous flow, for hot water far from room temperature, for partially full pipes or open channels, or at very high or very low velocities. For those cases — or where rigour matters — engineers use the Darcy-Weisbach equation with a friction factor from the Moody chart, which is physically based and works for any fluid. Hazen-Williams also gives only the straight-pipe friction loss; fittings, valves, and bends add "minor losses" that must be added separately, often as equivalent pipe lengths. And the chosen C value is an estimate that changes as pipes age and scale internally, so real systems carry a margin. This calculator is an educational and preliminary-design tool: use it to understand how flow, diameter, and roughness interact and to size pipes approximately, and rely on a qualified engineer and the governing code for any system that must perform.
10 Facts About Hazen-Williams
Published in 1905, still widely used for water.
SI: h_f = 10.67·L·Q^1.852 / (C^1.852·D^4.871).
The coefficient C bundles pipe roughness.
C ≈ 150 plastic, 130 PVC, 100 old cast iron.
Diameter is raised to nearly the fifth power.
Flow raised to 1.852 — loss climbs steeply with flow.
Valid for water near 15 °C in full pipes.
Not for other fluids — use Darcy-Weisbach instead.
Fittings add minor losses on top of pipe friction.
Aim for velocities around 0.6–2.5 m/s.
Frequently asked questions
It is an empirical formula for the friction head loss of water flowing in a pressurised pipe. In SI units, head loss = 10.67 × length × flow^1.852 ÷ (C^1.852 × diameter^4.871), with head loss in metres, length and diameter in metres, and flow in m³/s. The coefficient C captures the pipe's roughness. It's popular because it's simple and tuned for water, avoiding the friction-factor calculation that other methods need.
C is a roughness coefficient: higher means smoother and lower friction. Typical values are about 150 for new plastic, 140 for new steel or copper, 130 for PVC in service, 120 for concrete, 100 for older cast iron, and 80 or less for badly corroded or tuberculated pipe. Because pipes roughen and scale as they age, the C value drops over time, so designers often use a conservative figure to allow for the future condition.
Because diameter is raised to nearly the fifth power (4.871) in the denominator. A modest increase in bore slashes the head loss for the same flow — going from 100 mm to 150 mm cuts the loss to roughly a tenth. This is why the most effective remedy for a system that's short of pressure or flow is almost always a larger pipe, not a smoother one. It also means undersizing a pipe is punished severely.
Use Darcy-Weisbach when you need rigour or when the fluid isn't ordinary water — it is physically based and works for any fluid, temperature, and flow regime, using a friction factor from the Moody chart. Hazen-Williams is a convenient shortcut valid only for water near room temperature in turbulent, full-pipe flow. For hot water, oils, gases, viscous flow, or precise engineering, Darcy-Weisbach is the appropriate method.
No. Hazen-Williams gives only the friction loss along the straight pipe. Bends, valves, tees, reducers, and entries and exits add "minor losses" that must be calculated separately, commonly by converting each fitting into an equivalent length of straight pipe and adding it to the run. In systems with many fittings these minor losses can be significant, so they shouldn't be ignored when sizing for available pressure.
A common target for water service is roughly 0.6 to 2.5 metres per second. Too slow and sediment can settle and the pipe may be oversized and costly; too fast and you get high friction loss, noise, water hammer risk, and accelerated erosion or corrosion. The calculator reports the velocity so you can check it against these guidelines while balancing head loss and pipe cost.
Not accurately. The equation was calibrated for water at around 15 °C. Hot water has a different viscosity, and other liquids differ even more, so applying Hazen-Williams to them introduces error that grows the further you stray from cold water. For hot water systems or any non-water fluid, use Darcy-Weisbach with the fluid's actual properties. This tool assumes ordinary cold water.
Within its valid range it is generally good to within about 10–15%, which is fine for most water-distribution sizing. The biggest uncertainty is usually the chosen C value, since real pipe roughness varies and changes with age. The calculator's output is therefore an estimate; a conservative C and a sensible safety margin are used in practice, and final designs are checked by a qualified engineer against the relevant code.
Use it for learning and preliminary sizing. A real water system — plumbing, irrigation, fire protection, or mains — must be designed by a qualified engineer to the governing codes and standards, accounting for minor losses, peak demand, pressure requirements, and future pipe condition. Treat the output as an educational estimate, not a final design.
No. The values you enter are processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
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