What this calculator does
Hazen-Williams estimates friction loss in water pipes from the flow, diameter and a roughness coefficient C. Five litres per second through 50 metres of 100 mm pipe at C = 130 loses 0.263 metres of head.
The appeal is that C describes the pipe directly and needs no Reynolds number or iteration, unlike Darcy-Weisbach. The cost is that it is purely empirical and only valid for water at ordinary temperatures in turbulent flow. Use it for water distribution and fire systems; use Darcy-Weisbach for anything else.
The formula
The SI form of the formula gives head loss as 10.67 times the length times flow to the power 1.852, divided by C to the power 1.852 and diameter to the power 4.87. The diameter exponent of 4.87 is what makes pipe size so dominant. Velocity is reported alongside, since most design standards constrain it as well as the head loss.
| Term | Meaning |
|---|---|
| C coefficient | A roughness value for the pipe. Higher is smoother: 150 for plastic, 130 for new steel, 80 for corroded iron. |
| Head loss | Pressure lost to friction, expressed as metres of water column. |
| Darcy-Weisbach | The physically based alternative, valid for any fluid but needing a friction factor. |
| Design velocity | Usually kept between 0.6 and 3 m/s to avoid sedimentation at one end and noise at the other. |
The inputs explained
| Field | What to enter |
|---|---|
| Flow rate (L/s) | Flow rate in litres per second. |
| Internal diameter (mm) | Internal diameter in millimetres, not the nominal size. |
| Pipe length (m) | Pipe length in metres, along the run. |
| Hazen-Williams C | Hazen-Williams C for the pipe material and condition. It falls as pipes age and corrode. |
When to use it
Sizing a water main
Checking whether a candidate diameter keeps head loss and velocity within the design limits.
Diagnosing low pressure
Comparing calculated loss against measured pressure drop can reveal an undersized or degraded pipe.
Allowing for ageing
Running the same pipe at a lower C shows how much capacity corrosion will cost over time.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does pipe condition cost?
The same flow through pipes of different roughness.
| Hazen-Williams C | Head loss | Head loss per 100 m | Pressure drop |
|---|---|---|---|
| C = 150 | 0.202 m | 0.404 m | 2.0 kPa (0.29 psi) |
| C = 130 | 0.263 m | 0.527 m | 2.6 kPa (0.37 psi) |
| C = 100 | 0.428 m | 0.856 m | 4.2 kPa (0.61 psi) |
| C = 80 | 0.647 m | 1.295 m | 6.3 kPa (0.92 psi) |
Questions
When should I use Hazen-Williams rather than Darcy-Weisbach?
For water at ordinary temperatures in turbulent flow, which covers most water distribution and fire protection work. Darcy-Weisbach is the physically general method and should be used for other fluids, for unusual temperatures, or where laminar flow is possible.
What C value should I use?
150 for plastic, 140 for cement-lined ductile iron, 130 for new steel as a common design default, and 100 or lower for older tuberculated pipe. Designing with a lower C than the pipe currently has builds in an allowance for ageing over the asset life.
Why is diameter so influential?
Because it appears to the power 4.87. Halving the diameter multiplies head loss by roughly 29 at the same flow. This is why going one pipe size up is often the cheapest way to solve a pressure problem, and why undersizing is so costly to live with.
Does this include fittings?
No, it covers straight pipe only. Bends, valves and tees add minor losses, usually handled either by an equivalent-length addition to the pipe run or by a K-factor method. On a short run with many fittings, those losses can exceed the straight-pipe friction.
For the Darcy-Weisbach approach, see the friction loss calculator. For the friction factor itself, see the Darcy friction factor calculator.