Formula & Engineering Reference
The major-loss form above is the SI version of Hazen–Williams, with the constant 10.67 valid when Q is in m³/s and D in metres. The calculator converts your flow and diameter into those units before applying it.
| Symbol | Variable | Unit (SI) |
|---|---|---|
| hf | Major (friction) head loss | m |
| hm | Minor (fitting) head loss | m |
| L | Total pipe length (straight + equivalent) | m |
| Q | Volumetric flow rate | m³/s |
| C | Hazen–Williams roughness coefficient | — |
| D | Internal diameter | m |
| v | Mean flow velocity (Q / area) | m/s |
| ΣK | Sum of minor-loss coefficients | — |
| ΔP | Pressure drop (ρ·g·h) | Pa → kPa |
Internal diameter table. DN 15–300 with two bore options: Quick DN (ID = DN, handy for fast tender sizing) and Schedule 40 (the real bore of Sch 40 steel pipe). The selected bore feeds straight into D.
Take 10 L/s of water through a DN 100 line (Quick DN, so ID = 100 mm), 150 m long, new steel at C = 130, with fittings adding up to ΣK = 3.
Q = 0.010 m³/s, D = 0.10 m. Velocity = 0.010 / (π/4 × 0.10²) = 1.27 m/s.
Major loss = 10.67 × 150 × 0.0101.852 / (1301.852 × 0.104.87) = 2.85 m.
Minor loss = 3 × 1.27² / (2 × 9.81) = 0.25 m. Total head loss ≈ 3.10 m, which converts to ΔP = 1000 × 9.81 × 3.10 ≈ 30.4 kPa (about 0.30 bar). A comfortable result for a 150 m run at a sensible velocity.
Using it for anything but water. Hazen–Williams hides viscosity inside the C-factor, which was tuned for cold water. Run oil, glycol, or hot water through it and the answer drifts. For those, switch to Darcy–Weisbach.
Guessing the C-factor. Loss scales with C-1.852, so dropping C from 130 to 100 raises the head loss by roughly 60%. An old, scaled cast-iron main is a very different pipe from a new plastic one — pick C from both the material and its age.
Forgetting the fittings. On a short run full of elbows and valves, minor losses can rival the straight-pipe friction. Add them through ΣK or an equivalent length; don't leave them at zero out of habit.
Confusing Quick DN with the real bore. Quick DN assumes the bore equals the nominal size, which slightly understates loss for thick-walled steel. For steel pipe where the wall matters, use the Schedule 40 bore option.
Reading ΔP without checking velocity. A low pressure drop on an oversized line can still hide an uneconomic pipe; a tidy ΔP on a small line can still sit at an erosive velocity. Always glance at the velocity alongside the loss.
It estimates friction head loss for water flowing full and turbulent through a pipe. Because roughness collapses into one C-factor, it is fast and is the go-to method for water distribution, fire protection, and plumbing.
Roughly 150 for plastic, 140 for copper and cement-lined ductile iron, 130 for new steel and new cast iron, 120 for galvanised, and 100 or less for old tuberculated cast iron. The result is sensitive to C, so reflect both material and age.
There is no viscosity term — the C-factor was calibrated for water near room temperature. For oils, hot water, or any markedly more or less viscous fluid, Darcy–Weisbach is the right tool because it handles viscosity through the Reynolds number.
Either add their equivalent length to the pipe run or enter the summed coefficient ΣK so a minor-loss term ΣK·v²/2g is added. This page supports both; the equivalent-length value can come straight from the fittings calculator.
ΔP = ρ·g·h. For water at 1000 kg/m³, one metre of head equals about 9.81 kPa or 0.098 bar. The calculator reports ΔP in both kPa and bar automatically.
Hazen–Williams Engineering Guide
3 topics • Water friction loss referenceSooner or later every water system comes down to one question: will there be enough pressure left at the far end? Friction in the pipe eats away at the head you started with, and the Hazen–Williams equation is the workhorse that tells you how much. It has been used for water mains, riser stacks, and fire loops for over a century, mostly because it is honest about being simple — one coefficient for roughness, no fuss about temperature or viscosity, and an answer in seconds.
This guide explains what the calculator returns, how the famous C-factor controls the whole result, and where Hazen–Williams stops being the right tool and Darcy–Weisbach takes over. None of it is hard, but a couple of habits separate a number you can defend from one you just typed.