Hazen-Williams Calculator

Single-equation head-loss for cold water. Toggle between new and aged C-factors. Side-by-side with Darcy-Weisbach so you can see the disagreement at a glance.

Last reviewed 2026-05-06

Inputs

Flow rate GPM Inside diameter in Length ft Material Use aged C-factor (145)

PVC (smooth plastic) — Most common potable / irrigation material; very low roughness, ages well · Source: ASHRAE 2021 Ch. 22; AWWA C900

Hazen-Williams pressure drop v 5.11 ft/s — Safe operating velocity

0 psi

Head loss: 0 ft
In bar: 0 bar
C used: 0
Velocity: 0 ft/s

Side-by-side: Hazen-Williams vs Darcy-Weisbach

Same inputs, different physics. If the gap is wide, prefer Darcy-Weisbach.

Hazen-Williams

4.66ft

2.02 psi · empirical, water-only

Darcy-Weisbach @ 20 °C water

4.63ft

2 psi · Re 7.87e+4, f 0.0191

Δ vs HW

-0.6%

Within typical envelope; HW is fine for this case.

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How this works

Hazen-Williams (head loss in m, water): h_f = 10.67 · L · Q^1.852 / (C^1.852 · d^4.87) Q in m³/s, d in m, L in m. The exponents are empirical and the equation is dimensionally inhomogeneous — that's why the constant 10.67 has units.

Hazen-Williams is the standard for plumbing and irrigation work in North America: AWWA, ASPE, and IPC all spec it. Its appeal is being a single closed-form expression — no iteration, no fluid-property table.

The trade-off is that it is empirical and water-only. C bundles roughness, viscosity, and a great deal of fudge factor. Outside the validity envelope (Reynolds 10⁴–10⁷ in clean water near room temperature), the equation drifts from physically grounded values. The cross-check above shows you the disagreement explicitly.

C-factor library — when to use which value

	New	Aged	Use
PVC / CPVC / PEX / HDPE	150	145	New for any reasonable life
Copper	140	130	Aged for 25-year service
Carbon steel	130	100	Aged unless internally lined
Cast iron (unlined)	130	90	Aged — heavy capacity loss
Cast iron (cement-lined)	140	130	New stays close
Concrete (smooth)	130	—	Cement quality dominates

Common questions

When does Hazen-Williams give the wrong answer?

Outside the validity envelope: water far from room temperature, non-water fluids, very low Reynolds (laminar), very rough pipes, or unusual diameters. Hazen-Williams is empirical and bundles ν into the C coefficient; it has no physics for it to extrapolate.

Why not always use Darcy-Weisbach then?

Speed and convention. Hazen-Williams is one closed-form expression, no iteration. AWWA and ASPE plumbing standards still spec it for cold-water domestic and irrigation systems where it agrees with Darcy-Weisbach to within a few percent.

Where do the C-factor numbers come from?

The library cites ASHRAE 2021 Handbook Ch. 22, AWWA M11 (steel), Crane TP-410, and Cameron Hydraulic Data. New vs aged values are documented per material.

Should I size for new or aged C?

Sizing for "new" overstates capacity at end of life. Most municipal and industrial design uses aged C for 25-year service life. The toggle here switches between the two.