Hazen–Williams Calculator (Water Pipe Head Loss)

How to use the Hazen–Williams calculator

Enter the flow rate, the pipe internal diameter, its length and the C factor for the pipe material. You get the friction head loss, the equivalent pressure drop for water and the flow velocity. Add fittings with the minor losses calculator.

Hazen–Williams equation (SI)

h_f = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87)

Δp = ρ g h_f ≈ 9.81 kPa per metre of head

Example

50 m³/h of water through 100 m of 100 mm pipe, C = 130:

• Q = 50 ÷ 3600 = 0.01389 m³/s
• h_f = 3.498 m of head
• Δp = 34.30 kPa (4.97 psi)
• Velocity = 1.77 m/s

Head loss per 100 m at 50 m³/h (C = 130)

Pipe ID	Velocity	Head loss	Pressure drop
65 mm	4.19 m/s	28.507 m	279.56 kPa
80 mm	2.76 m/s	10.369 m	101.69 kPa
100 mm	1.77 m/s	3.498 m	34.30 kPa
125 mm	1.13 m/s	1.180 m	11.57 kPa
150 mm	0.79 m/s	0.485 m	4.76 kPa
200 mm	0.44 m/s	0.120 m	1.17 kPa

Typical C factors

Pipe material	C
PVC, HDPE, copper (smooth)	140–150
Cement-lined ductile iron	130–140
New welded steel	120
Galvanised steel	120
Old cast iron	80–100

Frequently asked questions

What is the Hazen–Williams formula in SI units?

h_f = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), with head loss and length in metres, flow in m³/s and diameter in metres.

When should I not use Hazen–Williams?

It is valid only for water at roughly 5–25 °C in turbulent flow. For other liquids, hot water, viscous fluids or gases, use the Darcy–Weisbach equation.

What C factor should I use?

About 150 for plastic pipe, 130–140 for copper and new lined pipe, 120 for new steel and 100 or less for old, tuberculated cast iron. Design codes often specify the value.

How does pipe size affect head loss?

Strongly — head loss varies with d^−4.87, so one size larger pipe often cuts friction loss by more than half.

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