How to use it
- Type the Flow rate, litres per second, for example 20.
- Type the Inside diameter, mm, for example 150, and the Pipe length, m, for example 300.
- Type the Roughness coefficient C of the pipe material, for example 130 for cast iron.
- Read the Head loss over the pipe, the loss per 100 m and the velocity, then the steps.
The Hazen-Williams formula
Water loses pressure as it rubs along the pipe wall. The Hazen-Williams formula gives that friction loss for water in a pipe under turbulent flow. The EPA's EPANET manual states it in US units, with head loss in feet and flow in cubic feet per second. The formula here is the same equation in metres and cubic metres per second (EPANET 2 Users Manual, section 3.1, Table 3.1).
hL = K × L × q^1.852 ÷ (C^1.852 × d^4.871)
hL = head loss over the pipe, m
K = 10.67 in SI units (the EPA's US form has 4.727)
L = pipe length, m
q = flow, m³/s
d = inside diameter, m
C = roughness coefficient
v = q ÷ (π d² ÷ 4), the mean velocity, m/s- The flow is raised to the power 1.852: twice the flow gives 3.61 times the loss.
- The diameter is raised to the power −4.871: half the diameter gives about 29 times the loss.
- The manual says the formula can be used only for water, and only for turbulent flow.
The roughness coefficient C
C depends on the material of the pipe. It falls as the pipe ages, because corrosion and deposits make the inside rougher, so an old pipe needs a lower value than the table gives for new pipe. The EPANET manual gives these values for new pipe (Table 3.2).
| Material of new pipe | C |
|---|---|
| Cast iron | 130 to 140 |
| Concrete or concrete lined | 120 to 140 |
| Galvanized iron | 120 |
| Plastic | 140 to 150 |
| Steel | 140 to 150 |
| Vitrified clay | 110 |
What this calculator does not include
- Minor losses at bends, tees, valves and fittings. The manual handles them with a loss coefficient on the velocity head, and in a short pipe they can be a large part of the total.
- Liquids other than water, and gases. The formula is for water only.
- Pumps, height changes along the pipe and the pressure needed at the outlet.
Worked example: 20 L/s in 150 mm over 300 m, C 130
The flow is 0.02 m³/s and the diameter 0.15 m, so the flow area is π × 0.15² ÷ 4 = 0.01767 m² and the mean velocity is 0.02 ÷ 0.01767 = 1.13 m/s. The head loss is 10.67 × 300 × 0.02^1.852 ÷ (130^1.852 × 0.15^4.871) = 2.86 m, which is 0.95 m for every 100 m of pipe.
| Figure | Value |
|---|---|
| Flow area | 0.01767 m² |
| Mean velocity | 1.13 m/s |
| Head loss over 300 m | 2.86 m |
| Head loss per 100 m | 0.95 m |
Frequently asked questions
Which C value should I use for a steel pipe?
For new steel pipe the EPA manual gives C between 140 and 150. For an old pipe with deposits, use a lower value, because the table is for new pipe only.
Why is the head loss given in metres?
The loss is the height of a column of water that the friction would use up, so 2.86 m means the water loses as much pressure as a 2.86 m column of water would give.
Does the calculator include bends and valves?
No. Add the losses of bends, tees and valves separately, because in a short pipe with many fittings they can be a large part of the total.
Can I use it for oil or for air?
No. The EPA manual says the Hazen-Williams formula applies only to water, and the calculator follows that rule. Other liquids and gases need a different formula, such as Darcy-Weisbach.
Is my data sent to a server?
No. The calculation runs in your browser, and nothing you type is sent to us or stored.
Sources
Figures and text last checked on Tuesday, November 10, 2026.