Pipe flow & head loss calculator
Work out the friction head loss and flow velocity in a water pipe from its diameter, length, flow rate and material — using the standard Hazen-Williams equation.
Pick a material preset (HDPE, PVC, GI, DI or CI) for the roughness coefficient, and the tool flags whether your velocity sits inside the usual 0.6–3 m/s window. A planning-level tool, computed in your browser.
Pipe parameters
Internal bore of the pipe.
Developed length of the run.
Design discharge through the pipe.
Roughness of the pipe wall: smoother pipe → higher C → less loss.
Head loss (friction)
3.66 m
Friction loss over 500 m of pipe
Velocity
1.13 m/s
Flow rate
20.0 L/s
Velocity
1.13 m/s
Hydraulic gradient
7.31 ‰
Loss per 1 km
7.31 m
Within the usual 0.6–3 m/s range for water mains.
| Head-loss equation | hf = 10.67·L·Q^1.852 / (C^1.852·D^4.87) |
| Converted units | D = 0.150 m · Q = 0.0200 m³/s · L = 500 m |
| Pipe cross-section area | π·D²/4 = 0.01767 m² |
| Velocity | 0.0200 / 0.01767 = 1.13 m/s |
| Friction head loss | 3.66 m |
The Hazen-Williams formula is an empirical relation calibrated for water near 15 °C in pressurised pipes flowing full. It does not account for minor (fittings/valve) losses, which you add separately, nor for very high or very low velocities where it loses accuracy. For non-water fluids or wide temperature ranges, use the Darcy-Weisbach equation instead. A planning-level estimate — verify against the current pipe standard.
From flow and pipe size to metres of head
Friction loss grows steeply with flow and falls sharply with diameter; the material's smoothness (C) sets how much the pipe wall resists the water.
Convert units
Diameter goes from mm to m and flow from L/s to m³/s, so the Hazen-Williams formula sees consistent SI units.
Head loss
hf = 10.67·L·Q^1.852 / (C^1.852·D^4.87). A smoother pipe (higher C) and a larger bore both cut the loss.
Velocity check
Velocity = Q / (π·D²/4). The tool flags whether it lands in the usual 0.6–3 m/s range for water mains.
Pipe head loss, answered
What is the Hazen-Williams formula for head loss?+
Head loss hf = 10.67 · L · Q^1.852 / (C^1.852 · D^4.87), where L is the pipe length in metres, Q is the flow in m³/s, D is the inside diameter in metres and C is the Hazen-Williams roughness coefficient. This calculator converts your flow from litres per second and your diameter from millimetres before applying the equation.
What Hazen-Williams C value should I use for each pipe material?+
Typical design values are about C = 150 for new HDPE and PVC (very smooth), 130 for ductile iron (DI) and cast iron (CI) with a cement-mortar lining, and around 120 for galvanised iron (GI). Older or tuberculated pipes use lower C values. A higher C means a smoother wall and less friction loss.
What is a good flow velocity in a water pipe?+
For water mains a velocity of roughly 0.6 to 3.0 m/s is generally acceptable. Below about 0.6 m/s sediment can settle; above about 3 m/s friction loss, water-hammer (surge) and wear rise sharply. This tool flags whether your velocity falls inside that window.
How is velocity calculated from flow and diameter?+
Velocity = Q / A, where A is the pipe cross-sectional area π·D²/4. With Q in m³/s and D in metres the result is in metres per second. Doubling the diameter quarters the velocity for the same flow.
When should I use Darcy-Weisbach instead of Hazen-Williams?+
Hazen-Williams is an empirical relation calibrated for water near normal temperatures in full, pressurised pipes, which makes it convenient for water supply and irrigation design. For other fluids, wide temperature ranges, very high or very low velocities, or where accuracy is critical, use the physically based Darcy-Weisbach equation with a friction factor.
Does this calculator include fitting and valve losses?+
No. It computes friction (major) loss along the straight pipe only. Minor losses from bends, tees, valves and entries/exits are added separately, usually as equivalent pipe lengths or K-factors times the velocity head.
Sources & data note
Based on the empirical Hazen-Williams head-loss relation hf = 10.67·L·Q^1.852 / (C^1.852·D^4.87), with velocity = Q/(π·D²/4). C-values (HDPE/PVC ≈ 150, DI/CI ≈ 130, GI ≈ 120) and the 0.6–3 m/s velocity window are typical design values — confirm against the manufacturer's data and the current pipe standard for your project. This is a planning estimate, not a substitute for detailed hydraulic design.