How to use it
- Choose what to calculate: the head loss for a known flow, or the flow from the available pressure.
- Choose the pipe material. For polyethylene (PE), choose the outside diameter and the SDR; the calculator works out the internal diameter. For other materials, enter the internal diameter.
- For head loss, enter the design flow. For flow, enter the pressure at the start and the pressure required at the end, or 0 if the water flows out freely.
- Enter the pipe length and, if the pipe climbs or drops, the rise.
- Enter the number of bends of each angle, and the sum of K for any other fittings such as tees and valves.
Worked example
A PE100 main, 110 mm SDR 11, has a 10.0 mm wall and a 90 mm internal diameter. It carries 10 L/s over 500 m with four 90° bends and two 45° bends. The velocity is 1.57 m/s, and friction loses 12.20 m of head. The bends add ΣK = 4 × 0.75 + 2 × 0.35 = 3.7, so the local loss is 3.7 × 1.57² ÷ (2 × 9.81) = 0.47 m. The total head loss is 12.66 m, a pressure drop of about 1.24 bar (124 kPa).
Calculating the flow from pressure
Take the same pipe and bends, with 4 bar at the start, 1 bar required at the end, and the end 5 m higher than the start. The available head is (400 − 100) ÷ 9.81 − 5 = 25.58 m. The calculator finds the flow at which friction and bend losses use up exactly that head: about 14.7 L/s (53 m³/h), at 2.31 m/s.
In US units, 150 gpm through 1,500 ft of 4 in PVC loses about 18.3 ft of head in friction, or 7.9 psi, at 3.8 ft/s.
Why SDR matters
SDR is the ratio of the outside diameter to the wall thickness. A lower SDR means a thicker wall and a higher pressure rating, but a smaller bore: a 110 mm pipe has a 90 mm bore at SDR 11 and a 97 mm bore at SDR 17. Because friction loss depends on the diameter to the power 4.87, that difference raises the friction loss by about 44%.
Choosing a C value
The Hazen–Williams C factor describes how smooth the pipe is. New plastic pipe is around 150, cement-lined ductile iron around 130 to 140, and steel around 120. Old unlined cast iron can drop to 100 or less as it corrodes, which is why older networks lose more pressure than their size suggests.
Frequently asked questions
What velocity should water flow at in a main?
Designers commonly aim for about 0.5 to 2.5 m/s (1.6 to 8 ft/s). Slower flow lets water sit and age in the pipe, and faster flow adds noise, wear and water hammer. Your water utility may set its own limits.
Can I calculate the flow if I know the pressure?
Yes. Choose Flow from available pressure, then enter the pressure at the start, the pressure you need at the end and the level difference. The calculator gives the flow and velocity the pipe can deliver.
Does the calculator include bends and valves?
Yes. Enter the number of bends of each angle, and add the loss coefficients of tees, valves and other fittings as a single ΣK value. The calculator uses standard bend coefficients, or your own K values if you choose custom values.
What does SDR mean for PE pipes?
Standard dimension ratio: the outside diameter divided by the wall thickness. For PE100, SDR 11 is rated PN16 and SDR 17 is rated PN10.
How do I convert head loss to pressure?
One metre of water head is 9.81 kPa, and one foot is 0.433 psi.
When is Hazen–Williams not suitable?
It is meant for water at normal temperatures in pipes of roughly 50 mm and larger. For other fluids, very small pipes or high velocities, use the Darcy–Weisbach equation instead.