Pipe Size Calculator & Velocity & Friction Loss
Sizes mainlines
Size your irrigation pipe right — from flow, diameter, length and material get the water velocity, the friction head loss in metres and bar, and the recommended pipe diameter.
Enter your pipe & flow
Next: you could downsize toward Ø 92 mm to save on pipe cost while staying near the 1.5 m/s target.
Hazen-Williams head loss; keep mains velocity ~1.5 m/s.
Pipe sizing — key facts
- Target velocity
- ≈ 1.5 m/s
- Velocity
- flow ÷ pipe area
- Method
- Hazen-Williams
- PVC / HDPE C
- ≈ 150
- Old steel C
- ≈ 100
- 1 L/s
- = 3.6 m³/h = 60 L/min
- 1 m head
- ≈ 0.0981 bar
- Privacy
- Runs in your browser; nothing uploaded
The pipe size that saves pumping energy
Pipe diameter is one of the cheapest long-term decisions on a farm and one of the most often got wrong. Too small, and water races through at high velocity, friction losses balloon, and your pump burns extra energy forever just to push against the resistance — plus you risk surge damage. The physics is unforgiving: friction loss rises with about the 4.87th power of diameter, so going one size smaller can multiply the loss many times over.
This tool computes the velocity and Hazen-Williams friction head loss for your flow, length and pipe material, and tells you the diameter that keeps velocity near the ideal 1.5 m/s. Use it to size a new mainline, check why pressure fades at the far end of a long run, or decide between a bigger pipe and a bigger pump. Pair it with the Pump Power and Drip Irrigation tools to design the whole system.
Right-size the main
Get the diameter that holds velocity near 1.5 m/s for your flow.
Cut pumping cost
See the friction head your pump must overcome — and lower it.
Diagnose pressure loss
Check why the far end of a long run loses pressure and flow.
Compare materials
See how PVC vs old steel changes friction via the C value.
Frequently Asked Questions
How do I size an irrigation pipe?+
Pick a diameter that keeps the water velocity around 1.5 m/s and the friction loss acceptable. Velocity = flow ÷ pipe area, and a too-small pipe causes high velocity, big friction losses and water hammer. This tool gives the velocity, the Hazen-Williams head loss and the recommended diameter for your flow.
What is a good water velocity in a pipe?+
Aim for about 0.6–1.5 m/s in mains and submains. Below ~0.6 m/s sediment can settle; above ~1.5–2 m/s friction and wear climb steeply and surge (water hammer) risk rises. The tool flags your velocity as low, good, high or excessive.
What is friction head loss?+
It's the pressure the pump must overcome to push water through the pipe against wall friction, expressed in metres of water (or bar). It rises sharply as diameter falls — halving the diameter raises loss roughly 30-fold — so the right pipe size saves pumping energy for years.
What is the Hazen-Williams formula?+
An empirical equation for head loss in water pipes: hf = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87), with Q in m³/s, D and L in m, and C the roughness coefficient of the pipe material. It's the standard for irrigation and water-supply pipe design, which this tool uses.
What is the C value for my pipe?+
The Hazen-Williams C is higher for smoother pipe: PVC and HDPE ≈ 150, new cast iron and concrete ≈ 130, aluminium ≈ 135, galvanised iron ≈ 120 and old steel ≈ 100. Smoother pipe (higher C) means less friction loss. Pick your material in the tool and it applies the right C.
How do I convert flow units?+
1 litre per second (L/s) = 3.6 m³/hour = 60 L/min. A borewell rated 120 L/min is 2 L/s. Enter the flow in L/s; if you only know L/min or m³/h, divide by 60 or by 3.6 respectively to get L/s first.
Why is my pump pressure dropping along the line?+
Friction loss. Every metre of pipe consumes a little head, and undersized or long runs eat a lot — so the far end gets less pressure and flow. Size the mainline generously, keep velocity ~1.5 m/s, and check the head loss here against your pump's spare head.
Bigger pipe or a bigger pump?+
Usually a bigger pipe. Upsizing one pipe class cuts friction loss dramatically and reduces the pumping head needed forever, lowering energy bills — often cheaper over time than a more powerful pump fighting friction. Use the recommended-diameter figure as a starting point.
Does pipe length matter?+
Yes — head loss is directly proportional to length. A 200 m run loses twice what a 100 m run does at the same flow and diameter. For long mains, upsize the diameter to keep total loss and pumping energy reasonable; the tool's per-100 m figure helps you scale it.
What about fittings and bends?+
Elbows, valves and tees add 'minor losses' on top of pipe friction. A common rule is to add 10–20% to the straight-pipe friction loss to allow for fittings, or compute equivalent lengths for each. This tool gives the straight-pipe loss; add a margin for a fitting-heavy layout.