Pipe Size Calculator & Velocity & Friction Loss

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.