Water Pipe Sizing Calculator
Bore from flow and velocity, with actual velocity and friction loss.
Inputs
Result
Formula
| Area | Q ÷ v, with Q in m³/s |
| Bore | √(4A ÷ π) |
| Actual velocity | Q ÷ (π d²/4) |
| Friction | λ × (L ÷ d) × (v² ÷ 2g) |
| Minor losses | friction × share |
| Per 100 m | loss ÷ length × 100 |
1.5 L/s at 1.2 m/s: 1250 mm² required, a 39.9 mm bore, so DN40 at an actual 1.19 m/s — 3.61 m of friction over 100 m and 4.69 m including 30% minor losses.
FAQ
Why keep velocity between 1.0 and 1.5 m/s?
It balances capital and comfort. Lower velocity means a bigger, dearer pipe; higher velocity raises friction loss with the square of velocity, pushing up pump energy, adding noise and erosion, and inviting water hammer. Fire mains allow 2.5 m/s because they only run during a fire, so long-term energy does not matter.
Why can't DN be used for PPR?
Because plastic pipe is designated on outside diameter and has much thicker walls. DN25 PPR is only 16.6-18.4 mm inside against 26.9 mm for DN25 galvanised — 40% smaller. Choose DN25 for both and the plastic line sees over twice the velocity and loss, which shows up as poor pressure at the top of a building. Size on bore, or on velocity.
What is water hammer and how is it avoided?
It is the pressure wave that runs back and forth when a valve shuts suddenly and the flow stops, converting kinetic energy into a surge up to three to five times working pressure — enough to split pipe and wreck valves. Prevent it with slow-closing valves or surge arrestors, velocities under 1.5 m/s, air release valves at high points, and non-return valves with bypass cushioning at pump outlets.
Related calculators
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.