# Water Pipe Sizing Calculator

Bore from flow and velocity, with actual velocity and friction loss.

- Page: https://jianzhujisuan.com/en/pipe-diameter
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/pipe-diameter.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Design flow 1.5 L/s, Economic velocity 1.2 m/s, Pipe length 100 m, Minor loss share 30%): **Calculated bore 39.9 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Design flow | number | 1.5 | L/s |  |
| Economic velocity | number | 1.2 | m/s | Domestic 1.0-1.5, fire 2.0-2.5 |
| Pipe length | number | 100 | m |  |
| Minor loss share | number | 30 | % | 40-60% with many fittings |

## 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

## Worked example

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.

## Questions

### 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.

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Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.