Pipe size and velocity calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Flow | 12 L/min |
| Pipe | 15 mm Copper |
| Internal diameter | 14 mm |
Velocity: 1.38 m/s
- Limit not yet confirmed Velocity within the limit: 1.377 m/s The limit for this check hasn't been confirmed yet: check the source. UK-wide: BS EN 806-3 / CIBSE Guide (design velocity). Check with the source.
| Result | Value |
|---|---|
| Velocity | 1.38 m/s |
| Internal diameter used | 13.6 mm |
| Pipe (mm) | Bore (mm) | Velocity (m/s) |
|---|---|---|
| 15 | 13.6 | 1.38 |
Show the working
- Flow: 12 L/min ÷ 60 ÷ 1000 = 0.2 L/s
- Velocity: 0.2 L/s in a 13.6 mm bore (your figure) = 1.38 m/s
How it works
- Flow Q (m³/s) = L/min ÷ 60 ÷ 1000; bore area A = π × d² ÷ 4 (d = internal diameter in m)
- Velocity v = Q ÷ A
- Head loss (Hazen–Williams) h = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), with L the run plus the fittings allowance
- Pressure loss (bar) = head loss (m) × 0.0981
Water pipes only. Gas pipework is sized by a Gas Safe registered engineer and isn't covered here.
Head loss is shown only when you enter the Hazen–Williams C for your pipe.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Velocity limit | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB2: velocity limit for choosing a pipe size. Researcher to source to BS EN 806-3 / CIBSE guidance. |
Regulations and standards
- BS EN 806-3 (sizing pipes for drinking water installations; Researcher to cite)
- BS EN 1057 (copper tube dimensions)
Explained: Approved Document G (hot water safety).
Pipe sizing methods are the same across the UK; water fittings rules differ by nation.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Pipe internal diameters by material and outside diameter TODO: Researcher to add the maker or standard URL | BS EN 1057 (copper); makers' data for PEX and MDPE (Researcher to fill) | UK-wide | 0/9 values entered Not yet checked against the source |