Pipe Size Calculator
Work out the pipe inner diameter a flow rate needs — and check the water velocity in an existing pipe — from flow rate and maximum velocity.
Pipe size is a trade-off: too small and the tap starves, too big and the water sits. This calculator finds the smallest inner diameter that keeps the water under your velocity limit.
Enter the flow and the velocity; the diameter follows. This is a planning guide — a professional must confirm the final size.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Enter the flow rate in litres per minute.
- Enter the maximum velocity (1.5 m/s is a common guide).
- Enter an existing pipe's inner diameter to check its velocity.
- Read the required diameter — and the velocity if you entered a pipe.
Worked example
| Flow rate | 12 L/min = 0.0002 m³/s |
|---|---|
| Max velocity | 1.5 m/s |
| Required area | 0.0002 ÷ 1.5 = 0.0001333 m² |
| Required diameter | √(4 × 0.0001333 ÷ π) = 13.03 mm |
| Velocity in a 15 mm pipe | 0.0002 ÷ (π × 0.0075²) = 1.13 m/s |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What velocity should water flow at in a pipe?
A common guide for supply pipes is up to 1.5 m/s; below 1 m/s is quieter. Above 2 m/s, noise and wear rise. The right figure depends on the pipe, the fittings and local code — this calculator uses the velocity you enter.
Why does pipe size matter?
Too small a pipe starves the tap (low flow, high velocity, noise); too big a pipe costs more and water sits longer. This calculator finds the smallest diameter that keeps velocity under your limit.
How do I know my flow rate?
Measure it: time how long a tap takes to fill a known container. A 10-litre bucket in 50 seconds is 12 L/min. Or use the water flow calculator.
Is this a substitute for plumbing design?
No. This is a planning guide. Pipe sizing, pressure and code compliance depend on local regulations and real installation conditions — see the safety notice. A professional must confirm the final size.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.