Pipe Volume Calculator
Work out how much water a pipe holds — from its inner diameter and length — in litres, US gallons, UK gallons and cubic metres.
How much water is inside that pipe? It matters for heaters, flushing and fill times — and it is a cylinder, so the maths is simple once the inner diameter is known.
Enter the inner diameter and the length; the volume follows in litres, gallons and cubic metres.
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 pipe's inner diameter.
- Enter the pipe length.
- Read the water volume in litres, gallons and cubic metres.
- Use the Copy results button to copy the answer.
Worked example
| Pipe | 100 mm inner diameter, 10 m long |
|---|---|
| Volume | π × (0.05 m)² × 10 m = 0.0785 m³ |
| Litres | 0.0785 × 1,000 = 78.54 L |
| US gallons | 78.54 ÷ 3.785411784 = 20.75 gal |
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
Do I use the inner or outer diameter?
The INNER diameter — the water only fills the inside. A 110 mm pipe with 3 mm walls has a 104 mm inner diameter, and that is the number to enter.
Why does pipe volume matter?
For sizing water heaters, working out how much water a system holds, flushing a pipe, or estimating how long a tap takes to run hot.
How many litres are in a metre of 100 mm pipe?
About 7.85 litres per metre (π × 0.05² × 1 m). A 10 m run holds 78.54 litres.
Is this a substitute for plumbing design?
No. This is a volume calculation for planning. Pipe sizing, pressure and code compliance depend on local regulations and real installation conditions — see the safety notice.
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.