1) What this calculator does
This page estimates how much water (or any fluid) a pipe can hold based on its internal diameter and length. It works with common UK units such as millimetres, metres, inches and feet, and outputs in cubic metres, litres and UK (imperial) gallons.
2) How the maths works
For a circular pipe the internal volume is V = π × r² × L where r is the internal radius and L is the length. If you provide outer diameter and wall thickness, the tool derives inner diameter = outer − 2 × wall before applying the same formula.
3) Supported units & conversions
All inputs are converted to metres internally. Litres are calculated from cubic metres (1 m³ = 1,000 L). UK gallons are based on the imperial measure (1 UK gal = 4.54609 L).
4) When to use this
Use it for pipe filling estimates, dosing calculations, flushing volumes, or to check tank-and-pipe capacities on small plumbing and heating jobs.
5) Accuracy tips
Use the inner diameter from the pipe specification (not nominal size). For plastic pipes, inner bore can vary by SDR/class—check the datasheet. Include extra volume for fittings, valves, and flexible hoses.
6) Example
A 10 m run with 100 mm internal diameter holds about 78.54 L (≈ 17.29 UK gal). Try the preset below and see the numbers update instantly.
7) Notes for UK audiences
This tool uses British spelling (litres) and imperial UK gallons. If you need US gallons for comparison, multiply UK gallons by approximately 1.20095 to get US gallons, or use litres directly.
