Pipe Volume Calculator
How much a pipe holds, what that weighs in water, and how long it takes to fill or flush at a given flow rate.
How much water is in the pipe.
How the pipe volume calculator works
A pipe is a cylinder: π(d/2)²L, using the internal diameter. Using the nominal or external size instead is the usual error, and on small pipe the difference is large — a "25 mm" pipe often has a 21 mm bore, which is 30% less volume.
The fill time is simply volume divided by flow rate, which is what matters for flushing, chlorinating or purging a line.
Formula: V = π (d/2)² L
Worked examples
| Inputs | Volume | Note |
|---|---|---|
| 25 mm bore, 50 m run | 24.54 L | 24.5 litres |
| 100 mm bore | 392.7 L | sixteen times the volume |
| A long garden hose | 3.98 L | 4 litres |
FAQFrequently asked questions
Should I use the internal or external diameter?
Internal, always. Nominal pipe sizes rarely match the bore, and using the outside diameter overstates the volume substantially on small pipe.
How much water is in a 25 mm pipe?
About 0.49 litres per metre.
Why does the volume grow so fast with diameter?
Because it goes with the square. Doubling the bore quadruples the volume — and a 100 mm pipe holds sixteen times a 25 mm one.
How long to flush a line?
Volume divided by flow rate, times two or three — one pipe volume rarely displaces everything, especially in a horizontal run.
Does this matter for pressure testing?
Yes. The volume sets how much water you need and how much a leak will drop the pressure. Small volumes lose pressure alarmingly fast.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.