Pipe Volume Calculator
Enter internal diameter and length to get the litres (or gallons) the pipe holds and what that water weighs — the number behind flushing volumes and recirculation design.
Last updated: 2026-09-28
How the calculation works
- Cylinder volume with the internal diameter: a 20 mm ID pipe holds 0.314 L per metre.
- Weight follows from water's density (≈1 kg/L) — relevant for hanger loads on long horizontal runs.
- The per-100 m row normalizes the figure for quick comparisons between sizes.
Formula
V = π/4 · d² · L
| Symbol | Meaning | Unit |
|---|---|---|
V | Internal volume | L |
d | Internal diameter | mm |
Worked example
Interpreting the result
Dead-leg volume is the hidden waste in domestic hot water: every litre of pipe between the heater and the tap is water you heat and pour away. It also sizes disinfection batches (chlorination needs the full system volume) and glycol changes (drain and refill quantities). Weight matters on long suspended runs: water adds ~1 kg per litre to hanger loads.
Assumptions
- Water at ambient conditions (≈1 kg/L).
- Internal dimensions — actual bore, not nominal size.
Limitations
- Does not include fittings, valves or fixtures — add their volumes for full-system numbers.
- For other liquids, adjust the density for the weight row.
Frequently asked questions
How much water is in a pipe per metre?
15 mm ID: 0.18 L/m. 20 mm: 0.31 L/m. 25 mm: 0.49 L/m. Multiply by the run length for the dead-leg figure.
Why does pipe volume matter?
Hot-water waste, disinfection dosing, glycol batching and hanger loading all derive from the water a run holds — it is the number behind recirculation economics.