The water a pipe holds is a cylinder: the bore area times the length. It sounds academic until you need to dose a system, purge air, add inhibitor, or work out how much cold water runs to waste before the hot arrives. Here is the method — and the one measurement people get wrong.
The formula
Volume = π × (bore ÷ 2)² × length. A ¾″ copper pipe (0.81″ / 20.2 mm bore) holds about 0.027 gallons per foot(0.32 L per metre), so an 80 ft (25 m) run holds roughly 2 gallons (8 litres).
Pipe Volume Calculator
Pick a pipe size and length for the gallons it holds, with a per-foot reference for every size.
Inside diameter, not outside
Only the bore holds water. That is the catch with plastic pipe: a ¾″ PEX pipe has a much smaller bore than ¾″ copper because its wall is thicker, so the same nominal size holds less. Standard sizes here use copper bores — use the custom-bore option for other materials.
Why it matters
- Dosing & disinfection — chlorine or inhibitor per gallon of pipework.
- Purging & draining — how much water comes out for a repair, or air to bleed.
- Antifreeze — how much to add to protect a run.
- Hot-water wait — the dead-leg volume you flush before hot arrives.
How long until hot water arrives?
Divide the dead-leg volume by the faucet’s flow rate. A 33 ft (10 m) run of ½″ (15 mm) pipe holds about 0.37 gallons (1.4 L); at a faucet flowing 1.5 GPM that is about 15 seconds of cold water down the drain every time — the clearest argument for short hot-water runs or a recirculation loop.
And the weight
A US gallon of water weighs about 8.34 lb (1 litre is 1 kg), so volume turns straight into weight. An 80 ft (25 m) run of 2″ (54 mm) pipe holds about 13.7 gallons (52 L) — roughly 114 lb (52 kg) of water hanging in the clips before you count the pipe. Support long large-bore runs accordingly.