Pipe Volume Calculator

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). The calculator does this for standard sizes or any bore you enter.

Dosing and disinfection need it (chlorine per gallon of pipework), so do purging air from a system, working out how much antifreeze or inhibitor to add, how much water drains down for a repair, and how long the dead leg makes you wait for hot water at the faucet.

Inside — 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. The standard sizes here use copper bores; use the custom-bore option for other materials.

A US gallon of water weighs about 8.34 lb (1 litre is 1 kg), so you can turn the volume straight into weight. That matters for supporting long large-bore runs — an 80 ft (25 m) run of 2″ (54 mm) pipe holds about 13.7 gallons (52 L), which is roughly 114 lb (52 kg) of water hanging in the clips before you count the pipe itself.

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 bathroom faucet flowing 1.5 GPM that is about 15 seconds of cold water down the drain every time. It is the clearest argument for short hot-water runs or a recirculation loop.