The volume of a pipe is calculated using the cylinder formula: Volume = π × r² × L, where r is the inner radius (half the inner diameter) and L is the pipe length. In metric: Volume (m³) = π × (d ÷ 2)² × L, with d and L in metres. Multiply by 1,000 to convert m³ to litres. This calculator does all that instantly.

Always use the inner diameter (also called the bore or ID) when calculating fluid volume. The outer diameter (OD) includes the pipe wall thickness. For example, a 22 mm copper pipe has an OD of 22 mm but an inner diameter of approximately 20.2 mm. Using the OD would overestimate the fluid volume. Check the manufacturer’s data sheet or a pipe schedule table for the exact inner diameter of your pipe.

Volume (litres) = π × (diameter in metres ÷ 2)² × length in metres × 1,000. Example: a 50 mm inner diameter pipe, 10 m long: Volume = π × (0.05 ÷ 2)² × 10 × 1,000 = π × 0.000625 × 10,000 = 19.635 litres. Alternatively: Volume (litres/metre) = π × (diameter in mm)² ÷ 4,000,000, then multiply by length in metres.

A standard 22 mm copper pipe has an inner diameter of approximately 20.2 mm. Using the formula: Volume = π × (0.0202 ÷ 2)² × 1 × 1,000 ≈ 0.320 litres per metre. So a 10 m run of 22 mm pipe holds around 3.20 litres. This is a useful figure for calculating inhibitor dosing in central heating systems.

Knowing total system water volume is essential for correctly dosing corrosion inhibitors and antifreeze, sizing the expansion vessel, selecting the right boiler output, calculating fill times, and determining the weight of pipework when full. Most inhibitor manufacturers specify a dose rate in ml per litre of system volume, so an accurate pipe volume calculation directly affects how much inhibitor you add.

Nominal pipe size (NPS or DN) is a dimensionless reference number that does not directly equal any physical dimension. The actual inner diameter depends on the pipe material, wall thickness, and schedule. For example, a 2-inch Schedule 40 steel pipe has an inner diameter of approximately 52.5 mm, not 50.8 mm (2 inches). Always check the manufacturer’s data sheet or consult a pipe schedule chart before calculating volume.

To convert litres to UK (imperial) gallons, divide by 4.54609. To convert litres to US gallons, divide by 3.78541. For example, 20 litres = 4.399 UK gallons = 5.283 US gallons. This calculator displays both automatically in the results panel.

This calculator works for any fluid (water, oil, gas, air) inside a circular-section pipe — the geometry is identical regardless of what is flowing through it. However, it is only suitable for circular cross-sections. For rectangular or square ducts, the formula is Width × Height × Length. For oval or elliptical pipes, use π × semi-major axis × semi-minor axis × Length instead.