ToolnozaUse the cylinder formula with the inside diameter
A straight, full circular pipe is modelled as a cylinder: V = π × r² × L. If you know the internal diameter D, use V = π × (D ÷ 2)² × L. Keep diameter and length in the same unit; the result is cubic units.
For a 10 m pipe with a 20 mm internal diameter, convert 20 mm to 0.020 m. V = π × 0.010² × 10 = 0.003142 m³, or about 3.14 litres because 1 m³ = 1,000 L.
Nominal size, outside diameter and bore
The labelled nominal pipe size is not necessarily the measured inside diameter. Wall thickness and pipe schedule reduce the bore. When only outside diameter and wall thickness are known, internal diameter = outside diameter − 2 × wall thickness. Use a manufacturer dimension table when the measured bore is unavailable; do not substitute a nominal label directly.
Convert the result to common units
One cubic metre is 1,000 litres. One US gallon is about 3.785 litres; an Imperial gallon is about 4.546 litres, so identify which gallon a specification uses. Keep extra digits during the calculation and round the displayed result to a practical precision for the application.
What the estimate includes and excludes
The formula describes the geometric capacity of a uniform, straight, circular pipe filled to its bore. It does not account for fittings, valves, reducers, sagging, deposits, trapped air, partial fill or flow rate. For a pipe network, calculate each straight section and add components separately when their capacity matters.
Check the result with the calculator
- Enter the pipe’s actual inside diameter and straight length.
- Select matching units and read the capacity in litres or gallons.
- Check whether your project needs total system volume, delivered flow, or pressure-loss analysis; these are different calculations.
The tool estimates capacity, not safe operating pressure, water-hammer effects or pipe sizing.