Capacity belongs to the inside of the pipe
A liquid-filled pipe is a long cylinder. Its capacity depends on the open bore, not the outside of the wall and not necessarily the nominal size printed on a fitting. Pipes with the same nominal designation can have different inside diameters because their wall thicknesses differ. Enter the actual internal diameter from the product specification or a reliable measurement. The length should cover the part of the pipe occupied by liquid. This calculator handles straight, constant-diameter runs and reports their geometric internal volume.
Cylinder volume and fluid density
Internal volume = π × inside diameter² × length ÷ 4; liquid mass = volume × densityMetric diameter uses millimetres and length uses metres. Imperial diameter uses inches and length uses feet. Both are converted to metres before the cylinder equation runs. Volume is then displayed in litres and US gallons, with 3.785411784 litres per US gallon. Density is entered in kilograms per litre in metric mode, or pounds per US gallon in imperial mode. The mass result includes the liquid only. Pipe walls, brackets, valves and insulation are not weighed. No waste allowance or flushing quantity is added to the internal capacity.
A ten-metre water-filled run
A pipe with an inside diameter of 50 millimetres has a diameter of 0.05 metres. Ten metres of that bore holds π × 0.05 squared × 10 ÷ 4 = 0.01963495 cubic metres, equivalent to 19.635 litres or approximately 5.187 US gallons. With the illustrative density of 1 kilogram per litre, its liquid mass is 19.635 kilograms. Doubling the length doubles the volume. Doubling the bore diameter multiplies the volume by four, because diameter is squared in the area equation. This makes accurate diameter data more important than a rough external label.
Combine runs and account for connected equipment
Calculate each diameter separately when a system changes bore along its route, then add the capacities. Include the actual path length around bends, but obtain valve bodies, tanks and equipment chamber volumes from their specifications rather than assuming every component is a straight cylinder. A partially filled gravity line needs a circular-segment calculation; this tool assumes a completely filled cross-section. Choose a fluid density appropriate to the composition and temperature if mass matters. The sample water density is a convenient approximate example, not a laboratory temperature correction. Capacity alone cannot tell you flow rate, pressure loss or pumping power. Those require velocity, roughness, fittings and system conditions that are outside the geometry used here.
Frequently asked questions
Should I enter nominal pipe size?
Enter actual inside diameter. Nominal names can refer to a standard family and may differ from both the inside and outside measurements.
Does this include the pipe weight?
No. Liquid mass is capacity multiplied by the entered fluid density. Add the pipe and fittings separately when assessing total support load.
How much does a 50 mm bore hold over 10 m?
It holds about 19.635 litres when completely filled, or approximately 5.187 US gallons. Wall thickness does not enter once the actual bore is known.
Can I calculate a half-full drain?
Not with this full-cylinder model. A partly filled circular pipe needs the actual filled segment area rather than the entire bore area.
Can volume tell me the flow rate?
No. Volume is stored capacity. Flow rate depends on velocity or the pumping and pressure conditions, which are not supplied on this page.
Last reviewed: 2026-10-03 · How we calculate