Enter the internal diameter and length of a pipe and get the volume it holds in gallons, litres and cubic feet, plus the approximate weight of that water.
Internal volume is the cross-sectional area of the bore multiplied by the length: pi times the radius squared times the length, worked in inches and converted at 231 cubic inches per US gallon. Results are also shown in litres and cubic feet, and the water weight uses the standard approximations of 8.34 lb per US gallon and about 1 kg per litre. The diameter to use is the internal diameter, not the nominal pipe size: nominal sizes are labels, and actual bores differ by material and wall schedule, so measuring the bore or using the manufacturer table for the exact pipe matters for accuracy. Fittings, valves and partially filled runs are not included, so the figure is the volume of a straight, completely full pipe.
How is pipe volume calculated? Volume is pi times the internal radius squared times the length. With the diameter and length in inches, that gives cubic inches; dividing by 231 converts to US gallons, and gallons times 3.785 gives litres.
Why does the inner diameter matter so much? Volume depends on the square of the radius, so a small diameter error has a doubled effect: a 10% larger bore holds about 21% more water. Nominal pipe sizes do not equal the bore, so the internal diameter of the actual pipe should be used.
How much does the water in a pipe weigh? About 8.34 lb per US gallon, or roughly 1 kg per litre. The calculator shows both, which matters when a filled pipe or tank is supported by a structure.