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Pipe Volume Calculator

A pipe volume calculator tells you how much water a pipe holds — in gallons and litres — from its inner diameter and length. It's the number you need to flush a line properly, dose it, or work out the water in a run of pipe, a tank or a feature.

Your measurements

Result

Enter the inside diameter (not the nominal size, which is a touch larger) and the length, and it returns the volume three ways.

How it is calculated

Volume = π × radius² × length, using the inner radius, converted to gallons (1 ft³ = 7.48 gal) and litres. Use the true inside diameter; nominal pipe sizes (e.g. '2 inch') are nominal and the bore varies by schedule.

Steps

Measure or look up the pipe's inner diameter.
Enter the length of the run.
Read the volume in gallons (and litres and cubic feet).
For several pipe sizes, calculate each and add them.
Use the inner diameter for accuracy — nominal sizes run slightly larger.

Worked example

A 2 in inner-diameter pipe, 50 ft long, holds about 8.2 gallons of water.

Common questions

How do I calculate the volume of a pipe?

Volume = π × (inner radius)² × length. The calculator converts it to gallons and litres for you.

Inner or outer diameter?

Inner — that's the water-carrying bore. Nominal sizes and outer diameters are larger, so use the actual inside measurement.

What's this useful for?

Flushing and chlorinating lines, dosing, estimating water in a system, and filling times for pipes, tanks and pool plumbing.

How many gallons per foot of pipe?

Depends on diameter — roughly 0.16 gal/ft for 2 in. The calculator scales it to any size.