How do I work out the volume of water in a pipe?

A pipe is a cylinder, so its volume is the circle area of the bore times the length. Use the inside diameter, not the outside — the wall does not hold water. A 2-inch inside diameter pipe 100 feet long holds about 16 gallons. This tool takes the bore and the length and gives you gallons and litres.

Should I use the inside or outside diameter?

Always the inside (the bore). Pipe is often named by a nominal or outside size, and the wall thickness can be a real fraction of a small pipe, so using the outside diameter overstates the volume. If you only know the nominal size, look up the actual inside diameter for that pipe and schedule and enter that.

What is this useful for on the job?

Sizing how much water a line holds for flushing or chlorinating, how much a loop of tubing carries, how long a purge takes, or how much a drain-down will spill into a bucket. It is the same cylinder volume behind a sonotube or a round tank, just expressed in gallons and litres for a fluid.

Pipe volume calculator

This is the geometric volume of a plain cylindrical bore. Fittings, valves and a part-full line all change what a pipe actually holds, so read it as the full-bore capacity and allow for the real run on site.

Enter the inside diameter and the length and the tool works out how much the pipe holds, in gallons and litres at once. The maths is the area of the bore circle multiplied by the length, converted from cubic feet or metres into fluid measure.

Worked example

A pipe with a 2-inch bore has a radius of 1 inch, or 1/12 foot, so the bore area is π × (1/12)² ≈ 0.0218 sq ft. Over 100 feet that is 2.18 cubic feet, and a cubic foot is about 7.48 gallons, so the line holds roughly 16.3 gallons — close to 62 litres.

The formula

volume = π × (bore ÷ 2)² × length

The bore area is π times the radius squared; multiply it by the length for the volume, keeping the diameter and length in the same unit first. One cubic foot is 7.48 US gallons and one cubic metre is 1000 litres, which is how the volume becomes a fluid figure.

Questions

How do I work out the volume of water in a pipe?: A pipe is a cylinder, so its volume is the circle area of the bore times the length. Use the inside diameter, not the outside — the wall does not hold water. A 2-inch inside diameter pipe 100 feet long holds about 16 gallons. This tool takes the bore and the length and gives you gallons and litres.
Should I use the inside or outside diameter?: Always the inside (the bore). Pipe is often named by a nominal or outside size, and the wall thickness can be a real fraction of a small pipe, so using the outside diameter overstates the volume. If you only know the nominal size, look up the actual inside diameter for that pipe and schedule and enter that.
What is this useful for on the job?: Sizing how much water a line holds for flushing or chlorinating, how much a loop of tubing carries, how long a purge takes, or how much a drain-down will spill into a bucket. It is the same cylinder volume behind a sonotube or a round tank, just expressed in gallons and litres for a fluid.

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