Pipe Volume Calculator

Pipe Volume formula

Gallons = pi x (Inside diameter (in) / 2)² x Length (ft) x 12 / 231
Cubic feet = Gallons x 231 / 1,728   (7.48 gallons per cubic foot)
Weight (lb) = Gallons x 8.34
Seconds to clear = Gallons / Flow (gpm) x 60

Worked example

A 60 ft run of 1/2 in PEX (0.475 in inside diameter) to a bathroom: pi x (0.475 / 2)² = 0.1772 sq in of bore. Multiply by 60 ft x 12 = 720 in for 127.6 cu in, and divide by 231 to get 0.55 gallons. At a 2 gpm faucet the cold water clears in 0.55 / 2 x 60 = 17 seconds.

A 250 ft run of 2 in Schedule 40 PVC (2.067 in inside diameter) to a barn: pi x (2.067 / 2)² = 3.356 sq in. Times 250 x 12 = 3,000 in is 10,067 cu in, or 10,067 / 231 = 43.6 gallons. That is 10,067 / 1,728 = 5.83 cubic feet and weighs 43.6 x 8.34 = 363 lb, which matters if the line hangs from a ceiling or has to be drained for winter.

Quick reference

Why the volume of a pipe matters

The most common reason to know how much water a pipe holds is the wait for hot water. Every gallon sitting in the line between the water heater and the shower has to be pushed down the drain first, and at 2 gpm that is 30 seconds per gallon. A 3/4 in trunk holds twice the water of a 1/2 in line per foot, so running 1/2 in branches to individual fixtures, keeping the heater central, or adding a recirculation loop all cut the wait.

The same number sizes a recirculation pump, tells you how much glycol a hydronic loop or a winterized irrigation main needs, how much a drained line will release into a basement, and how much weight a long overhead run adds to its hangers. For a hydronic system, add the boiler, radiators and expansion tank to the pipe volume.

Use the inside diameter, not the nominal size

Nominal pipe size is a label, not a measurement. A 3/4 in copper tube is 0.875 in outside, and its bore depends on the wall thickness: 0.785 in for type L and 0.811 in for type M. PEX of the same nominal size has a much thicker wall and a 0.671 in bore, while 3/4 in Schedule 40 PVC is 1.050 in outside and 0.824 in inside. The built-in list uses the ASTM standard inside diameters; pick Custom and enter a measured bore for anything else, such as galvanized steel, CPVC or polyethylene.

The result is water at rest. Actual delivery also depends on fittings, elevation and pressure, and any plumbing change should follow the plumbing code adopted in your area and be checked by a licensed plumber.

Key facts

• One US gallon is 231 cubic inches, and one cubic foot holds 7.48 gallons, about 62.4 lb of water.
• Type L copper has a thicker wall than type M: 3/4 in type L has a 0.785 in inside diameter and 3/4 in type M 0.811 in.
• PEX is sold by copper tube size but has a smaller bore: 3/4 in PEX is 0.671 in inside, so it holds about 27% less water per foot than 3/4 in type L copper.
• Volume rises with the square of the diameter, so doubling the inside diameter holds four times as much water per foot.
• Water weighs 8.34 lb per gallon, so 100 ft of 4 in Schedule 40 PVC holds about 66 gallons and 550 lb of water.

Frequently asked questions

Sources and references

• •Copper Tube Handbook (A4015), tube dimensions and physical characteristics (Copper Development Association)
• •Pressure Dimensional Catalog, PVC Schedule 40 and 80 pipe (Charlotte Pipe)
• •NIST Handbook 44, Appendix C, General Tables of Units of Measurement (NIST)