Pipe Volume Calculator

Pipe volume calculator: gallons held inside a run of pipe

A pipe volume calculator treats the pipe as a long thin cylinder. The formula is π × radius² × length, with the radius taken from the inside diameter. Divide the inside diameter in inches by 24 to get the radius in feet, then multiply the cubic feet by 7.48 to get gallons. A 2-inch pipe, 50 feet long, holds 1.09 cubic feet, or 8.16 gallons.

Only the bore counts. Wall thickness does not hold any water.

The formula in plain words

Imagine pouring water into a long drinking straw laid flat. The straw’s opening is a circle. The length is how far the water stretches. The water fills the circle all the way along.

• Radius in feet = inside diameter in inches ÷ 24
• Cubic feet = π × radius² × length in feet
• Gallons = cubic feet × 7.48

The 24 is 12 inches per foot times 2 to turn diameter into radius.

Worked examples

1. Irrigation main, 2 in inside diameter, 50 ft long

• Radius: 2 ÷ 24 = 0.0833 ft
• Cubic feet: π × 0.0833² × 50 = 1.091 ft³
• Gallons: 1.091 × 7.48 = 8.16 gal

2. Drain line, 4 in inside diameter, 100 ft long

• Radius: 4 ÷ 24 = 0.1667 ft
• Cubic feet: π × 0.1667² × 100 = 8.727 ft³
• Gallons: 8.727 × 7.48 = 65.28 gal

Example 2 uses a pipe only twice as wide and twice as long as example 1, yet it holds eight times the water. Doubling the width quadruples the area, and doubling the length doubles it again, so 4 × 2 = 8.

Gallons per foot of pipe

Inside diameter	Gallons per foot	Gallons per 100 ft
1 in	0.041	4.1
1.5 in	0.092	9.2
2 in	0.163	16.3
3 in	0.367	36.7
4 in	0.653	65.3
6 in	1.469	146.9

Nominal size versus actual inside diameter

Pipe sizes are names, not measurements. For Schedule 40 PVC, the inside diameters are about 1.049 in for 1 in, 1.610 in for 1.5 in, 2.067 in for 2 in, 3.068 in for 3 in, 4.026 in for 4 in, and 6.065 in for 6 in. Using nominal sizes instead of these makes the volume come out low, by about 1 percent for 4 in pipe and about 9 percent for 1 in pipe. Other materials, such as copper or PEX, have their own inside diameters, so check the manufacturer’s data.

Putting it to use

Hot-water wait time is a good example. If a 0.785 in inside diameter copper line is 40 ft long, it holds π × (0.785 ÷ 24)² × 40 × 7.48 = 1.01 gallons. At a fixture flow of 2 gallons per minute, that is about 30 seconds of water running before hot water arrives.

How to measure

• Find the true inside diameter. Look it up from the pipe markings or product data sheet.
• Add up the path. Sum each straight run and each vertical drop.
• Split different sizes. If a line steps down from 2 inch to 1 inch, calculate each section separately.
• Check units. Diameter goes in inches, length in feet.
• Treat fittings as zero. Include them only if you need exact fill volume.

Common mistakes

• Using the outside diameter. The outside includes the wall, which overstates volume.
• Entering nominal size. A “2-inch” pipe is not exactly 2.000 inches inside.
• Mixing inches and feet. Entering length in inches makes the answer 12 times too small.
• Forgetting vertical lines. A tall riser can hold as much as a long horizontal run.
• Assuming the pipe runs full. Drain lines often flow partly full, so the volume is a maximum.

Related calculators

To see how fast water moves, use the pipe velocity calculator or the GPM calculator. For pressure loss over a long run, try the pipe friction loss calculator, and for larger vessels, the tank volume calculator.