How do you calculate the volume of water in a pipe?

Multiply the cross-sectional area of the pipe by its length: V = pi x (d/2)^2 x L, using the inner diameter. This calculator does the math in cubic inches and converts to US gallons (231 cubic inches per gallon) and liters, so you just pick the pipe size and enter the length.

Should I use the inner or outer diameter?

Always the inner diameter (ID). Water only occupies the inside of the pipe, and wall thickness varies a lot between materials. A 3/4" Copper Type L pipe has an ID of 0.785", while 3/4" PEX is only 0.671", so the same nominal size holds noticeably different amounts of water.

How much does the water in a pipe weigh?

Water weighs about 8.34 pounds per US gallon, or 1 kilogram per liter. The calculator shows the weight alongside the volume, which matters when you are draining systems, supporting long horizontal runs, or estimating what a full riser adds to a structure.

Why does pipe volume matter on a plumbing job?

Pipe volume tells you how much water to expect when draining a system, how much treatment chemical or glycol to dose, and how long a hot water line takes to deliver. It also feeds flushing and chlorination calculations for new water service lines.

Pipe Volume Calculator

Calculate the water volume in any pipe run: PEX, copper, or Schedule 40. Get US gallons, liters, water weight, and gallons per foot instantly.

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How much water does your pipe hold?

Pick a pipe type and size, or enter a custom inner diameter, then add the run length. Results update as you type.

Pipe Material

Nominal Size

Pipe Length

Enter a pipe length to see the volume

You'll get US gallons, liters, and water weight instantly.

Water volume per foot: Copper Type L

Handy reference for flushing, purging, and glycol or chlorination dosing on Copper Type L runs.

Nominal Size	Inner Diameter	Gallons / ft	Liters / m
1/2"	0.545"	0.0121	0.151
3/4"	0.785"	0.0251	0.312
1"	1.025"	0.0429	0.532
1-1/4"	1.265"	0.0653	0.811
1-1/2"	1.505"	0.0924	1.148
2"	1.985"	0.1608	1.997

Volumes use standard published inner diameters (PEX SDR-9, Copper Type L/M, Schedule 40). Fittings, valves, and equipment add a little extra. Round up when dosing.

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