A pipe is a cylinder, so its volume is area times length — the catch is that the area has to come from the inside diameter, and pipe is sold by nominal size, which for steel and PVC is neither the inside nor the outside measurement. A "1 inch" schedule 40 pipe measures 1.049 inches inside, and that difference squares when it becomes an area. This calculator carries the real inside diameters for common schedule 40 sizes, and reports the volume in US gallons, imperial gallons, litres, cubic feet and cubic metres at once. Add a flow rate and it also gives the fill or drain time and the water velocity, which is the number that decides whether a run will be noisy.
Features
Volume in US gallons, imperial gallons, litres, cubic feet and cubic metres
Inside diameters for schedule 40 pipe from half an inch to eight inches
Weight of the water the run holds, in pounds and kilograms
Fill or drain time from a flow rate in GPM, LPM, L/s or m³/h
Water velocity in feet per second and metres per second
Flags velocities above the point where erosion and noise start
Totals several identical runs at once
How to use the Pipe Volume Calculator
Enter the inside diameter, or pick a schedule 40 nominal size
Enter the length of the run and how many identical runs there are
Read the volume in whichever unit you need
Add a flow rate to see the fill time and the water velocity
Example
Input
3/4" schedule 40, 100 ft
Output
2.77 US gal · 10.5 L · 23.1 lb of water
The inside diameter of 3/4" schedule 40 is 0.824", not 0.75" — that is the figure the volume comes from.
Common errors & troubleshooting
The answer is far larger than the plumbing chart says. — You have probably used the nominal or outside diameter. Use the inside diameter — the schedule 40 picker fills in the real figure for common sizes.
US and imperial gallon figures differ. — They are different units. A US gallon is 3.785 litres and an imperial gallon is 4.546, so the same pipe holds about 20 per cent fewer imperial gallons. Both are shown.
The fill time looks wrong for a pump rating. — Pump ratings are quoted at a given head. The real flow into a long run with fittings will be lower, so treat the fill time as a best case.
The velocity warning appears on a small pipe. — That is the point of it. Above roughly 5 ft/s copper erodes at the fittings and the run gets noisy — the same flow through the next size up brings the velocity back down.
Frequently asked questions
How many gallons are in 100 feet of 1-inch pipe?
About 4.5 US gallons for schedule 40, whose real inside diameter is 1.049 inches. Using the nominal 1 inch instead would understate it by roughly a tenth.
Why does pipe size not match the actual diameter?
Nominal pipe size is a historical label, not a measurement. It once described the inside diameter of a particular wall thickness, and it stuck even as wall thicknesses changed, so today it matches neither the inside nor the outside.
How much does the water in a pipe weigh?
About 8.3 pounds per US gallon, or 1 kilogram per litre. A 100-foot run of 2-inch pipe holds around 17 gallons, so roughly 140 pounds hanging off the hangers once it is full.
What water velocity is too fast?
Common guidance is to stay under 5 feet per second on the cold side and 8 on short runs, mostly to limit noise and erosion at fittings. Copper is the fussiest; plastic tolerates more.
Can I use this for a pool hose or an irrigation line?
Yes — any round pipe or hose works, as long as you use the inside diameter. Flexible hose usually states it directly, since it has no nominal size convention to confuse things.
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