How to find the volume in a pipe
A pipe is a cylinder, so its volume is π × radius² × length. Convert the inner diameter to a radius in feet, keep length in feet, then multiply by 7.48 to get gallons. A 4-inch pipe 10 feet long holds about 6.53 gallons (0.873 cubic feet). A pipe is just a long cylinder, which the cylinder volume calculator also handles.
How to use this calculator
Enter the pipe's inner diameter in inches and its length in feet. The result shows the water volume in gallons and cubic feet. Use the inner diameter, not the nominal pipe size — wall thickness reduces the actual bore. Flow rather than capacity is the job of the GPM calculator.
Volume per foot by pipe size
| Inner diameter | Gallons per foot |
|---|---|
| 1 in | 0.041 |
| 2 in | 0.163 |
| 4 in | 0.653 |
| 6 in | 1.469 |
Why inner diameter matters
Nominal pipe sizes don't equal the actual bore. A "1-inch" pipe has thicker or thinner walls depending on schedule, changing the inner diameter and therefore the volume. For accurate results — draining, dosing, or pressure-testing — measure or look up the true inner diameter.
Volume scales with the square of the diameter, so small errors compound quickly: a 6-inch pipe holds more than twice as much per foot as a 4-inch pipe, not one and a half times as much. For a run that changes size along its length, calculate each section separately and add the results together. To express the result in metric units, use the gallons to liters calculator.
Frequently asked questions
- How do I calculate the volume of water in a pipe?
- Use π × radius² × length with the inner radius and length in feet, then multiply cubic feet by 7.48 for gallons. A 4-inch pipe 10 ft long holds about 6.53 gallons.
- How many gallons per foot is a 4-inch pipe?
- About 0.653 gallons per foot of length, based on a 4-inch inner diameter.
- Should I use nominal or inner diameter?
- Use the inner diameter (the actual bore). Nominal pipe size doesn't account for wall thickness and will overstate the volume.
- How do I convert cubic feet to gallons?
- Multiply cubic feet by 7.48 to get US gallons.
- How do I convert the result to litres?
- Multiply US gallons by 3.785. A 4-inch pipe 10 feet long holds about 6.53 gallons, which is roughly 24.7 litres.