Operator math, worked
How to calculate the volume of a pipe in gallons
A water main is just a long round tank lying on its side. To find how many gallons it holds, change the diameter to feet, find the circle end with 0.785, multiply by the length for cubic feet, then multiply by 7.48 for gallons.
Last reviewed 2026-08-04
The formula
gallons = 0.785 x diameter_ft x diameter_ft x length_ft x 7.48
The steps
Change the diameter to feet
Pipe size is usually in inches. Divide by 12 to get feet. A 6-inch main is 0.5 ft; a 12-inch main is 1 ft.
Find the circle end
The end of the pipe is a circle. Its area is 0.785 times the diameter times the diameter, in square feet.
Multiply by the length
Multiply the circle area by the pipe length to get the cubic feet the pipe holds.
Turn cubic feet into gallons
Multiply the cubic feet by 7.48, because one cubic foot of water is 7.48 gallons.
Worked example
How many gallons are in 1,000 feet of 6-inch main?
- Diameter: 6 in / 12 = 0.5 ft
- Circle: 0.785 x 0.5 x 0.5 = 0.19625 sq ft
- Cubic feet: 0.19625 x 1,000 = 196.25
- Gallons: 196.25 x 7.48 = 1,468
Answer: About 1,468 gallons
Common questions
- Why change inches to feet first?
- Every other length is in feet, so the pipe diameter has to be in feet too. Divide inches by 12.
- Is a pipe the same as a tank?
- Yes, a pipe is a round tank on its side. The length plays the part of the depth.
- What if I only want cubic feet?
- Stop before the last step; skip the times 7.48 and the answer stays in cubic feet.