Pipe Volume Calculator (Gallons)
Find how many gallons of water a pipe holds from its inside diameter and length — useful for sizing purges, flushes and chlorination doses.
How it works
Volume = π ÷ 4 · d² · L · 1 gallon = 231 in³ · (d in inches, L in inches)
A pipe is just a cylinder, so its volume is the circular cross-section area times the length. Converting cubic inches to US gallons uses the exact definition of 231 cubic inches per gallon. Because volume scales with the square of the diameter, a 2 in pipe holds four times as much water per foot as a 1 in pipe — which is why large mains need surprisingly large flush and disinfection volumes.
Worked example
A 1 in inside-diameter line 100 ft long: cross-section is π/4 × 1² = 0.785 in², times 1,200 in of length = 942.5 in³. Divide by 231 and it holds about 4.08 gallons — the amount you must flush to fully exchange the water in that run.
Frequently asked questions
Should I use nominal or inside diameter?
Use the actual inside diameter. A "1 inch" copper or PVC pipe rarely has a 1 in bore — Type L copper is about 1.025 in and Schedule 40 PVC about 1.049 in — so nominal size will slightly misstate the volume.
Why do I need pipe volume?
It sizes flushing after repairs, purge times before sampling, chlorine or antifreeze doses, and how long stagnant water sits in a line. For chlorination you dose per gallon of pipe volume.
How do I convert to cubic feet?
Divide gallons by 7.4805, since one cubic foot equals 7.48 US gallons. The 4.08 gallons above is about 0.55 cubic feet.
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This calculator is for educational purposes. Double-check important results before acting on them.