Pipe Volume & Flow Calculator
Gallons standing in the line, and gallons moving through it — from the inside diameter, which is the only diameter that holds water.
Nominal pipe size is not inside diameter — a "6-inch" pipe's true ID depends on material and schedule/wall. Enter the actual ID (stamped on the pipe or in the manufacturer table) or the gallons will be off. Velocity guidance is not a code sign-off.
How the math works
Volume = π × (ID/2)² × length, × 7.48052 for gallons. Flow = area × velocity, × 448.831 to turn cubic feet per second into gallons per minute. The trap this page is built around: nominal size is not inside diameter. A “6-inch” pipe’s true ID depends on material and wall/schedule — enter the actual ID from the pipe stamp or the manufacturer table, or every gallon downstream is fiction.
A worked example
A true 6 in ID line, 100 ft long, holds 19.63 cu ft — 146.9 gallons standing in it. At 5 ft/sec velocity the same bore moves 440.6 gallons per minute. And a tank that drains 1,000 gallons in 20 minutes is flowing at an average of 50 GPM, whatever the pipe is.
Questions from the field
How much water is in a pipe? π × (ID/2)² × length × 7.48052 gallons per cubic foot. A 100 ft run of true 6 in ID holds about 147 gallons.
How do I convert velocity to GPM? Area in sq ft × velocity in ft/sec = cu ft/sec; × 448.831 = GPM. Six inches at 5 ft/sec is about 441 GPM.
Why does the inside diameter matter so much? Volume scales with the SQUARE of the diameter. A half-inch of wall error on a small line is a double-digit percentage error in gallons.
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