Pipe volume formula
The internal volume of a pipe is the volume of a cylinder: pi times the internal radius squared times the length. When working in inches for diameter and feet for length, convert units before multiplying, then convert cubic inches to gallons (one US gallon equals 231 cubic inches).
Use the internal diameter, not the nominal pipe size. A nominal 1-inch copper pipe has an internal diameter of about 1.055 inches (Type M) or 0.995 inches (Type L).
V = pi x (d/2)^2 x L
V is volume, d is internal diameter, and L is pipe length. All dimensions must use the same units.
1 US gallon = 231 cubic inches
Conversion factor for translating pipe volume to gallons.