How much liquid volume in ten feet of 12 inch copper pipe?

There are 231 Cubic inches in One U.S. Gallon. Assuming your pipe has an ID of 12" a 10 foot length has 13564.8 Cubic inches. To calculate this you must first calculate the area of the 12" circle and then multiply it by the length (in inches) It would look like this:

Area = Pie x radius squared. The radius is 1/2 the diameter or 6" Sooo....

Area = 3.14 x 36 which comes to 113.04 square inches.

Volume = Area x length

Volume = 113.04 x 120" which is 13561.8 cubic inches

to convert to gallons simply divide 13561.8 by 231 = 58.72 gallons.

Hope that helps

Scott