How many gallons of water per foot will a 24 inch pipe hold?

I will work with the assumption that you mean an 18 inch diameter pipe.

The radius (r) is 9 inches and the length (l) is 12 inches per foot.

The volume contained will be calculated by ∏r2l = ∏ x 92 x 12.

If we use 3.1416 as the value of ∏, that gives us 3.1416 x 81 x 12, which works out at 3,053.6352 cubic inches of water.

Now, an Imperial gallon and a US gallon are different - an Imperial gallon holds 277.42 cubic inches and a US gallon holds 231 cubic inches

So in Imperial gallons the 12 inches of pipe holds 3,053.6352 / 277.42 = 11.0072 gallons

and in US gallons it holds 3,053.6352 / 231 = 13.2192 gallons.