Pipe is generally cylindrical and the dimension refers to the inside diameter.
One cubic foot contains 7.48 gallons. The volume of a one foot long 20" diameter pipe is: pi(1.667/2)^2 = 2.18 cubic feet.
At 7.48 gallons per cubic foot the total volume is 16.3 gallons.
A 6-inch pipe holds 1.47 gallons per foot of length. A 6-foot pipe holds 211.5 gallons per foot of length.
106.12 US gallons per foot of 51-inch-diameter pipe.
2.61153 gallons.
7.481 gallons
There are 1.6293 UK gallons per foot in two inch diameter pipe. 1.6293 UK gallons converted into US gallons is 1.95671
0.3672 gallon per foot of pipe.
STD wall pipe holds 2.5 gallons a foot.
1.2
A 12 inch diameter round pipe will hold 4.89 UK gallons or 5.88 US gallons per foot.