You can put this solution on YOUR website! How many gallons of water could a 5" diameter pole hold if it's 336 feet long
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I doubt a pole holds any.
If you mean a pipe, and the ID is 5" --> r = 2.5
---- cubic inches
Vol =~ 79168 cu inch
1 gal =~ 231 cu inches
--> 79168/231 =~ 342.7 gallons