Oh, I think it is a math question, is that 1/2 inch diameter water pipe? Figure out the volume of 1/2 gallon of water and the corresponding volume in the 1/2" pipe? Sorry, don't know my formula's, and my dad is not around today. If no one answers, I'll ask him this weekend. If that is the question.
I would just get some pipe and fill it with 1/2 gallon of water and mark where it goes up to.... lazy, I know!
Yes that is correct, as a rule of thumb 100 feet of 1/2" will hold 1 gallon of water. With each 1/4" size increment doubling that volume. 100' of 3/4" holds 2 gallons and 100' feet of 1" holds 4 gallons.