Math CentralQuandaries & Queries


Question from tim:

I need to know how many gallons of water a 16" pipe that is 3 miles long holds?

Hi Tim,

I know there are 231 cubic inches in a US gallon so I would first 3 mile measurement to inches. There are 12 inches in a foot and 5280 feet in a mile so 3 miles is 3 × 12 × 5280 inches.

Your pipe is long cylinder that is 16 inches in diameter so its radius is 8 inches. The volume of a cylinder is π r2 L where r is the radius, L is the length and π is approximately 3.1416. Thus the volume of your pipe is

π × 82 × 3 × 12 × 5280 cubic inches.

Since there are 231 cubic inches in a US gallon divide the volume in cubic inches by 231 to obtain the volume in gallons.


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