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circular pond

pi X radius squared X height = volume of a cylinder

so

3.14159 times half-the-width times half-the-width again times the height
(measured in feet) equals the cubic footage

there are 7.45 gallons in a cubic foot

example:
if you have a circular pond
10 feet across and 2 feet deep
your pond contains
1,170 gallons_____________
_______________________

rectangular pond

length times width times height times 7.45 equals gallons

example:
if you have a square pond
10 feet east-to-west and 10 feet north-to-south and 2 feet deep
your pond contains
1,490 gallons_____________