Use the formula to determine the cubic feet of area and multiply it by 7.48 gallons per cubic foot. Here is just an example: Say the pond is a perfect circle with a diameter of 20ft and 10 ft deep, .785x20x20x10=3140 cu. Ft. X 7.48 = 23,487 gallons. But the pond is probably sloped like a cone shape, then you will measure from the edge of the bank to the center of the bottom, lets use 18 ft as an example So, .785x20x20x18=5652 cu. Ft. Divided by 3 =1884 cu. Ft. X 7.48 =14,092 gallons. Get the rough measurements of your pond, use this formula, and your answer will be a very close estimate.