For ponds that are significantly square or rectangular, use this formula:
Length x Width x Average Depth x 7.5 = Gallons
Example:
A rectangular pond is 90' long by 40' wide with an average depth of 4'.
90 x 40 x 4 x 7.5 = 108,000 gallons of water
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For ponds that are significantly circular, use this formula:
Diameter x Diameter x Average Depth x 6 = Gallons
Example:
A circular pond is 125' across with an average depth of 5'.
125 x 125 x 5 x 6= 468,750 gallons of water
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For ponds that are significantly elliptical, use this formula:
Length x Width x Average Depth x 6 = Gallons
Example:
An elliptical pond 200' long by 80' wide with an average depth of 6'
200 x 80 x 6 x 6 = 576,000 Gallons of Water
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If you need to know how to figure Surface Acreage, read this page.