An acre is 43560 square feet. A 60' x 50' pond would be 3000 square feet. So, 43560/3000 equals approximately 0.07 acres. If we assume a 4' average depth, it would be about 4000 cubic feet. That is about 30,000 gallons.
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3000 sq ft x 4 ft deep = 12,000 cubic feet.