You need three dimensions to calculate the volume: Width, Length, and Height. Multiple the three dimensions together to find the volume then multiply by 7.481 (gallons of water per cubic foot). HINT: Be sure and convert yards to feet before multiplying. +++ More +++ Given that the tank is cylindrical, the 2 yards wide would be the diameter. The Area must be multiplied by the height to find the volume. Area for Diameter is A = Pi x R-squared where Pi is 3.1416 and R is Radius. The Radius is half of the diameter, hence a diameter of 6 ft (2 yds) would have a radius of 3 ft. Use the formula to find the Area: 3.1416 x (3x3) = 3.1416 x 9 = 28.2744 sq ft. Next multiply the area by the height to find the volume: 28.2744 x 15 = 424.116 cu ft. Next multiply the volume by number of gallons per cubic foot (7.481): 424.116 x 7.481 = 3,172.8117 gallons capacity (assuming the dimensions given are inside dimensions).