What is the formula for round tank to gallons?

A round tank 10 feet in diameter. That's the measurement from one side to the other. And 3 feet high.

.785 x 3 x 10 x 10 = feet cubed. If you take a square and draw a circle in it. The area of the circle is less than area of the square. It's 78.5% of the area of a square.

So the area of a circle is .785 x dia squared. ( .785x3ft x 10x10 =cubic feet) = 235.5 cubic ft. Now multiply 235.5 x 7.48 to get gallons. 235.5 x 7.48 = 1,761.54 gallons of water in the tank. There's 7.48 gallons in a cubic foot of water. So the formula is

.785. X 3 x 10 x 10 x 7.48 = gallons in a round tank.