Volume of water in casing

        assume 18 ft. depth of water. 40 ft. depth of well.
        Casing is 7.5 inch diameter. 3.75 radius.
        22 feet of water.  264 inches of water
        
        11663.0 cubic inches of water in casing
        11663.0 / 144.0 => 81 cu. ft. of water in casing.
        
        1 US gallon = 231 cubic inches
        11663.0 / 231.0 => 50.5 gallons of water in casing
        

calculator.net/volume-calculator

        Volume = π×3.752×264 = 11663.162726452 inches3
        11663.2 cu. in.
        
        11663.2 / 231.0 => 50.490043290043296
        50.5 gallons