Convert Excel column to Integer

public static int ColumnNumber(string colAdress)
        {
            int[] digits = new int[colAdress.Length];
            for (int i = 0; i < colAdress.Length; ++i)
            {
                digits[i] = Convert.ToInt32(colAdress[i]) - 64;
            }
            int mul = 1; int res = 0;
            for (int pos = digits.Length - 1; pos >= 0; --pos)
            {
                res += digits[pos] * mul;
                mul *= 26;
            }
            return res;
        }