The solution lies with using Excel's array feature to traverse a data list automatically and extract missing sequential numbers, even if the number list is out of order.

Inserting the Next Consecutive Integer in Excel. Creating a list of consecutive numbers is one of the easiest ways to create unique IDs for product lists, transaction IDs or event numbers. Of ...

A. Microsoft Excel is best known for crunching numbers, but it’s also a powerful tool for organizing tasks. Checkboxes can ...

Now, create a new number list in the next adjacent column (see the above screenshot). You have to use this number list to reverse the list in Excel.