How to autofill series in Microsoft 365 Excel.

Excel is treating A and B as a pattern to repeat because the cells contain text, not a numeric series. Likely the easiest solution is to use a formula that generates the letters based on the row number.

If A1 should contain A, enter this formula in A1:

=SUBSTITUTE(ADDRESS(1,ROW(),4),"1","")

Then fill the formula downward. It will produce A, B, C, D, ... Z, AA, AB, AC, and so on, continuing through EK.

If you specifically want the sequence to stop at EK, EK is column 141, so you need 141 rows, from row 1 through row 141.

Alternatively, if you want to use Excel's built-in series feature rather than a formula, enter A and B in the first two cells, select both, then go to Home > Fill > Series. Choose Columns, select Linear, and set Step value to 1. Note though that Excel's Series feature generally works with numbers, dates, and similar values, so the formula method is much more reliable for letter sequences.

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hth

Marcin