A formula way, try :

1] Assume your datas located at "Sheet x, A1:A20" and "Sheet y, A1:A20"

2] Sheet x, A21, enter the array formula and filled down :

{=INDEX('Sheet y'!A:A,SMALL(IF(ISERROR(MATCH('Sheet
y'!A$1:A$20,$A$1:$A$20,0)),ROW($1:$20),65000),ROW( 1:1)))&""}


Regards
Bosco


"Chel" wrote:

Is it possible to write a formula that will autopopulate a column with any
missing numbers. I believe it should be something really simple, but I can't
get it to work.

Example: Sheet x, column A - contains employee numbers
Sheet y, column A - contains employee numbers

I want to update Sheet x, column A with any employee numbers that are on
Sheet y and not sheet x.

Thank you in advance.