[excel] Format numbers in thousands (K) in Excel

Enter this in the custom number format field:

[>=1000]#,##0,"K€";0"€"

What that means is that if the number is greater than 1,000, display at least one digit (indicated by the zero), but no digits after the thousands place, indicated by nothing coming after the comma. Then you follow the whole thing with the string "K".

Edited to add comma and euro.