At this risk of raising more controversy over this matter...!

I want to use this function to test that the user has entered a
"correct" ISBN by validating the check digit.

I want to use
=if(right(a1)<(11-MOD(SUMPRODUCT(MID(A1,ROW(INDIRECT("1:9")),1)*{10; 9;8;7;6;5;4;3;2}),11)),"Invalid","")
but every possible last digit give the result "invalid".

To complicate matters further in some cases the result of the original
function could be 11 which is transalted into "X" on a book. How would
I need to change my formula to cope with that?

Thanks

Colin


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