Thank you John and Bondi - it works perfectly now! :)

"John Michl" wrote:

Bondi is correct. In order to use SUMPRODUCT, each array must have
exactly the same number of members. In your case, the first two arrays
have 59,999 members and the third 60,000. Note that the don't have to
be the same rows, just the same size. Also, I've not used semi-colons
in my Sumproduct formulas. If you are still having problems try a
comma.

- John Michl