Originally Posted by GrannyGear
Go to
http://www.sheldonbrown.com/gears/ for an automated calculator.
*** Changing chains along with your new cassette is a good idea...you may well need some extra links anyway to accomodate "bigger" cogs.
I like this calculator better.
http://www.panix.com/~jbarrm/cycal/cycal.30f.html
It gives more info and allows you to change the front chainrings as well. Use sheldon's for the listing of the number of teeth on each ring of standard cassettes.