Inspired by Francois Hochede's ratings calculator, I wrote my own version. It does the same thing with a few extras.
In the left-hand column, enter both players' rating and experience, plus the match length. Then press a win/lose button. The output will appear in the right-hand column.
If you've got experience of 400 or over, you can leave the experience field blank.
If you click the "You win" or "You lose" buttons, the program will display its output and do nothing else. That might be all you need.
If you click the "Win + update" or "Lose + update" buttons, the program will display its output and will reflect the changes in both players' rating and experience fields, just as if you had actually played the match. With these buttons, you can play matches over and over, and the ratings will update as they would on FIBS. You can change any field, like the match length, whenever you like.
The right-hand column shows the result from one match each time. The large text area at the bottom shows your rating change for lots of match lengths. It uses the current ratings and experiences for both players. It also shows the FIBS rating formula's assumed probability that the lower-rated play will win the match - this changes at different match lengths, of course.
If you click the "New form" button, both players will be reset to rating=1500, experience=0 and matchlength=1.