This calculator takes the average Elo rating of each team, computes the resulting Elo expectation for a "one player" vs "one player" match using that average Elo rating, and then uses that average to compute a new Elo rating for each person on the original teams. See the page about Spades card game rankings for more details.
Results if your team wins:
Results if your team loses:
There is a very detailed page about how Elo ratings are computed at the site. The calculator above can help you understand how the ratings work. For the default inputs, notice that the highest rated (1600) player gains exactly as much as the lower rated (1400) player when winning - they both get 25 points.