Your Online Elo is calculated using the same formula as the over-the-board (OTB) FIDE rating, with a coefficient specific to each time control:
Rn = Ro + (ΔR × K)
Rn: your new rating
Ro: your rating before the game
ΔR: the difference between your actual and expected result
K: the development coefficient (K-factor)
The full regulations, including K-factors, are published in the FIDE Handbook.
Where can I see how my rating changes?
Go to Ratings in your profile. There you can see your rating chart for All time, 365 days or 30 days, and a weekly breakdown with your rating, number of games, wins, losses, draws and your rank worldwide and in your country.
Can I check which games affected my rating?
Yes. All your rated games are saved in your profile and are public, so anyone can see exactly which games affected your rating. Rated games are marked with the FIDE icon in your game list.
If you have a Platinum or Black membership and a connected FIDE ID, your rating data is also sent to FIDE daily and shown on your fide.com profile. See Can I see my World Chess Title and rating on fide.com?