We are glad you asked! Your FOA rating is calculated by the same formulae as the over-the-board rating, with coefficient related to the specific time control. The actual formula is Rn = Ro + (ΔR x K), and full regulations, including K-factor, can be found on in FIDE Handbook under Online Arena regulations.
Is my rating data safe?
All official games are stored in your profile, and they are public, adding full transparency to the games that affected your rating. Also, the rating data is exchanged with FIDE on daily basis so that the rating change can reflect on your FIDE player’s card.
Arena also publishes useful hints in the profile showing how many ELO points you gained or lost during a specific period, such as a week, a month, or a year.