The site uses Glicko rating. The formulas can be found on the following site.
http://www.glicko.net/glicko.html
It is possible the site uses some approximation of the formulas and not the exact ones, but that would require staff to answer.
chess rating calculation
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If I know my rating and my opponent's rating, how can I calculate my new rating if I win (or lose) the current game. Please give the formula.