Why I made this
When I watch Levy Rozman, aka GothamChess, play "Guess the Elo", I often wonder what chance I would actually have at beating him. I realized that the Elo rating system describes exactly that. For example, a 400-rating-point difference means the favorite will win about 91% of the points. Although this might fall apart at extreme gaps, it still gives you a good estimate of just how far you are from beating some of these higher-ranked players.
How the calculation works
E = 11 + 10(Ropponent − Ryou) / 400
The result is your expected score: a win is 1 point, a draw is ½, and a loss is 0. So a 1% expected score could still mean you draw twice and lose 98 times out of 100.