I did some very quick and rough research a few days ago to see how chess24 ratings relate to FIDE ratings, and I share my findings here quickly. In short, the formulas that best matched the data were the following:
FIDE Standard rating = chess24 Classical rating - 201
FIDE Rapid rating = chess24 Classical rating - 160
Method of research: I went through the chess24 user list quickly and quite randomly (3-4 hours), focusing mainly on Premium members, and was able to identify 36 users whose corresponding FIDE profile could be identified with reasonable certainty. For each one of these, I took note of their various FIDE ratings (Standard, Rapid, Blitz) whenever available, as well as of their various chess24 ratings (Classical, Blitz, Bullet) whenever available and whenever the user had played a reliable number of games in each mode. This gave a database that enabled cross-comparing ratings over a reasonable number of users in order to identify quick formulas for estimating a user's playing strength in terms of an estimated corresponding FIDE rating based on his chess24 performance. (I have here been looking for a simple mathematical format such as: [estimated FIDE rating] = [chess24 rating] minus/plus [a specific constant].)
(1) As for a formula for determining a person's estimated FIDE Standard rating based on his chess24 Classical rating, the simple formula that best matched the collected user data was:
[estimated FIDE Standard rating] = [chess24 Classical rating] - 201
This formula was based on the data of 29 users who had both a chess24 Classical rating and a FIDE Standard rating. The real FIDE Standard ratings of these 29 users deviated from the predicted value with an average of 174 points, where their real FIDE Standard rating ranged from 798 points below the predicted value (in 1 extreme case) to 429 points above the predicted value.
(2) As for a formula for determining a person's estimated FIDE Rapid rating based on his chess24 Classical rating, the formula that best matched the collected user data was:
[estimated FIDE Rapid rating] = [chess24 Classical rating] - 160
This formula was based on the data of 14 users who had both a chess24 Classical rating and a FIDE Rapid rating. The real FIDE Rapid ratings of these 14 users deviated from the predicted value with an average of 178 points, where their real FIDE Rapid rating ranged from 641 points below the predicted value (in 1 extreme case) to 230 points above the predicted value.
Obviously, the formulas should be taken with a big grain of salt. They are based on very limited data (29 and 14 users respectively) and it's clear that there can be great individual variations depending on the particular conditions of each user. For instance, a person who has not participated in FIDE tournaments for a while but has been training intensively since then can be expected to have an official FIDE rating much lower than their estimated rating according to these formulas.
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