How the rating works, explained in plain language
No formulas you need a degree to read. Two numbers go in, one number comes out, and here is exactly what happens in between.
By Aurora Sicat
A rating is a guess. It is the federation's best estimate of how strong a player is, based only on the games they have played and who they played them against.
Here is the whole idea in one sentence: before a game, we work out how likely each player is to win, and afterwards we move their ratings by how far the result differed from that.
Say you are rated 1600 and your opponent is 1800. The 200-point gap says your opponent should score about 0.76 out of 1 — roughly three wins in four. If you draw, you did better than expected, so your rating goes up and theirs goes down. If they win, almost nothing happens to either of you, because that was already the expected result.
How much a rating moves is set by one number called K. New players have K of 40, so their rating finds its real level quickly. After thirty rated games it drops to 20. For players above 2400 it is 10, because at that level a rating is already well established and should not swing on a single game.
That is all of it. Multiply K by the difference between what you scored and what was expected, round to the nearest whole point, and that is your change. Open any profile on this site and the Calculations tab shows those numbers for every game. You are meant to check them.