How ratings are calculated

This calculator uses the KNSB (Dutch chess federation) system. For each game, your rating changes by:

rating change = K × (your score − expected score)

• Your score — 1 for a win, ½ for a draw, 0 for a loss.
• Expected score — how much you were expected to score, based on the rating difference (your rating minus the opponent’s). It is computed with the KNSB normal-distribution formula (standard deviation 2000÷7).
• K — the development factor (verversingsfactor). It depends on age and how many rated games you have played.

The result is rounded to one decimal place. Games against an unrated opponent (rating 0) do not change your rating.

What is K? What is Nv?

K is the verversingsfactor (development factor): how many rating points you gain or lose per game for a given surprise. A higher K means bigger swings.

Nv is the number of rated games your current published rating is based on (your rating history, capped at 100).

• Youth (junior categories through category A), rating ≤ 2100: if Nv < 30, K = 216 ÷ √Nv (higher when you are new); from Nv ≥ 30 onward, K = 40.
• Adults, rating ≤ 2100: if Nv < 75, K = 216 ÷ √Nv; from Nv ≥ 75 onward, K = 25.

So 40 for youth (Nv ≥ 30) means an established junior player uses K = 40. The calculator defaults to 40; change it below if yours is different (you can look up your K on ratingviewer.nl).