What this calculator does
FIDE, the international chess federation, rates players using the Elo system, but with a specific rule for choosing the K-factor, the number that controls how much a single result moves a rating. A newer or lower-rated player's rating swings more per game than an established, higher-rated player's, by design, so that ratings settle quickly for newcomers but stay stable for experienced players.
This calculator applies the published FIDE K-factor categories directly: K = 40 for players new to the rating list or under 18 and rated below 2300, K = 20 for established players rated below 2400, and K = 10 for established players rated 2400 or above. Picking the wrong category is the most common source of a rating-change estimate that does not match FIDE's own calculation.
The formula
The expected score is worked out from the rating gap between the two players using the standard Elo formula. The rating change is then the K-factor for the chosen category, multiplied by the difference between the actual result (1 for a win, 0.5 for a draw, 0 for a loss) and that expected score.
| Term | Meaning |
|---|---|
| K-factor | A number set by FIDE's rating rules that controls how much one result can move a rating; higher K means bigger swings. |
| Expected score | The probability of winning implied by the rating gap between the two players, from 0 to 1. |
| Established player | Under FIDE rules, a player who has completed at least 30 rated games and so has moved past the new-player K-factor. |
The inputs explained
| Field | What to enter |
|---|---|
| Player rating | The player's FIDE rating before the game. |
| Opponent rating | The opponent's FIDE rating. |
| Result for player | The result of the game from the player's perspective. |
| FIDE player category | Choose the category that matches the player's current status; this sets the K-factor FIDE applies to the result. |
When to use it
Estimating a rating change after a tournament game
Plugging in the pre-game ratings and the result gives a close estimate of how much a single FIDE-rated game moves a rating, ahead of the official update.
Understanding why a new player's rating moves so much
A newcomer's K = 40 means an early result can swing their rating by up to 40 points, far more than the same result would move an established player rated below 2400 (K = 20) or above it (K = 10).
Comparing to a generic Elo calculation
A general Elo calculator lets you pick any K-factor by hand; this one applies FIDE's specific, published category rules instead, which is the more accurate choice for an actual FIDE-rated game.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the same result change ratings differently by K-factor category?
The identical game and result, run through each FIDE player category.
| Player category | Rating change | K-factor used |
|---|---|---|
| New to rating list (K=40) | +20.00 | 40 |
| Under 18, rated below 2300 (K=40) | +20.00 | 40 |
| Established, below 2400 (K=20) | +10.00 | 20 |
| Established, 2400+ (K=10) | +5.00 | 10 |
Questions
What K-factor does FIDE use for a brand-new player?
K = 40 applies to a player who has not yet completed 30 rated games, and separately to any player under 18 whose rating is still below 2300, regardless of games played.
When does a player move from K = 20 to K = 10?
Once a player has completed at least 30 rated games and their rating has reached 2400 or above, FIDE applies K = 10 for their subsequent games, reflecting that an established, higher-rated player's true strength is already well established.
Does this replace FIDE's official rating calculation?
No. This gives a close estimate for a single game using the published formula and K-factor rules, but the official calculation can involve rounding conventions and rating-period batching that this calculator does not attempt to replicate exactly.
How does this differ from a generic Elo calculator?
A general-purpose Elo rating calculator lets you type in any K-factor directly. This calculator instead selects the K-factor for you based on FIDE's own published player categories, which is more accurate for an actual FIDE-rated game.
For a generic Elo calculation with any K-factor you choose, see the Elo rating calculator.