Blunder checker
Find the moves that lost the game
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Find the moves that lost the game
- Paste a PGN or import from Chess.com and Lichess
- Every mistake and blunder with the move you should have played
- Jump from any blunder straight into the full review
How the checker decides
For every position in the game the engine produces an evaluation. We convert evaluations into a win probability for the side to move, using the same logistic curve Lichess fitted on millions of games: 0.00 is 50%, +1.00 is roughly 64%, +3.00 about 86%. Then we compare the probability before your move with the probability after it. The drop is the cost of the move.
| Label | Win-probability lost |
|---|---|
| Inaccuracy | 5 to 10 points |
| Mistake | 10 to 20 points |
| Missed win | From 85%+ down to below 60%, but not below 30% |
| Blunder | 20 points or more |
Why probability instead of pawns? Because a pawn is not worth the same everywhere. Dropping from +0.5 to -1.5 flips the likely result of the game. Dropping from +7 to +5 changes nothing except the engine's enthusiasm. Pawn-based checkers flag the second case and bury the first in noise.
For each flagged move we also store the engine's preferred line from the position before it. That is the “better was” suggestion in the list, with a few moves of continuation so you can see the idea rather than just the move. The full methodology, including how brilliant and great moves are awarded, is on the accuracy calculator page.
What a blunder looks like
Black to move after 4.Qh5 in the Italian. The natural developing move 4...Nf6 (red) walks into 5.Qxf7 checkmate. The engine prefers 4...Qe7 or 4...g6 (green), covering f7 and h5. In a report, 4...Nf6 is graded a blunder: Black's winning chances fall from about 45% to zero in one move.
Most blunders are less dramatic: a piece left undefended, a pawn push that opens a diagonal, a trade into a lost endgame. The checker finds all of them the same way, by measuring the swing.