Check a chess game for blunders

1. 1Bring the game. Paste the PGN, or type a chess.com or lichess username and pick from the recent games that come back. Any PGN works, from any source — a club game typed out by hand is as welcome as an online one.
2. 2Let the engine read it. Stockfish runs in your browser, twice: a quick pass over the whole game, then a deeper look at the moments that mattered. A normal game takes well under a minute, and you can play through it while it works.
3. 3Look at the marks. Every move is graded on the board. Blunders, mistakes and misses are marked where they happened, each with what the position was worth before and after, and the move that should have been played.

A blunder is a move that throws away at least thirty percentage points of winning chance. That is deliberately measured in winning chance rather than material: dropping a rook while a queen ahead barely changes the result, and giving away a single pawn in a level rook ending can be the whole game. Material alone cannot tell those apart.

There is a separate grade for the other bad thing that happens to club players — letting a won game slip without losing it outright — because “you were winning and now you are only slightly winning” is a different mistake with a different cure. All eleven grades are set out in the guide.

Finding the blunder is the easy half. What changes your results is what comes next:

• Play the position on again. The board stays live — try the move you wish you had played and the engine judges it against the one you did.
• Check several games at once. A review of ten or twenty finds the mistakes you keep making rather than the one you made on Tuesday.
• Drill your own blunders. The positions you got wrong become puzzles, so the thing you practise is the thing you actually miss.

Check a game now, or read about how this compares with a paid game review and what an accuracy score is worth.