Blunder, mistake, inaccuracy: what move ratings mean

The engine rates a position in pawns: +1.0 means White is about a pawn better. That number becomes a chance of winning, from 0 to 100%. A pawn matters a lot in an equal position and hardly at all when you are a rook up, and the percentage captures that.

Each move is then judged by how many percentage points it lost compared with the best move.

• Best move: the engine’s first choice.
• Excellent: loses less than 2 points; good: less than 5.
• Inaccuracy: loses 5 to 10 points. Not a disaster, but there was something better.
• Mistake: loses 10 to 20 points, usually a pawn or a clearly worse position.
• Blunder: loses 20 points or more, often a piece or the whole game.
• Forced move: the only legal move; it is not judged.

A blunder rarely looks like one. Here Black develops a knight, a natural move, and loses at once: the knight attacks the queen but does nothing for f7.

Two more labels go to strong moves. An only good move (!) is the best move when every alternative is much worse. A brilliant move (!!) is a sound sacrifice: the move gives up material worth at least two pawns, and the position stays good.

• A label measures how much a move lowered your chance of winning, compared with the best move.
• Inaccuracy 5–10 points, mistake 10–20, blunder 20 or more.
• Read the reason, not just the label: it tells you what to learn.