Chess Blunder Checker

How a blunder is measured

We turn the engine score into a winning chance, called Win%, from the point of view of the player who moved. A move is judged by how many Win% points it gives away compared with the best move.

• Inaccuracy: a drop of 5 points or more.
• Mistake: a drop of 10 points or more.
• Blunder: a drop of 15 points or more.

If you played the engine’s top move, it is never marked as an error. Small differences between two searches are noise.

Two rules that surprise people

A blunder needs the deeper pass. The first pass is quick and covers every move. Moves that look like blunders are then searched much deeper. If the deep search does not confirm it, the label is downgraded to a mistake.

Allowing mate is a blunder, with one exception. If your move lets the opponent force mate, it is a blunder, but only when you still had at least 10% winning chances before it. If you were already below that, allowing mate does not make it a blunder on its own.

Miss. A miss is a drop of 10 to 15 points right after your opponent’s mistake or blunder. You were handed an advantage and let part of it go. A larger drop is still a blunder or mistake.

An example: the Immortal Game

We ran the 1851 Anderssen–Kieseritzky game through the engine. At move 18 the engine flags 18.Bd6, which takes White from 70.7% to 50.0%, because it allows a fork.

Two moves later the engine flags 20...Na6, which goes from 30.5% to 0.0%, because it allows a forced mate.

Each note comes with a line you can play on the board in the review, so you can see the refutation instead of taking our word for it.

Use it on your own games

Paste a PGN in the box at the top. In our tests a typical game took under half a minute on a laptop, and then you get the labelled moves and key moments. Need the file first? See the PGN analyzer guide.

One game shows one afternoon. To see what you do again and again, enter your Lichess or Chess.com username on the report page. It groups your recent games' errors into patterns, such as leaving pieces undefended, missing pieces your opponent left hanging, or letting won positions slip, so you know what to practise.

What the checker cannot do

It runs Stockfish 18 lite, a compact build for browsers. In a complex middlegame a larger engine on a server might rate a close move differently. Treat the labels as a guide, then check the line yourself.