Analyze your chess games with Stockfish, in the browser

Paste a game, step through it move by move, and see exactly where it turned.

Free. No account required. Nothing you analyze leaves your device.

What it does

Chess Analyzer takes a game in PGN form — the format every online chess site exports — and replays it against the Stockfish engine. For each move you get an evaluation, a classification (was that a blunder, or merely an inaccuracy?), and the move the engine would have preferred instead. The evaluation bar beside the board shows who is ahead and by how much, so a collapse is visible as a shape before you read a single number.

You do not need a finished game to use it. Set up any position from a FEN string, or just start playing from the opening position on the front page: every move you make is scored as you go, and a green arrow shows the engine's preferred continuation. When you play something the engine dislikes, you can undo it and play its choice instead with one tap — which makes it a reasonable way to grind through a position you keep getting wrong.

How the analysis works

The engine is Stockfish, compiled to WebAssembly and running in a worker thread inside your own browser. That has a practical consequence worth stating plainly: by default your games are not uploaded anywhere. It also means analysis speed depends on your device, and that you can pick how hard the engine thinks — four depth settings, from a quick pass to a deep one.

Some accounts can additionally switch on Cloud Analysis, which sends the board positions of the game being analysed to our server so a much faster machine can do the work. It is off unless you turn it on; the privacy policy sets out exactly what is sent and what is kept.

Moves are graded by how much evaluation they throw away compared to the best available move, with special handling for the cases where raw centipawns mislead: walking into a forced mate is a blunder no matter what the numbers say, and finding a forced mate from a position you were losing is marked brilliant.

Guides

• How moves are classified — what separates an inaccuracy from a mistake from a blunder here, with the actual thresholds.
• Reading the evaluation bar — centipawns, mate scores, why the bar stops at ten pawns, and how the material count handles promotions.
• Choosing an analysis depth — what Fast through Very Deep really change, and why deeper analysis grades you harsher.
• Importing a game or a position — exporting a PGN from Lichess or Chess.com, what the parser discards, and how FEN fields are filled in for you.
• Free play and diverging from a game — scoring your own moves as you make them, and using the Best chip to drill a position.

What's new

The analyzer changes regularly. Recent releases pinned the opening name so it stays visible while you scroll the moves, and added opening names to free play — see what's new.

Also on Android

The same analyzer is available as an Android app on Google Play, with no ads at all. The web version is the quickest way to try it without installing anything.