How to use the chess analysis board
This chess analysis board reviews a whole game with Stockfish, the open-source chess engine, running in your browser. Paste a game in PGN (the text format that Lichess, Chess.com and every chess database export), press Load game, then Review the game. The engine evaluates every position of the main line, draws the evaluation graph, grades each move and puts an arrow on the board for the best move in every position. No account, nothing uploaded.
Step through the game with the arrow buttons under the board or with the left and right arrow keys on a keyboard (Home and End jump to the start and the end). Tap any move in the move list, any chip under Jump to the mistakes and blunders, or any point of the graph to go straight there.
You do not need a PGN: you can also play the moves of a game on the board from the starting position, or switch to Set up position to use the board editor. Place pieces from the palette, choose the side to move and the castling rights, and press Analyze this position. The editor refuses impossible positions and says why (a missing king, a pawn on the first rank, the side not to move in check, and so on).
Trying other moves: variations
At any point of the game, play a different move on the board. It is added as a variation in brackets in the move list and the game itself stays untouched. Turn on Analyze every position I visit to see the engine's top lines for the position on the board, including positions inside your variations, and press Play beside a line to follow it. Make this line the main line promotes a variation; Delete from this move removes a move and everything after it. Variations that were in the pasted PGN are kept, as are its comments and annotation symbols.
How moves are graded
For every position the engine gives an evaluation. We turn it into a winning chance from 0 to 100 for the side to move with the formula 100 / (1 + e−0.00368 × centipawns), where a forced mate counts as 100 and being mated as 0. A move's loss is how many points of winning chance the player gave away: their chance before the move minus their chance after it. These are our own thresholds, chosen for this tool:
| Grade | Rule | Near equality, about |
|---|---|---|
| Best | the engine's own first choice (or the only legal move) | no loss |
| Good | loss under 5 points | under half a pawn |
| Inaccuracy (?!) | loss of 5 to 10 points | half a pawn to a pawn |
| Mistake (?) | loss of 10 to 20 points | a pawn to two and a quarter pawns |
| Blunder (??) | loss of 20 points or more | more than two pawns, or allowing mate |
Working in winning chances rather than raw pawns means that dropping a pawn in an equal position counts, while dropping a pawn when you are already a rook up barely matters, which matches how the game feels. The summary table counts the grades for each side and shows the average loss per move.
Depth, time and what the engine is
The review depth sets how far ahead Stockfish looks in each position. Fast (depth 10) is the quickest; Deep and Very deep take several times longer but catch more tactics. Positions already evaluated at the chosen depth or deeper are not searched again. Before a move is graded Inaccuracy, Mistake or Blunder, the review takes a second look: it searches the position before that move again at least six plies deeper (up to 3 seconds per position), then scores the move that was played on its own at the same depth. A move the deeper search rates as best, or as good as best, is graded Best. That is why the queen sacrifice 15. Bxd7+ in the example game comes out Best and not as a mistake, which a plain depth 12 search would say. For a single position, the live engine thinks for 1, 3 or 10 seconds, to depth 20, or until you press stop.
The engine is Stockfish 19 in its compact single-threaded WebAssembly build (the lite version from the stockfish.js project, about 1.8 MB), running on your own device in a background thread. It is weaker than full desktop Stockfish with its large network and many threads, and short searches can misjudge deep tactics, but it is still far stronger than any human player. It loads only when you first ask for an evaluation.
Saving and sharing your analysis
The game, your variations and the evaluations are kept in this browser, so the page shows them again next time. Copy link to this puzzle creates a link that opens the same game and review on another device. Under FEN, PGN and export you can copy the FEN of the current position, start a new game from a FEN, or copy the whole game as PGN, with the grades added as ?!, ? and ?? and a note with the engine's preferred move. Use the board to study your own games after you play them, not during a game.
For a single position, such as a puzzle from a book, the chess move calculator shows the top lines and looks for forced mates.
Solver tested . Found a problem? Tell us.
Questions
How do I analyze a chess game from Lichess or Chess.com?
Export or copy the game as PGN (both sites have it in the share or download menu of a finished game), paste it in Paste a game and press Load game, then Review the game. Clock times and other comments in the PGN are kept as comments.
Is this Stockfish online for free?
Yes. The page runs Stockfish 19 in its lite WebAssembly build directly in your browser, so there is no account, no limit on games and nothing is sent to a server. It needs a current browser (on iPhone, Safari 16 or later).
Why does a move I like show as an inaccuracy?
The grade only measures how much winning chance the move gave away compared with the engine's choice, at the depth you picked. Practical, human-friendly moves can lose a few points. Try a deeper review or turn on the live engine on that position to see the line the engine prefers.
Can I set up a position instead of a whole game?
Yes. Switch to Set up position to use the board editor, or paste a FEN under FEN, PGN and export. You can then play moves from that position and review them like a game.
Are variations in my PGN kept?
Yes. Variations in brackets, comments and annotation symbols are read and shown in the move list. The review grades the main line; variations are evaluated when you visit them with the live engine on.
How accurate is the review?
It is as accurate as the search depth allows. Depth 10 to 12 finds simple tactics and clear blunders; for close positions use Deep or Very deep, or the live engine with more time. Evaluations near 0 can change by a few tenths from one search to the next.