Stockfish online: free analysis in your browser

Which Stockfish is it?

Stockfish 18, packaged as stockfish.js 18.0.8. It is the "lite" single-thread WebAssembly build. Lite means a smaller neural network, so the engine file is about 7.3 MB. Single-thread means it needs no special browser settings, so it runs in a normal web page.

The trade-off is plain. A lite single-thread engine is lighter than a full desktop install with many threads. It is a good fit for reviewing your own games, and we do not claim it matches a desktop engine given hours.

How it runs on your device

The engine lives in a Web Worker, a background thread of your browser tab. We talk to it with plain UCI text messages. Your page stays responsive while it thinks.

To go faster we start several workers. The number is your CPU core count minus one, never fewer than one and never more than four. A four-core laptop gets three workers, and each one analyses different positions of your game in parallel. Each worker gets 16 MB of hash memory.

How a game gets analysed

We search by node count, not by depth or time. A node is one position the engine looks at. That keeps results steady across fast and slow devices, although slower devices take longer.

1. Quick pass. Every position gets 200,000 nodes, with two candidate lines kept.
2. Deep pass. Positions that look like mistakes, blunders or standout moves get 1,000,000 nodes, again with two lines. The second line is how we tell a move that was the only good one from a move that was merely fine.

The deep pass is capped at 36 positions per game, but blunder-level candidates are never dropped. In our own tests on one laptop, games of 33 to 47 half-moves took roughly 10 to 19 seconds, and a 77-half-move game about 29. Your times will differ.

Where your data goes

Your game is analysed by the worker in your browser. We do not send moves to a server for analysis, and the analytics events we record describe actions such as "review finished", not your moves. There is no account.

One exception is import. If you enter a Lichess or Chess.com username, your browser asks that site for your public games. That request goes to Lichess or Chess.com, not through us.

Licence

Stockfish is licensed under the GPL-3. We ship the engine unchanged as its own files, and the page talks to it only through messages. The licences page links to the source and the full licence text.

What you do with it

The engine output becomes accuracy, move labels and key-moment sentences. See free chess analysis for the full list and chess accuracy for the maths, or open an example review to see the output.