Chess engines explained
What is Stockfish? The chess engine, explained simply
Stockfish is a free, open-source chess engine and the tool behind most “best move” arrows you see online. This guide explains what it does, how to read its evaluation, and how to use it to learn from your own games rather than just copy its moves.
What Stockfish is
A chess engine is a program that searches through possible moves and scores the positions it reaches. Stockfish is open-source software developed by a community of volunteers since 2008. It combines a deep search of candidate moves with a neural-network evaluation of positions, known as NNUE, added in 2020.
It is widely regarded as the strongest chess engine available, far above any human player. That strength is exactly why it is useful for analysis, and why playing a full-strength game against it teaches little.
How to read a Stockfish evaluation
The evaluation is an estimate in pawns. Positive numbers favor White, negative numbers favor Black. The deeper the search, the more reliable the number, but it remains an estimate.
| Evaluation | What it means |
|---|---|
| +0.3 to -0.3 | Roughly equal position |
| +1.0 | White is about a pawn better |
| +3.0 or more | White is winning in practice (roughly a piece up) |
| -1.5 | Black is better by about a pawn and a half |
| M3 / #3 | Forced mate in 3 moves for the side shown |
Read the best line, not only the number. A +2.0 that depends on a long sequence of only-moves is much harder to realize in a real game than a +2.0 based on an extra piece.
How to use Stockfish to improve
- Think first, then check. Choose your own move, then compare it with the engine.
- Review your games. Let the engine find the moments where the evaluation dropped, and ask why.
- Understand, don't memorize. For each mistake, find the idea you missed: a tactic, a weak square, a loose piece.
- Practice the patterns. Turn your mistakes into positions to solve again later.
Use Stockfish free in your browser
On Chessdesk, Stockfish runs directly in your browser with no account. Explore any position on the analysis board, get the best move from a screenshot, FEN or PGN with the next-move calculator, and find the mistakes in a whole game with the free game review. To understand the labels it gives, see our guide to move classifications.
To play rather than analyze, pick a human-like bot near your rating instead of full-strength Stockfish.
Frequently asked questions
What is Stockfish in chess?
Stockfish is a free, open-source chess engine: a program that analyzes positions and suggests the best moves. It is widely regarded as the strongest chess engine available and is used by players, coaches and chess websites for analysis.
Can I play against Stockfish?
At full strength, Stockfish is far stronger than any human, so a game against it teaches little. It is more useful as an analysis tool. To play, choose a bot near your level: Chessdesk's bots are human-like opponents with fixed ratings, from three free bots between 723 and 1086 Elo to a ladder from about 400 to 2400 with a free account.
Is Stockfish free?
Yes. Stockfish is open-source software. On Chessdesk it runs in your browser on the analysis board, the next-move finder and the game review, with no account needed.
What does the Stockfish evaluation mean?
The number is the engine's estimate in pawns: positive favors White, negative favors Black. +1.0 is roughly an extra pawn. An M or # followed by a number means a forced mate in that many moves.
Review your last game with Stockfish
Free, in your browser, no account. Create a free account to save your games and turn your mistakes into puzzles.