Stockfish: the world's best free chess engine

Stockfish is the strongest open-source chess engine in the world. It’s free, runs in the browser without installing anything, and you can use it right now to analyze your games. If you want to play against the computer, it’s already built in here on ajedrez.pro.

What is Stockfish?

Imagine having a coach at your disposal who never gets tired, calculates millions of positions per second, and never makes mistakes. That’s Stockfish.

It’s an open-source chess program: anyone can see how it works, improve it, and use it without paying a cent. Thousands of programmers have contributed to it over the years, and the result is the strongest engine that exists.

How does it calculate so well? It uses a technique called Alpha-Beta, which lets it quickly discard bad moves and focus on the ones that actually matter. On top of that it adds an evaluation function: it analyzes the board position and assigns it a score based on factors like piece value, king safety, or pawn structure.

The number you see while it’s analyzing —like +1.3 or -0.7— is that score. A positive value favors White; a negative one favors Black. The higher the number, the bigger the advantage. Once you understand that, the engine’s output stops being a mystery.

The history of Stockfish (and why it matters)

Stockfish was born in 2008, created by Tord Romstad, Marco Costalba, and Joona Kiiski from an earlier engine called Glaurung. Since then, the open-source community hasn’t stopped improving it.

Here are a few milestones that shaped its history:

• 2018: Google unveiled AlphaZero, an AI-based engine that learned to play chess on its own in a matter of hours. It beat Stockfish in a series of games and left everyone stunned. Was this the end of Stockfish?
• 2019: Leela Chess Zero (LC0), another machine-learning engine, won the Chess.com Computer Chess Championship against Stockfish. AI engines were starting to compete on equal footing.
• 2020: At the Lichess Quarantine Championship, held during the pandemic, Stockfish got its revenge and beat LC0 in the final.

The lesson is clear: competition has pushed Stockfish to keep getting better. Today it incorporates neural network techniques (NNUE) that make it stronger than ever.

How to use Stockfish to improve your game

This is where the real value lies. It’s not about watching the computer win: it’s about using it to understand your own mistakes.

Here’s a three-step process I suggest:

1. Play your game. It can be on Lichess, Chess.com, or right here on ajedrez.pro.
2. Analyze with Stockfish. Load the game and watch for moments where the engine shows a sharp drop in your evaluation. Those are your key mistakes.
3. Study those positions. Ask yourself: why was my move bad? What did the engine see that I didn’t?

How many times have you lost a game without knowing exactly why? With Stockfish, that ends.

Interpreting the evaluation

The engine shows you a number in pawns (or centipawns, one hundredth of a pawn). Here’s how I interpret it:

• 0.0 to ±0.3 — balanced position.
• ±0.5 to ±1.0 — slight advantage, but the game is still open.
• ±2.0 or more — decisive advantage. The side with it should win with good technique.
• M5, M3… — mate in that many moves. End of discussion.

If you want to dig deeper into how player strength is measured, I explain it all in the guide on ELO.

Analyzing positions with FEN

Have a specific position you want to study? You can load it directly with its FEN notation. It’s a text string that describes the board at a given moment. Paste it into the analyzer and Stockfish gets to work instantly.

Where to use Stockfish for free

You don’t need to install anything. You have several options:

• Right here — on ajedrez.pro you can play against the computer directly in your browser.
• Lichess — a free, unlimited analyzer at lichess.org.
• Local download — if you want to use it on your PC with an interface like Arena or ChessBase, download it at stockfishchess.org.

My recommendation if you’re starting out: use it in the browser. No installs, no hassle. Once you get the hang of it, you can decide if you want to go further.

Stockfish as a training tool

The engine won’t make you a better player by itself. What it does give you is information that used to be available only to professional coaches.

Use it wisely:

• Don’t look at the analysis during the game. Learn to make decisions on your own.
• Analyze afterward, calmly, looking for the key moments.
• Don’t memorize lines blindly. Try to understand why the engine prefers each move.

If you want to know more about how to structure your training, I cover it in the guide on how to improve at chess.