Stockfish Elo and skill levels

Stockfish has two built-in ways to play weaker. Both are UCI options, settings that a chess program sends to the engine before it searches.

• Skill Level, from 0 to 20. 20 is full strength and 0 the weakest setting. Below 20, Stockfish compares several candidate moves and sometimes picks a weaker one on purpose, with an element of chance. The lower the level, the earlier in the search it makes this choice and the larger the mistakes it accepts.
• UCI_LimitStrength and UCI_Elo. With UCI_LimitStrength switched on, a program can ask for a target strength in Elo with UCI_Elo. In Stockfish 19, the allowed range is 1320 to 3190. Internally, Stockfish converts this Elo into a skill level.

The range is set in the source code (search.h in Stockfish 19). According to a comment there, the conversion is fitted to games between Stockfish at different skill levels and another engine, and covers roughly 1320 to 3190 on the CCRL Blitz scale. CCRL is a rating list for engines, not for people.

There is a third way that every program can use: limiting the search itself. A fixed number of nodes (positions the engine looks at), a fixed depth or a short time per move all make Stockfish weaker, because a shallow search misses tactics that a deeper search finds. Many apps combine such limits with the skill level.

An Elo number only means something inside its own pool of players. Engine levels are measured in games between engines. Your rating comes from games between people, on one particular site or in one federation.

• Different pools. UCI_Elo is anchored to an engine rating list. Lichess and Chess.com ratings come from games among their own players, FIDE ratings from rated over-the-board games.
• Different mistakes. A limited engine plays mostly strong moves and then, now and then, a weaker one chosen by chance. People make other kinds of mistakes: they overlook a threat, misjudge a plan or run short of time. A level that matches your rating on paper can feel much harder or much easier.
• Different conditions. The time per move and the speed of the device change how strong a level plays.

Even human ratings do not match each other. The same player has different numbers on Lichess, on Chess.com and in FIDE, as the comparison in chess rating conversion shows. So treat the Elo of an engine level as a step on a ladder, not as a rating you can compare with your own.

Far above any human. With Stockfish 18, the Stockfish team wrote that the engine is stronger than any human player, even on older or low-end hardware (announcement). On the CCRL 40/15 rating list of 16 October 2026, Stockfish 19 on four processor cores is in first place with 3650 Elo.

That number is on an engine scale, measured against other engines under fixed conditions. It does not tell you exactly how a match against a person would go. What it does tell you: at full strength, Stockfish is a tool for analysis, not a training partner for people. To play against it, you need a limited level. What changed in the latest version is summed up in Stockfish 19.

On Lichess you can play against the computer at levels 1 to 8. The computer moves come from fishnet, the distributed Stockfish service of Lichess. At the time of writing, its source code (api.rs) sets three limits for each level:

Three things stand out. Levels 1 to 3 use skill values below 0, outside the range of the official Stockfish option. At level 8, the skill limit is off (20), but depth and time are still capped, so level 8 is very strong without being Stockfish at full strength. And the code sets skill, depth and time, not an Elo. Any Elo you read for a Lichess level is an estimate from elsewhere, not a number from Lichess's code.

ChessWOW uses Stockfish 19's own settings for its five levels:

The Beginner level needs a different method. UCI_Elo cannot go below 1320, and Skill Level 0 on its own still plays too well for someone who is learning the game. So ChessWOW combines Skill Level 0 with a small limit on the number of nodes. The engine then searches so little that it also misses short tactics. All Elo values are targets on Stockfish's scale, not Lichess or Chess.com ratings, and the device and the time control influence how a level plays.

You can try the same five levels in your browser on the board at the top of play against Stockfish. The browser runs a smaller version of Stockfish 19 with a smaller network.