When Stockfish analyses a position, it shows a depth: 12, then 18, then 24, climbing the longer it thinks. Here is what that number means, and how deep is deep enough.
What depth means
Depth is how far ahead Stockfish searches, counted in plies: a move by one side. Depth 18 is eighteen plies, nine moves for each side. That is the nominal depth. Stockfish follows promising lines much further and cuts weak ones short, so it also reports a selective depth (seldepth), the longest line it looked at.
Stockfish deepens step by step. It searches to depth 1, then 2, then 3, reusing what it learned each time. The lines you see early are real, only shallower, and they settle as the depth grows.
Why each ply costs more
Each extra ply multiplies the work, so the time grows much faster than the depth. Going from depth 18 to depth 24 can take ten times as long or more, depending on the position and the device. A quiet ending goes deep quickly; a sharp middlegame full of captures goes slowly.
That is why analysis boards stop at a depth, or after a time, instead of searching forever. The extra depth usually changes the evaluation very little, while the processor, the fan and the battery pay the whole cost.
How deep is deep enough?
It depends on the question you are asking.
- Checking a book’s line, or your own move, for blunders: depth 16 to 20. At that depth modern Stockfish sees tactics far beyond human reach, and it takes seconds in a browser.
- Comparing two good moves in a quiet position: depth 22 to 26, or more time. Small differences between moves, a few tenths of a pawn, are where extra depth matters.
- Deep opening preparation: far more depth and time, ideally on a strong computer. A browser tab is not the place for it.
Keep in mind that a difference of 0.2 in the evaluation is rarely something a human can use. When the top moves are close, play the one whose idea you understand.
Lines, time and the other settings
- Lines (MultiPV): how many different moves Stockfish shows. Each line is searched to the same depth, so each one costs time. One to three lines are plenty for study.
- Maximum time: a limit in seconds, whatever depth has been reached. Useful on phones, and on laptops running on battery.
- Threads and hash: in a desktop program, more threads and a bigger hash table search faster, at the cost of memory and heat.
In GreyChess
GreyChess runs Stockfish 18 in your browser, on your own device. It searches to depth 18 by default and then stops, so the fan stays quiet. It runs on one thread with a small hash table, which keeps the book and the board beside it smooth. In the board’s settings you can choose the depth (from 12 to 24), how many lines to show and a maximum time. The best line appears under the board, and each move in it plays the line out on the board. When you want a second opinion, the position opens on Lichess in a click.
Try it on the positions in your own chess books, or on any diagram.