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There is no universal Elo value for one extra ply. In LK Forge’s 2026 fixed-depth self-play benchmark, chess gained +321 Elo from depth 1 to 2, +315 from 2 to 3, and +225 from 3 to 4. Those are steps between versions of one engine, not ratings against people or other engines.
What one ply means
A ply is one half-move: one move by either White or Black. Search depth counts those half-moves from the root position. A depth of four, for example, looks ahead through four individual moves, rather than four complete turns.
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In a simple minimax search, the engine would examine every line up to the chosen depth. Practical engines do not search every branch uniformly: they prune low-priority lines, reduce some branches and extend others. Stockfish’s FAQ therefore treats reported depth as a useful guide, not a guarantee that every move was examined to that ply.
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LK Forge tested fixed-depth negamax engines for Connect 4, Checkers, Othello and Chess. Its 2026 article reports 2,400 headless self-play games in total. Each adjacent depth pair played 120 colour-balanced games: chess covered depths 1–4, Connect 4 and Checkers 1–7, and Othello 1–6. Games used random legal opening moves; each opening was played with the sides swapped, and draws counted as half a point. The author reports using master seed 20260819 and calculates each Elo step from the colour-balanced score using 400 × log10(p/(1−p)).
#1 Best Overall
| Game | Depth range tested | First extra ply, depth 1 to 2 |
|---|---|---|
| Chess | 1–4 | +321 Elo |
| Connect 4 | 1–7 | +953 Elo |
| Checkers | 1–7 | +546 Elo |
| Othello | 1–6 | +610 Elo |
All figures in this table are LK Forge’s self-play Elo steps between adjacent depths in its own engine, not absolute engine ratings. The unusually large first-ply differences in Connect 4, Checkers and Othello illustrate how strongly the outcome depends on the game. A depth-1 player can miss immediate tactical consequences that a single additional half-move reveals.
How the value changes with chess depth
For chess, LK Forge reports +321 Elo from depth 1 to 2, +315 from 2 to 3, and +225 from 3 to 4. The first two gains are close; the third is smaller. This is evidence against a simple rule that every extra ply is worth the same amount—or that the first ply must always be the most valuable.
Rank #2
LK Forge attributes the pattern in chess to its large branching factor: even after a shallow increase in depth, much of the position tree remains unexplored. The benchmark’s result applies to its engine, seeded openings and tested depths, not to every chess engine or position.
What these Elo steps do—and do not—tell you
They compare depth settings within one implementation
The benchmark changes search depth while keeping the engine implementation in view. It is not a contest between different engines, and its Elo figures are not transferable to human ratings or to commercial-engine rating lists. LK Forge notes that high draw rates in chess and checkers make those Elo gaps approximate.
Rank #3
Depth is not the same as time or nodes
LK Forge uses depth rather than elapsed time as its comparison knob, reducing the direct dependence on hardware speed that a milliseconds-per-move test would have. But fixed depth alone does not make engines generally comparable: evaluation functions, search methods, hardware, positions, time controls and draw rates can all affect practical playing strength.
Stockfish’s advanced documentation describes nodes per second (NPS) as useful for comparing speed, while warning that NPS differences between engine versions do not necessarily track differences in playing strength. Speed is not a substitute for a match-strength result.
Different search methods need different interpretation
Leela Chess Zero (Lc0) provides a useful contrast, but it was not one of the four engines in this test. Its official FAQ describes a neural-network engine using Monte Carlo Tree Search. Lc0 can run on a CPU, but is significantly slower than on a modern GPU; comparisons depend on hardware and testing conditions. Its search counts or depth should not be compared directly with LK Forge’s fixed-depth negamax results.
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Analysis depth is not a universal strength threshold
Chess.com’s August 3, 2026 support article says its server-side analysis uses Stockfish 18 with NNUE and reports diminishing returns in engine quality past the low 20s for broadcast analysis. That is Chess.com’s own analysis context, not a universal point at which more depth stops helping.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to evaluate another “Elo per ply” claim
Before applying a reported gain to your own engine or analysis, check what was held constant and what the rating measures. A useful benchmark report should make clear:
- Whether games use fixed depth, fixed time or a fixed node budget.
- The search method and evaluation model for each engine.
- Hardware and thread count.
- Which positions or openings were tested, and whether colours were balanced.
- The number of games, how draws were scored and how much uncertainty remains.
- Whether Elo is an internal self-play step or a rating against an external pool.
LK Forge reports seeded, colour-balanced self-play and fixed-depth comparisons, which makes its setup interpretable on those terms. It does not establish a universal conversion from one additional ply to a particular Elo gain.
Exploring engine analysis yourself
Stockfish is free software available for desktop and mobile platforms; its official use page describes ways to run it. Changing depth in an analysis interface can help show how a position’s evaluation or best line changes as search proceeds. Treat that as an illustration of one position, not as a match-strength measurement: a strength comparison requires games under controlled conditions.
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