How we rate difficulty: the SE grade
Difficulty should be measured, not claimed.
By The Hardest Sudoku Team · Last updated
TL;DR
SE grade (the Sudoku Explainer standard) rates a puzzle by the hardest logic technique it forces you to use. The scale runs from about 1.0 for the easiest puzzles to roughly 11.0+ for the monsters.
What is the SE grade?
SE grade is the difficulty rating produced by Sudoku Explainer, a well-known open solver. It works by finding the hardest technique required to crack a puzzle by pure logic and assigning a number for it. SE grade is the difficulty number from Sudoku Explainer: the higher the grade, the more advanced the logic a puzzle demands. Easy puzzles sit near 1.0–2.0; the hardest monsters reach roughly 11.0 and above.
Because the rating is tied to a specific, reproducible solving method, two people grading the same puzzle get the same number — which is exactly why we use it instead of vague labels like "expert".
What does each band mean?
Roughly, the scale climbs from singles-only puzzles to forcing-chain monsters. Each band below links to its own page.
| Band | SE range | What it takes |
|---|---|---|
| SE grade 1 | 1.0–2.0 | The gentlest puzzles. Solvable end-to-end by spotting naked and hidden singles — no candidate bookkeeping required. |
| SE grade 2 | 2.0–3.0 | Easy puzzles that still resolve through singles, but you may need to scan more carefully before the next placement appears. |
| SE grade 3 | 3.0–5.0 | Intermediate. Pairs and locked candidates (pointing/claiming) start to matter — you eliminate candidates rather than place values directly. |
| SE grade 5 | 5.0–7.0 | Hard. Basic fish (X-Wing) and consistent candidate elimination across rows and columns become the bread and butter. |
| SE grade 7 | 7.0–8.5 | Expert. Wings (XY-Wing) and larger fish (Swordfish) appear; progress often depends on chaining one elimination into the next. |
| SE grade 8 | 8.5–10.0 | Diabolical. Simple techniques run dry early; you lean on forcing chains and careful candidate logic to break the puzzle open. |
| SE grade 11 | 10.0–12.0 | Monster territory. These puzzles have no easy entry point and demand long forcing chains end to end — the home of AI Escargot, Golden Nugget and Platinum Blonde. |
Why does Hardest Sudoku use it?
Most Sudoku never gets hard enough — we go all the way up to the hardest; we measure difficulty directly and refine it with data.
We also run a controlled experiment comparing SE grade against other difficulty signals using real solver outcomes — so the metric earns its place rather than being assumed.
Sources: Sudoku Explainer (SukakuExplainer) rating engine; SudokuWiki.org technique documentation. Band ranges follow the conventional SE rating tiers.