What is the hardest Sudoku?
The objective answer — measured on the SE grade scale, not a marketing label.
By The Hardest Sudoku Team · Last updated
TL;DR
The hardest Sudoku puzzles are graded around SE grade 11 and above — 'monsters' like AI Escargot that have no simple entry point and require long forcing chains. Here is what that means and the famous examples.
What makes a Sudoku 'the hardest'?
Difficulty is best measured by the most advanced logic a puzzle forces you to use, not by how few clues it shows. 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.
The very hardest puzzles — the 'monsters' — sit around SE grade 11 and above. They offer no naked singles to start with and cannot be solved without long forcing chains, the deepest standard technique.
What are the most famous monster puzzles?
These are widely documented in the Sudoku community. Each links to its own page with the grid and full story.
| Puzzle | SE grade | Creator | Year |
|---|---|---|---|
| Arto Inkala's 2012 puzzle | 11.9 | Arto Inkala | 2012 |
| AI Escargot | 11.0 | Arto Inkala | 2006 |
| Easter Monster | 11.0 | JPF (forum.enjoysudoku.com) | 2007 |
How do I train on hard Sudoku?
Hardest Sudoku grades every puzzle on the SE grade scale and goes up to the monster bands. See how we rate difficulty or the techniques you'll need.
Play the hardest puzzles →Sources: Sudoku Explainer (SE) ratings; SudokuWiki.org; the enjoysudoku.com forum ("hardest sudoku" threads); contemporaneous press coverage of Arto Inkala's puzzles.