The puzzle most often called the world's hardest sudoku is a 21-clue grid by Finnish mathematician Arto Inkala, published by The Daily Telegraph and other papers in June 2012. It is shown below. It is very hard: it cannot be solved with the standard pattern techniques most players know, and our own grader cannot place a single digit in it. It is not, however, the hardest puzzle that exists. Sudoku enthusiasts have since rated other puzzles harder, and there is no official way to decide the title.
As a string, row by row with 0 for blanks: 800000000003600000070090200050007000000045700000100030001000068008500010090000400. The digits match on SudokuWiki and in the ABC News coverage from June 2012.
What happened when we graded it
Sudoku Royale grades every puzzle before a player sees it. The grader tries techniques from easiest to hardest: naked and hidden singles, pointing and claiming, naked and hidden pairs and triples, X-Wing, swordfish, jellyfish, XY-Wing and simple colouring. That set is enough for 91% of the puzzles we generate. We ran Inkala's two famous puzzles through it on 24 September 2026.
- Inkala 2012: zero digits placed. After filling in every candidate, none of the 13 techniques applies anywhere on the grid.
- AI Escargot (2006): one hidden single, then stuck.
- For comparison, the example puzzle on Wikipedia's sudoku page needs nothing but 51 naked singles.
Both Inkala puzzles need chain-based logic: long "if this cell is 5, then that cell cannot be 3" sequences, sometimes several of them nested. Humans who solve them usually rely on forcing chains or a lot of patient trial and elimination. In the app, puzzles like these are filtered out. Of every puzzle our generator has produced and graded, 42,750 (8.6%) were rejected for needing techniques beyond the grader, so players never meet them.
AI Escargot, the 2006 puzzle
Inkala first published AI Escargot in 2006 and later wrote a book about it. It spread widely online as the hardest sudoku ever. Players on sudoku forums soon produced puzzles that software rated as harder, and Inkala's 2012 puzzle was his own answer to that.
So which puzzle really is the hardest?
Nobody can say for certain, because "hardest" depends on how you measure it. Common approaches:
- Technique ratings. Rating software scores a puzzle by the hardest step a logical solver needs. Puzzles that need very long or nested chains score highest.
- Trivialisation. SudokuWiki's Andrew Stuart tested how many random extra clues a puzzle needs before a standard solver can finish it. By that measure Inkala's 2012 puzzle came third among the puzzles he tested, behind two by David Filmer (SudokuWiki).
- Human solve time. The fairest test for players, but nobody has timed thousands of people on the candidate puzzles.
One thing is settled: clue count does not decide difficulty. The minimum is 17 clues (see how many sudoku puzzles are there), and many 17-clue puzzles only need singles. Inkala's grid has 21.
Should you try it?
If you enjoy a project, yes, with notes and patience. If you want to improve, you will learn more from puzzles one step above your level. A useful ladder: master hidden singles, then pointing pairs, then naked pairs, then the X-Wing and swordfish. Puzzles in Sudoku Royale's top practice levels use those techniques and stop short of the chains Inkala's puzzles demand. Our solve-time data shows how long each level takes real players.
Frequently Asked Questions
What is the hardest sudoku ever?
The best known is Arto Inkala's 21-clue puzzle from 2012, publicised by The Daily Telegraph. Other puzzles made by enthusiasts rate as hard or harder on some measures, so there is no undisputed hardest sudoku.
Who made the world's hardest sudoku?
Arto Inkala, a Finnish mathematician. He published AI Escargot in 2006 and a harder 21-clue puzzle in 2012.
Does the world's hardest sudoku have a unique solution?
Yes. We checked Inkala's 2012 puzzle and AI Escargot with a brute-force solver; each has exactly one solution.
Can the hardest sudoku be solved without guessing?
It can be solved by logic, but the logic is long chains of implications that go far beyond the usual pattern techniques. Many people who solve it by hand end up doing structured trial and error.
Is a sudoku with fewer clues always harder?
No. The minimum is 17 clues and many 17-clue puzzles are easy. Difficulty depends on which techniques the puzzle forces you to use.