Sudoku Hardest Sudoku Ever: Play the Legendary Grids Online
Hardest Sudoku Ever is a phrase people often use for Arto Inkala's famous extreme sudoku. It is a fair starting point: Inkala's puzzle became widely known because it resists ordinary scanning, pairs, triples, and many of the comfortable patterns that carry players through normal hard sudoku.
This page starts with the Arto Inkala puzzle, then adds two David Filmer puzzles that advanced sudoku communities and SudokuWiki's difficulty discussion have treated as even tougher under one solver-based metric. In other words, this is not just one hard grid. It is a small gallery of sudoku puzzles built to make expert players slow down.
The Three Levels on This Page
Arto Inkala is the default level because it is the best-known candidate for the title "hardest sudoku ever." Inkala, a Finnish mathematician, has been described in mainstream coverage as the creator of exceptionally difficult sudoku puzzles, including the famous 2010 and 2012 grids that circulated internationally.
David Filmer #28 and David Filmer #49 are included for players who want to go beyond the famous headline puzzle. SudokuWiki's article on difficult-puzzle metrics ranked Filmer's two puzzles ahead of Inkala's under a "trivialization" test: the fewer extra clue pairs that unlock a puzzle, the more stubborn the puzzle appears to be for that method.
There is no single official world ranking for sudoku difficulty. Human difficulty, computer search difficulty, and named-strategy difficulty can produce different lists. That is why this page presents Inkala as the famous benchmark and Filmer's two puzzles as serious community challengers.
About Arto Inkala
Arto Inkala is a Finnish mathematician and sudoku constructor. Coverage from outlets such as The Guardian and ABC News helped spread his reputation for designing extreme sudoku grids. His puzzles are famous not simply because they have few clues, but because the clues are arranged to frustrate easy progress.
About David Filmer
David Filmer is known in the online sudoku community for very hard classic sudoku puzzles. The two Filmer grids here are the ones highlighted by SudokuWiki's difficulty metric article, where they outscored Inkala's puzzle in that particular analysis. Public biographical information is limited, so the most useful thing to know is the reputation of the puzzles themselves: they are brutal, sparse in useful openings, and designed for solvers who are comfortable with deep candidate work.
Why 17 Clues Does Not Mean Hardest
Our 17 Clue Sudoku page focuses on a different kind of extremity: the mathematical minimum number of starting clues for a uniquely solvable standard 9x9 sudoku. A 17-clue grid is rare and elegant, but clue count alone does not measure solving difficulty.
A 21- or 22-clue puzzle can be harder than many 17-clue puzzles because the givens can be placed antagonistically. Instead of giving you a clean first deduction, the clues can keep many candidates alive at once. The result is a high branching factor, greater elimination depth, and a need to look several consequences ahead before a contradiction or forced digit appears.
What Makes a Sudoku Extremely Hard?
- Low-quality openings. Easy singles and obvious locked candidates do not appear early.
- High branching factor. Many cells keep several plausible candidates, so the grid offers too many tempting paths.
- Deep eliminations. The important contradiction may sit ten or twenty logical steps away from the first choice.
- Unfriendly clue placement. The givens do not merely reduce the grid; they shape it into bottlenecks.
- Human memory pressure. Even if a computer can backtrack quickly, a human has to track chains, assumptions, and exclusions without losing the thread.
Use Start Notes immediately. On these puzzles, pencil marks are not a convenience; they are the playing field.
How to Approach the Hardest Sudoku Ever
Begin with full notes, then search for hidden singles, locked candidates, naked pairs, hidden pairs, and strong links. If nothing moves, do not panic. These puzzles are famous because they create long quiet stretches. Use the hint button when you want a nudge, or the solve button when you want to study the final grid.
For the most authentic challenge, start with Arto Inkala. When you want a sterner test, try David Filmer #49, then David Filmer #28. Under SudokuWiki's metric, #28 is the nastiest of the three.
Frequently Asked Questions
The best-known answer is Arto Inkala's extreme sudoku, but some solver-based community metrics rank two David Filmer puzzles as harder. This page includes all three.
It can be. Seventeen clues is the minimum for a unique sudoku, but clue count is not the same as solving difficulty. Inkala's clue placement creates deeper bottlenecks than many 17-clue puzzles.
The two extra levels are attributed to David Filmer, whose #28 and #49 puzzles are cited by SudokuWiki as stronger than Inkala under one difficulty metric.
That depends on what you count as logic. Many solvers use deep chains, forcing nets, or trial-and-error style look-ahead when ordinary named techniques stall.
Start with Arto Inkala because it is the famous benchmark. Then try David Filmer #49 and David Filmer #28 for the tougher community challengers.