Hardest Sudoku
Play the hardest free Sudoku on the web — and meet the world’s hardest Sudoku ever. The board below is our toughest 9×9 tier, free, in your browser, with no signup. Every puzzle has one unique solution.
The world’s hardest Sudoku ever
In 2012, Finnish mathematician Arto Inkala published a puzzle that news outlets around the world dubbed the hardest Sudoku ever made. It followed his earlier 2006 creation, “AI Escargot.” Both rate around 11.0 on the Sudoku Explainer difficulty scale — far beyond a typical evil puzzle — and the 2012 puzzle reportedly took him about three months to construct. Here it is, exactly as published, as a benchmark you can study:
| 8 | ||||||||
| 3 | 6 | |||||||
| 7 | 9 | 2 | ||||||
| 5 | 7 | |||||||
| 4 | 5 | 7 | ||||||
| 1 | 3 | |||||||
| 1 | 6 | 8 | ||||||
| 8 | 5 | 1 | ||||||
| 9 | 4 |
Why is it so hard?
Inkala engineered the puzzle so that almost no move can be trivially deduced. Where an ordinary board lets you scan and place numbers quickly, his demands an advanced technique at nearly every step — long chains, fish patterns and colouring stacked together. That low “ease of deduction” is exactly what drives its rating to the top of the scale. It is the technique depth, not the 21 starting clues, that makes it brutal.
Can it be solved by logic?
Yes. Like every well-formed Sudoku, Inkala’s puzzle has exactly one solution, reachable by deduction — it simply requires very long chains and advanced patterns to get there. The same is true of our generated hardest boards: each carries one unique solution. (We don’t claim our boards are guess-free — the honest promise is a single unique solution per puzzle.)
Think you can beat puzzles like this?
Play our hardest Sudoku ↑What makes a Sudoku the hardest?
The hardest required technique sets the difficulty — not the clue count. It’s a common myth that fewer givens make a harder puzzle; they don’t. A board is hard when the toughest method you need to crack it is itself advanced, and when those hard steps come up again and again.
That’s why our generated “hardest” puzzles are evil-grade: the toughest the engine can produce while still guaranteeing a single unique solution. Below 17 givens a 9×9 Sudoku loses its unique solution, so the hardest puzzles we can honestly serve all sit at that one ceiling — which is why our hardest, impossible and evil tiers serve the same grade of board.
To finish one, expect advanced patterns. We don’t re-teach strategy here — each links out to sudoku247wiki.com:
Play more impossible challenges
Hardest sits at the very top of the ladder, alongside our other toughest tiers. They serve the same evil-grade puzzles — pick the name that fits:
Impossible Sudoku
The same top grade, branded for the wall it puts up.
Evil Sudoku
Our flagship hardest tier, with daily and printable challenges.
Expert Sudoku
A step down — build your toolkit before climbing back up.
Hard Sudoku
Ease in with hard puzzles, then work your way up.
Prefer paper? Our sister print site has printable packs, and sudoku247wiki.com covers every technique in depth.
Hardest Sudoku FAQ
What is the hardest Sudoku ever?
The puzzles designed by Finnish mathematician Arto Inkala are widely cited as the world’s hardest Sudoku. His 2006 puzzle, "AI Escargot," and his 2012 puzzle both rate around 11.0 on the Sudoku Explainer difficulty scale — far above a typical "evil" puzzle — and the 2012 puzzle in particular is reported to have taken him roughly three months to construct. Both have exactly one solution.
What makes a Sudoku hard?
The hardest technique a puzzle requires, not how many clues it starts with. A puzzle is hard when almost every step forces you to use an advanced method — fish patterns, chains, colouring — rather than simple scanning. Inkala’s puzzles are engineered so that few moves can be trivially deduced, which is what pushes their rating to the top of the scale.
How many clues does the hardest Sudoku have?
Clue count does not set difficulty — a 30-clue puzzle can be far harder than a 24-clue one. Inkala’s 2012 puzzle starts with just 21 given numbers, but its difficulty comes from the depth of logic each step demands. For the record, 17 is the proven minimum number of clues for any uniquely solvable 9×9 Sudoku (McGuire, Tugemann and Civario, 2014) — a property of all Sudoku, not a difficulty rule.
Can the hardest Sudoku be solved by logic?
Yes. The hardest puzzles, including Inkala’s, have a single unique solution that can be reached by deduction — but they require long chains and advanced fish patterns, not just scanning. For our own generated boards, the honest promise is one unique solution per puzzle; we do not claim every board is guess-free.
Are these the hardest Sudoku puzzles I can play?
The board above is our hardest generated tier — evil-grade puzzles, the toughest we can produce while guaranteeing a single unique solution. Below 17 givens a 9×9 Sudoku loses that guarantee, so "hardest," "impossible" and "evil" on this site all serve the same top grade. Inkala’s famous puzzle is featured here as a benchmark, not as one of our generated boards.
Who created the world's hardest Sudoku?
Arto Inkala, a Finnish applied mathematician, designed the puzzles most often called the world’s hardest — "AI Escargot" in 2006 and a further record-setting puzzle in 2012.
The 17-clue minimum is proven in the literature — see the mathematics of Sudoku.