17-Clue Sudoku
Play minimal 17-clue evil Sudoku — boards that start with just 17 given numbers, the proven minimum for a unique 9×9. Free, in your browser, with no signup.
Why 17 clues is the minimum
For years, solvers wondered how few starting numbers a Sudoku could have and still point to one and only one answer. In 2014, mathematicians Gary McGuire, Bastian Tugemann and Gilles Civario settled it: their exhaustive computer search proved that no 16-clue Sudoku has a unique solution. That makes 17 the fewest clues a standard 9×9 can carry while staying uniquely solvable.
Around 49,158 essentially distinct 17-clue puzzles have been catalogued (a collection assembled largely by Gordon Royle), and that catalogue is thought to be very nearly complete. You can read the full story in the mathematics of Sudoku and the 17-clue proof write-up.
Are 17-clue puzzles the hardest?
Not automatically. It is tempting to assume fewer clues always means a harder puzzle, and on average sparser boards do trend tougher — but the honest truth is that difficulty is set by the hardest technique a puzzle forces you to use, and that depends on where the 17 clues sit, not just on the count. A well-placed 17 can demand chains and fish; a different 17 might unravel sooner than a denser evil board. That is why we serve these as evil-tier puzzles, not as a guaranteed “world’s hardest”.
Techniques for 17-clue Sudoku
With so little to start from, expect to lean on full pencil marks and advanced patterns. We don’t re-teach strategy here — each one links out to sudoku247wiki.com, where it’s explained step by step:
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17-Clue Sudoku FAQ
What is a 17-clue Sudoku?
A 17-clue Sudoku is a puzzle that starts with only 17 given numbers — the proven minimum for a standard 9×9 with a single, unique solution. The other 64 cells are empty, which makes the board look strikingly sparse and leaves you very little to build on early.
Why is 17 the minimum number of clues?
In 2014, Gary McGuire, Bastian Tugemann and Gilles Civario used an exhaustive computer search to prove that no 16-clue Sudoku has a unique solution — meaning 17 is the fewest clues a uniquely solvable 9×9 can have. It is one of the most famous results in the mathematics of Sudoku.
Are 17-clue Sudoku always the hardest?
No. Fewer clues tends to make a puzzle harder, but it does not guarantee it — what really sets the difficulty is the hardest technique a puzzle forces you to use, which depends on where the clues sit, not just how many there are. Some 17-clue puzzles are tough; others fall faster than a denser evil board.
How many 17-clue Sudoku puzzles exist?
Around 49,158 essentially distinct 17-clue Sudoku puzzles have been catalogued (a collection assembled largely by Gordon Royle), and the catalogue is believed to be very close to complete. No 16-clue puzzle with a unique solution exists at all.
Can 17-clue Sudoku be solved by logic?
Every 17-clue puzzle we serve has a single, unique solution, so there is always one correct answer to find. Cracking one usually takes full pencil marks plus advanced patterns such as fish and chains. We do not claim it is guess-free — the honest promise is one unique solution, every time.