17-Clue Sudoku: The Minimal Puzzle
A standard sudoku needs at least 17 given clues to have exactly one solution — and in 2012 that was proven: there is no valid 16-clue puzzle. This is a real 17-clue grid, playable below, with the story of the proof and the full solution.
Why 17 is the minimum
For years solvers collected thousands of 17-clue puzzles but never found a valid 16-clue one. In 2012, researchers Gary McGuire, Bastian Tugemann and Gilles Civario settled it with an exhaustive computer search: every 16-clue arrangement either has no solution or more than one. So 17 is the hard floor for a proper, unique classic sudoku.
Fewer clues does not automatically mean harder, though. Many 17-clue puzzles are only moderately difficult — the sparse grid just means you must lean on scanning and candidate work early. This one is on the tougher end.
Playing a minimal puzzle
With only 17 clues you will find very few immediate placements, so fill in full pencil marks straight away. Hunt for hidden singles across rows, columns and boxes first, then pairs and pointing pairs to thin the candidates.
Every 17-clue puzzle still has exactly one logical solution. If you stall, the difficulty rater will score this grid and the step-by-step solver reveals the next move.
Frequently asked questions
What is the minimum number of clues in a sudoku?
17. A unique classic 9×9 sudoku must have at least 17 given clues. This was proven in 2012 by an exhaustive computer search showing no valid 16-clue puzzle exists.
Is there a 16-clue sudoku?
No. A 2012 study by McGuire, Tugemann and Civario proved that no 16-clue sudoku has a unique solution. Any 16-clue grid has either zero solutions or more than one, so 17 is the minimum.
Are 17-clue sudokus the hardest?
Not necessarily. Clue count and difficulty are different things. Many 17-clue puzzles are moderate; a few are very hard. Difficulty depends on the logic required, not the number of starting clues.
How many 17-clue sudokus are there?
Tens of thousands of essentially different 17-clue puzzles are known, collected by enthusiasts over many years. New ones are still occasionally found, but the set is thought to be nearly complete.