Very hard sudoku
Very hard sudoku is the top level on the site. These boards were checked one by one: our logic solver knows every technique of hard sudoku and still gets stuck. To go on you need chains of deductions or a tested hypothesis that leads to a contradiction. Every board still has exactly one solution.
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Frequently asked questions
Does very hard sudoku need guessing?
Not blind guessing, but testing a hypothesis: pick a cell with two options, place one and carry on with logic. If you reach a contradiction, the other option is correct. This is a legitimate part of the solution at this level.
Can a very hard sudoku have more than one solution?
No. Every board on the site is checked by a computer that makes sure it has exactly one solution, even at the hardest level.
How long does it take?
Even experienced players usually need half an hour to an hour. The timer stops when you pause or switch to another tab.
Explore more: online sudoku, all levels, Hard sudoku, and the sudoku solver.