Hard sudoku
Hard sudoku starts exactly where the simple steps run out. Every board here has at least one moment with no cell that has a certain number, so you have to write notes and use techniques such as naked pairs, hidden pairs or locked candidates. Even so, it is all solved with logic, with no guessing, and there is only one solution.
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Frequently asked questions
What makes a sudoku hard?
Not the number of empty cells, but the kind of thinking it needs. A board is marked hard here only if our solver, which works like a person, has to use note-based techniques: pairs, locked candidates, triples or X-wings.
Where can I get hard sudoku for free?
Right here. Every board is free, with no sign-up and no pop-up ads in the middle of a game. Every tap on "New sudoku" gives you a different board.
What is the difference between hard and very hard?
A hard sudoku can be solved entirely with logic, using the well-known techniques. In a very hard sudoku those are not enough, and at some point you must test a hypothesis ("if this is a 4, what happens?") until you reach a contradiction.
Explore more: online sudoku, all levels, Very hard sudoku, Medium sudoku, and the sudoku solver.