Sudoku Generator
Printable Sudoku at four difficulties, every puzzle verified to have exactly one solution, so no puzzle here ever requires guessing.
Or check a puzzle you already have
Paste 81 digits, using a zero or a dot for a blank. Row by row, spaces and line breaks ignored. It will tell you whether the puzzle is legal, whether it has exactly one solution, and what that solution is.
Exactly one solution, every time
A real Sudoku has one solution and needs no guessing. This generator removes clues one at a time and, after every single removal, checks whether the puzzle would still be unique — putting the clue straight back if it would not. That is slower than removing clues at random, and it is the difference between a puzzle and a frustration.
The clue count is a target rather than a promise. Some grids cannot be stripped to 25 clues and still be unique, so the sheet reports what it actually achieved rather than the number you asked for.
Things worth knowing
- A real Sudoku has exactly one solution. This generator checks after every single clue it removes and puts the clue back if the puzzle would become ambiguous — which is why it never needs guessing.
- The clue count is a target rather than a promise. Some grids cannot be stripped to 25 clues and still be unique, and the tool reports what it actually achieved rather than the number you asked for.
- Seventeen clues is the proven minimum for a unique Sudoku. It took a computer search to establish that, and puzzles anywhere near it are genuinely brutal.
- Difficulty is not the same as clue count, though they correlate. A 30-clue puzzle where every step is a simple scan is easier than a 36-clue one that needs chains.
- If you are stuck, switch from asking "what can go in this cell" to "where can this digit go in this box". Most stalls break the moment you change which question you are asking.
How to use
- Choose a difficulty and generate a puzzle.
- Print it, or solve on screen.
- Use the answer key on its own page.
- Work from the most constrained cells first.
Frequently asked questions
Does every puzzle have exactly one solution?
Every puzzle here does, and it is checked before the puzzle is offered. This matters more than it sounds: a puzzle with multiple solutions cannot be solved by logic alone, so at some point you would have to guess, and a published puzzle that does that is considered defective.
What is the fewest clues a valid puzzle can have?
Seventeen. That there is no valid 16-clue puzzle was proved in 2012 by exhaustive computer search — a genuinely substantial computation. Seventeen-clue puzzles exist but are rare and not necessarily hard, since clue count and difficulty are only loosely related.
What makes a puzzle difficult?
Which solving techniques it requires, not how many clues it has. An easy puzzle yields to scanning rows and columns; harder ones need candidate elimination, and the hardest require chains of conditional reasoning. Difficulty ratings describe the hardest technique needed at any point.
Does Sudoku involve arithmetic?
None at all. The digits are simply nine distinct symbols, and the puzzle would work identically with nine letters or nine colours. It is a constraint satisfaction problem wearing numbers as a costume, which is why being poor at mental arithmetic is no obstacle.
What is the best solving strategy?
Scan for cells where only one digit can fit, and for digits that can only go in one cell within a row, column or box. Exhaust those before pencilling in candidates, since candidate notation is slow and the easy deductions usually crack most of the puzzle first.
Where did Sudoku come from?
From Number Place, published in an American puzzle magazine in 1979 and devised by Howard Garns. It was taken up in Japan in the 1980s, given its current name there, and returned to the West as a phenomenon in 2004 — by which time Garns had died without knowing what he had started.
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