Sudoku is pure logic: fill a 9 × 9 grid so that every row, every column and every 3 × 3 box contains the digits 1 to 9 exactly once. This version generates a fresh puzzle whenever you like, at four difficulty levels, and every puzzle has exactly one solution. Pencil notes, conflict highlighting, undo and hints make it as comfortable as paper — and you never run out of puzzles.
How to play
- Choose a difficulty and press New puzzle, or start with the puzzle already on the board.
- Select a cell by clicking or tapping it, or move with the arrow keys.
- Type a digit 1–9, or tap it on the number pad. Tapping the same digit again clears it.
- Turn on Notes (or press N) to pencil in candidates. Shift + digit adds a note without switching modes.
- Erase (Backspace or Delete) clears a cell; Undo (Ctrl/⌘ + Z) steps back.
- Press Check to cross out wrong entries, or Hint to reveal the selected cell (or the first wrong or empty one).
Cells that repeat a digit in a row, column or box turn red immediately — a conflict is always a mistake, though a cell can be wrong without conflicting yet. The number pad shows how many of each digit are still to place.
Difficulty levels
| Level | Givens | What you need |
|---|---|---|
| Easy | about 37–38 | Naked and hidden singles only |
| Medium | about 30–31 | Naked and hidden singles only, with more searching |
| Hard | about 26–28 | At least one technique beyond singles |
| Expert | about 24–25 | Advanced techniques and patience |
The generator fills a complete valid grid at random, then removes clues in symmetric pairs (and single cells for expert), checking after every removal that the solution is still unique. For easy and medium it also confirms that singles alone can finish the puzzle; for hard and expert it confirms that they cannot.
Solving techniques
- Naked single: a cell where only one digit fits, because the other eight already appear in its row, column or box.
- Hidden single: a digit that can go in only one cell of a row, column or box, even if that cell has other candidates.
- Naked pair: two cells in a unit that share the same two candidates; those digits can be removed from every other cell in the unit.
- Pointing pair: if a digit’s candidates in a box all lie in one row (or column), it can be removed from the rest of that row outside the box.
- X-wing: a digit confined to the same two columns in two rows can be eliminated from those columns everywhere else.
Start every puzzle by scanning each digit across the boxes for hidden singles; they solve most of an easy grid.
Worked example: finding a hidden single
Suppose the top-middle box needs a 5. Row 1 already contains a 5 in the left box and row 2 has a 5 in the right box, so the 5 in the middle box must sit in row 3. If two of the three row-3 cells in that box are already filled, the 5 has only one place to go — even if that cell could also hold a 3 or a 7. Place it, and the new 5 may unlock another single elsewhere.
Sudoku by the numbers
- There are 6,670,903,752,021,072,936,960 valid completed grids, about 6.67 × 10²¹, first counted in 2005.
- Treating rotations, reflections, relabeled digits and similar symmetries as the same, there are 5,472,730,538 essentially different grids.
- The minimum number of clues for a puzzle with a unique solution is 17, settled by an exhaustive search in 2012.
Counting arguments like these rely on factorials and permutations — explore them with the permutations calculator or the factorial calculator.
Puzzles are generated in your browser and your progress is stored only on this device. Clearing your browser data removes the saved game.
Frequently asked questions
Does every puzzle have a unique solution?
Yes. After removing each clue the generator runs a solver that counts solutions, and it keeps a removal only if exactly one solution remains. So every puzzle can be solved by logic alone, without guessing.
What makes a puzzle easy or hard here?
Two things: the number of givens and the techniques required. Easy and medium puzzles (about 38 and 30 givens) can always be finished with naked and hidden singles. Hard and expert puzzles (about 27 and 24 givens) always need at least one more advanced step.
How do pencil notes work?
Press Notes (or N) to switch the number pad to note mode, or hold Shift while typing a digit. Small candidates appear in the cell. When you place a number, that digit is erased from the notes in the same row, column and box automatically.
What is the minimum number of clues a Sudoku can have?
17. In 2012 researchers completed an exhaustive computer search showing that no proper Sudoku with only 16 clues exists, while many 17-clue puzzles do.
Is my game saved?
Yes, in your browser's local storage, including notes and elapsed time. Come back later on the same device and browser to continue where you left off.