Generate fresh 9x9 sudoku puzzles in easy, medium, or hard difficulty, each verified to have a single unique solution. It is ideal for printable puzzle sheets, classroom packs, or a quick brain-teaser.
How it works
The generator runs three stages entirely in your browser:
- Build a full grid. A randomised backtracking algorithm fills an empty board so every row, column, and 3x3 box contains 1–9 once — a complete, valid solution.
- Remove clues. Cells are cleared one at a time according to the chosen difficulty’s clue target (about 40 easy, 32 medium, 26 hard).
- Guarantee uniqueness. After each removal a solver checks the board still has exactly one solution. If clearing a cell would allow a second solution, that cell is restored.
This means every puzzle is solvable by logic alone, with no guessing.
Difficulty levels and what they demand
| Difficulty | Approx. given clues | Typical solve effort |
|---|---|---|
| Easy | ~40 | Straightforward scanning |
| Medium | ~32 | Some candidate elimination |
| Hard | ~26 | Advanced techniques |
The number of clues is the main driver of difficulty, but placement matters too. A puzzle can have 30 clues heavily clustered in one quadrant and still be harder than a well-spread 26-clue puzzle. The generator removes clues in a varied order to avoid clustering.
Solving techniques by difficulty
Understanding what the difficulty levels demand helps you approach each puzzle with the right strategy:
Easy puzzles can usually be solved with a single technique: naked singles. Scan each row, column, and box; any cell where only one digit is still possible can be filled immediately. Working through the grid repeatedly this way is enough to complete most easy puzzles without any notation.
Medium puzzles require hidden singles — cells where a digit can only go in one place within a row, column, or box, even though the cell itself has multiple candidates. A pencil-mark notation (writing small candidate digits in each empty cell) helps spot these quickly.
Hard puzzles often need more advanced techniques such as naked pairs (two cells in a unit that together contain only two candidates, ruling those digits out from the rest of the unit) or pointing pairs (a candidate confined to one row or column within a box, eliminating it from the rest of that row or column). Well-constructed hard puzzles never require guessing — a logical deduction chain, however long, always resolves them.
Printing and classroom use
Click Generate puzzle to build a new puzzle, then use your browser’s Print or Save as PDF function. The solution remains hidden until you click Show solution, so you can safely print the puzzle page alone and keep the answer key on screen. For classroom packs, generate several puzzles in sequence and save each as a separate PDF.
Everything happens in your browser — nothing is uploaded and no account is needed.