How Sudoku Puzzles Are Made
How a Sudoku is generated: build a full solved grid, then remove clues while checking the solution stays unique.
Every Sudoku you play was built by a generator, and understanding the process demystifies the puzzle. It happens in two phases: first create a completely filled, valid grid, then carve clues away from it until you reach a puzzle that is still uniquely solvable but no easier than it needs to be.
Our puzzles are produced by the shared engine from a seed, which is why the same seed always yields the same board — and why the practice grids in this wiki are real, generated puzzles rather than hand-typed examples.
Step 1: build a full solution
The generator fills an empty grid with a complete, valid arrangement of digits — every row, column and box correct. This is done with a randomised backtracking fill, so each run produces a different finished grid.
Step 2: remove clues, checking uniqueness
Clues are then removed one at a time. After each removal the generator re-solves the grid to confirm it still has exactly one solution; if a removal would create a second solution, that clue is put back. The difficulty target controls how many clues come out and which solving techniques are needed.
- Fill a complete, valid grid (randomised backtracking).
- Remove clues one by one, re-checking uniqueness each time.
- Stop when the difficulty target is met and the solution is still unique.
Frequently asked questions
- Why does the same seed always give the same puzzle?
- The engine is deterministic: a seed fixes the random choices, so every generation from that seed produces an identical grid. It is what lets this wiki embed reproducible practice puzzles.
- How does a generator know a puzzle is hard?
- It targets a clue count and checks which techniques are needed to solve the result. Fewer clues and harder required deductions push a puzzle up the difficulty ladder.
Related reading
Practice online
Put it into practice on free puzzles with hints, notes and four difficulty levels.
Play Sudoku