When you click Play at Pro Sudoku, you get a fresh Sudoku puzzle that was generated just for that request. No two sessions need to be the same. Here is how we create those puzzles so that they are valid, solvable, and matched to the difficulty you choose.
One puzzle, one solution
A proper Sudoku puzzle has exactly one solution. That means there is only one way to fill the grid so that every row, column, and 3×3 box contains the digits 1–9 once. Our generator builds a full grid first (a completed Sudoku) and then removes numbers in a controlled way. After each removal, we check that the remaining puzzle still has exactly one solution. If removing a number would create multiple solutions or make the puzzle unsolvable, we do not remove it. So every puzzle you play is guaranteed to have a unique solution.
How difficulty is chosen
We offer four levels: Easy, Medium, Hard, and Expert. Difficulty is mainly determined by how many cells are left as “givens” (pre-filled numbers). Easy puzzles have more givens, so there are more obvious moves and fewer steps where you must use advanced techniques. Expert puzzles have fewer givens, so the solver has to rely on patterns like pairs, triples, X-Wing, and other strategies described in our Techniques section. The generator is tuned so that each difficulty band produces puzzles that feel consistent within that level.
Generated on demand
We do not store a fixed list of puzzles. When you start a game or request a new puzzle, the server runs the generator and returns a new grid. That means you can play as many puzzles as you want without repeating, and we do not need to maintain a huge database. It also means that every puzzle is checked for a unique solution before it reaches you.
Hints and validation use the same logic
When you ask for a hint, we use the same solving logic that we use to generate and validate puzzles. The hint system looks at the current grid and finds the correct value for the cell you selected, so the number we reveal is always the one from the unique solution. Similarly, when you finish the grid, we validate it by solving the puzzle and comparing the result to your entries. That keeps hints and win/lose detection fair and consistent.
Printable puzzles work the same way
The printable Sudoku feature uses the same generator. You choose a difficulty and how many puzzles you want (1–4). Each one is generated with a unique solution and the same quality checks. So whether you play on screen or on paper, you are getting the same kind of puzzle.
We want every game at Pro Sudoku to be a fair, logical challenge. If you have ideas for how we could improve the puzzles or the generator, we would love to hear from you—visit our Contact page. To try a puzzle yourself, play free Sudoku here. For beginner advice, see our Sudoku tips for beginners.