Sudoku Generator

The generator loads with an empty state ready to create your first puzzle. All controls are in the left sidebar — the grid updates instantly.

Choose Your Difficulty

Five levels are available, each controlling how many clues (pre-filled cells) the puzzle starts with:

Generate and Play

Click New Puzzle to generate. The timer starts automatically. Click any cell to select it, then use the number pad or your keyboard (1-9) to place a number. Use arrow keys to navigate between cells. Mistakes are highlighted in red immediately — the counter in the sidebar tracks your total errors.

Pencil Marks

Toggle Pencil Mode in the Controls panel to enter candidate notation. When active, clicking a number adds or removes it as a small candidate in the cell — essential for advanced solving techniques like naked pairs, hidden triples, and X-wing patterns. Pencil marks are automatically cleared from related cells when you place a confirmed number.

Our generator uses a backtracking algorithm — one of the most studied recursive approaches in computer science. The same algorithm is taught in CS courses at Stanford University (CS 106B), MIT (6.006), and is documented extensively in 'Introduction to Algorithms' by Cormen, Leiserson, Rivest, and Stein (the 'CLRS' textbook, 4th edition, 2022).

How It Works

The algorithm follows three phases:

1. Generation: Starting with an empty board, the solver fills cells one at a time, testing numbers 1-9 in random order. If a number violates Sudoku constraints (row, column, or 3×3 box uniqueness), it backtracks and tries the next number. This randomized backtracking ensures every generated puzzle is unique.
2. Solution Storage: Once the board is fully solved, the complete solution is stored separately.
3. Clue Removal: Cells are removed randomly until the target clue count is reached. For Hard, Expert, and Evil difficulties, the generator verifies unique solvability after each removal using a second backtracking pass — ensuring exactly one valid solution exists.

Unique Solution Guarantee

Easy and Medium puzzles skip the uniqueness check for faster generation (the high clue count makes multiple solutions extremely rare). Hard+ difficulties enforce uniqueness by attempting to solve the puzzle after each cell removal — if more than one solution is found, the cell is restored. This approach, documented by Professor Peter Norvig (Google Director of Research) in his influential 2011 essay 'Solving Every Sudoku Puzzle,' ensures mathematical validity while maintaining acceptable generation speed.

Based on Google's Related Searches showing 'sudoku generator pdf' and 'sudoku generator printable' as top queries, we built robust export functionality.

PNG Export

Click the PNG button to download a high-resolution image of the current puzzle state. The export includes the full 9×9 grid with proper 3×3 box borders, given numbers in bold white, player-entered numbers in violet, the difficulty label, and a watermark. The image is rendered via the HTML5 Canvas API at a resolution suitable for sharing on social media, embedding in documents, or printing at up to 300 DPI.

Print View

Click Print to open a clean, printer-friendly page. The print view strips all dark-mode styling and renders a classic black-and-white Sudoku grid with thick box borders and proper number sizing — optimized for standard letter and A4 paper. Teachers frequently use this feature to generate worksheet packets with varying difficulty levels.

Copy as Text

Click Copy to copy the puzzle as a text grid using dots for empty cells. This format is compatible with most online Sudoku solvers, forums, and the standard Sudoku interchange format used by competitive puzzle communities.

Show Solution

Toggle Show Solution to reveal all answers. The solution view is also available for PNG export and printing — useful for creating answer keys alongside puzzle worksheets.

The World Puzzle Federation classifies Sudoku solving techniques into four tiers based on complexity (WPF Sudoku Instruction Booklet, 2023). Here's how each maps to our difficulty levels:

Easy (42 clues) — Naked Singles

Every cell can be solved by finding the one number that fits based on direct row, column, and box elimination. No advanced techniques needed. Average solve time for experienced solvers: 3-5 minutes.

Medium (33 clues) — Hidden Singles

Requires identifying cells where a number can only go in one position within a row, column, or box, even if other numbers could also fit that cell. Average solve time: 8-15 minutes.

Hard (27 clues) — Naked/Hidden Pairs & Triples

Demands identifying groups of 2-3 candidates that must occupy specific cells, allowing elimination of those candidates elsewhere. Our pencil marks feature is essential at this level. Average solve time: 15-30 minutes.

Expert (23 clues) — X-Wing & Swordfish

Requires pattern recognition across rows and columns — identifying when a candidate appears in exactly two positions forming a rectangular pattern. Expert puzzles may also need XY-Wing and coloring techniques. Average solve time: 30-60 minutes.

Evil (20 clues) — Forcing Chains & Bifurcation

The hardest puzzles that may require trial-and-error approaches or advanced chain logic. According to research by Gordon Royle at the University of Western Australia, the minimum number of clues for a valid Sudoku with a unique solution is 17 — our Evil difficulty uses 20 clues, placing it firmly in the 'extremely difficult' category while remaining solvable without guessing for expert solvers.