Sudoku is a logic-based number placement puzzle played on a 9×9 grid divided into nine 3×3 subgrids (called "boxes" or "regions"). The goal is to fill every row, column, and 3×3 box with the digits 1 through 9 exactly once. Some numbers are pre-filled as clues — your task is to deduce the rest using logic alone, without guessing.

Difficulty is primarily determined by the number of clues given and the complexity of solving techniques required. Easy puzzles (40-45 clues) can be solved with basic scanning. Medium (31-36 clues) requires single-candidate spotting. Hard (26-31 clues) needs advanced techniques like pairs and triples. Expert (21-26 clues) demands mastery of X-Wing, Swordfish, and other advanced strategies.

Pencil marks (or notes) are small candidate numbers written in empty cells to track which digits could potentially go there. They are essential for solving harder puzzles. In our tool, toggle Notes mode to add/remove candidate numbers. When Notes mode is on, tapping a number adds it as a candidate; when off, the number is placed directly.

A properly constructed Sudoku puzzle should have exactly one unique solution. Puzzles with multiple solutions are considered invalid. Our generator uses a backtracking algorithm to ensure every puzzle has a unique, logically deducible solution — so you can trust that each puzzle is solvable without guessing.

Playing Sudoku regularly can improve logical reasoning, concentration, pattern recognition, and short-term memory. Studies suggest it helps maintain cognitive function and may delay age-related mental decline. It's also a relaxing, screen-friendly mental workout that provides a satisfying sense of accomplishment upon completion.

Our generator first creates a complete valid Sudoku solution using a randomized backtracking algorithm. It then strategically removes cells based on your selected difficulty level while ensuring the puzzle remains uniquely solvable. Each "New Game" click produces a fresh, random puzzle — no two games are alike!

Key strategies include: Naked Singles (only one candidate fits), Hidden Singles (a digit fits only one cell in a row/column/box), Naked Pairs/Triples (eliminating candidates from related cells), Pointing Pairs (candidates confined to one row/column within a box), and advanced techniques like X-Wing and Swordfish for Expert-level puzzles. Use Notes mode to track candidates systematically.