What Is Sudoku?

Sudoku is a logic-based number-placement puzzle played on a 9×9 grid. The grid is divided into nine 3×3 boxes, and the goal is to fill every cell with a digit from 1 to 9 so that each row, each column, and each 3×3 box contains every digit exactly once.

A well-formed sudoku puzzle has exactly one solution. You only need pure logic — never guesswork — to solve it.

The 3 Golden Rules

1. Rows: Each of the 9 rows must contain digits 1–9 with no repetition.
2. Columns: Each of the 9 columns must contain digits 1–9 with no repetition.
3. Boxes: Each of the 9 small 3×3 boxes must contain digits 1–9 with no repetition.

Beginner Strategy: Step by Step

1. Scan for naked singles. Find any cell where only one digit can fit because the others already exist in its row, column, or box.
2. Look for hidden singles. Pick a digit and check each box: if there is only one cell where it can possibly go, place it.
3. Use pencil marks. When a cell has multiple candidates, jot them down as small notes. They will guide your next deductions.
4. Eliminate with pairs. If two cells in a unit share the same two candidates, those digits are locked there — remove them from other cells.
5. Move to advanced techniques. For tougher puzzles, learn X-Wing, Swordfish, XY-Wing and other patterns in our technique library.

Tips & Best Practices

• Start with rows, columns, or boxes that already have many digits filled in — they shrink the candidate space the fastest.
• Never guess. If you cannot find a logical move, scan again or use pencil marks more aggressively.
• When stuck, focus on a single digit (e.g. all 7s) and trace where it must go in each box.
• On harder puzzles, the answer often hides in cells with exactly two candidates — they fuel most chain techniques.

Common Mistakes to Avoid

• Guessing under pressure. A wrong guess propagates through dozens of cells before you notice it — and back-tracking is painful.
• Forgetting pencil marks. Without notes you keep re-scanning the same cells. Mark candidates and update them as you go.
• Skipping the easy stuff. Even on expert puzzles, naked singles and hidden singles unlock most of the grid before advanced techniques are needed.
• Ignoring the box. Beginners focus only on rows and columns. The 3×3 box is just as restrictive — always check all three.

Frequently Asked Questions