How to Play Sudoku

Sudoku is a logic-based number placement puzzle. The goal is simple: fill a 9×9 grid so that every row, every column, and every 3×3 box contains each of the digits 1 through 9 exactly once.

No math is required — only logical reasoning. Every puzzle has a unique solution that can be reached through pure deduction, without guessing.

The 9×9 grid is divided into nine 3×3 boxes (also called regions or blocks). The three core constraints are:

1. Each row must contain the digits 1–9 with no repetition.
2. Each column must contain the digits 1–9 with no repetition.
3. Each of the nine 3×3 boxes must contain the digits 1–9 with no repetition.

These three rules interact with each other, which is what makes Sudoku challenging and satisfying. A digit placed in one cell immediately restricts what can go in every other cell that shares its row, column, or box.

When you open a new puzzle, some cells are already filled in — these are called "givens" or "clues." Your job is to fill in the remaining empty cells.

Start by scanning for rows, columns, or boxes that are nearly complete. If a row already has eight digits, the missing one is trivially determined. These easy wins build momentum and reveal more information for harder cells.

Next, pick a single digit — say, the number 5 — and scan the entire grid to see where it can still legally go. If a box has only one empty cell that isn't blocked by a 5 in its row or column, you've found a placement.

Keep a light touch early on. Avoid guessing. Every placement should be justified by the rules, not intuition.

As puzzles get harder, a few core techniques will carry you through the vast majority of them.

Elimination: For each empty cell, list the digits that are already present in its row, column, and box. Any digit not on that list is a candidate. When only one candidate remains, place it — this is called a "naked single."

Scanning: Systematically sweep rows and columns for a specific digit. If a digit can only fit in one cell within a box (even if that cell has multiple candidates), it must go there. This is called a "hidden single" and is one of the most common techniques in beginner and intermediate puzzles.

Candidate Marking (Pencil Marks): For harder puzzles, write small candidate digits in each empty cell. As you place digits elsewhere, erase the eliminated candidates. Patterns in the remaining candidates reveal advanced moves.

Naked Pairs and Triples: If two cells in the same row, column, or box share exactly the same two candidates, those digits can be eliminated from all other cells in that unit. The same logic extends to triples.

With these strategies, you can solve most easy and medium puzzles — and make solid progress on hard ones. Practice is the best teacher: the more puzzles you solve, the faster these patterns become second nature.