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Sudoku
Sudoku is a logic-based number puzzle played on a 9×9 grid. If you’re searching for Sudoku rules, how to play Sudoku, or why a move is invalid, this guide explains the fundamentals clearly.
Sudoku is popular because it requires no guessing, rewards careful thinking, and can be enjoyed at any pace, from casual play to serious problem-solving.
How to Play Sudoku
Sudoku is a logic-based number puzzle played on a 9×9 grid. This guide explains the rules of Sudoku, how to make valid moves, how to recognize mistakes, and how to complete a puzzle correctly. It is written for beginners and for players who want a clear reference to standard Sudoku rules.
Objective
The goal of Sudoku is to fill the grid so that:
- Each row contains the numbers 1 through 9 exactly once
- Each column contains the numbers 1 through 9 exactly once
- Each 3×3 box contains the numbers 1 through 9 exactly once
A completed Sudoku puzzle satisfies all three conditions at the same time.
The Sudoku Grid
- The grid has 9 rows and 9 columns
- Each cell contains a single digit from 1 to 9, or is empty
- The grid is divided into nine 3×3 boxes
Each number you place must follow the row, column, and box rules.
Making a Valid Move
A number can be placed in a cell only if:
- The same number does not already appear in the same row
- The same number does not already appear in the same column
- The same number does not already appear in the same 3×3 box
If any of these rules are broken, the move is invalid.
Example: Puzzle in Progress with an Invalid Placement
The diagram below shows a Sudoku puzzle partway through solving.
The highlighted cell is currently selected, and some cells are marked as invalid because they break Sudoku rules.
In this example, the selected cell causes a row conflict: there are two 5s in the top row.
If you see invalid cells, correct them before continuing. Sudoku is easiest when you catch mistakes early.
Understanding Common Rule Violations
Row conflicts
A row conflict happens when the same number appears more than once in a single row.
Column conflicts
A column conflict happens when the same number appears more than once in a single column.
Box conflicts
A box conflict happens when the same number appears more than once in a 3×3 box.
Many beginner mistakes happen by overlooking the box rule, especially in the center boxes.
Common Solving Techniques
These techniques are enough to solve most easy and medium Sudoku puzzles.
Scanning
Look at a row, column, or box and identify which numbers are missing. Then check where those numbers can legally fit.
Candidate elimination
For an empty cell, eliminate numbers that already appear in the same row, column, or box. The remaining numbers are possible candidates.
Naked singles
If a cell has only one possible number that can fit without breaking the rules, that number must go in that cell.
Hidden singles
If a number can only fit in one cell within a row, column, or box, that cell must contain that number.
Difficulty Levels
Sudoku difficulty labels vary by source, but they usually mean:
- Easy: frequent naked singles and simple scanning
- Medium: hidden singles and more careful elimination
- Hard: multi-step logical reasoning
- Expert: advanced patterns and deeper analysis
How a Sudoku Puzzle Ends
A Sudoku puzzle is complete when:
- All cells are filled
- Every row, column, and 3×3 box follows the rules
- No duplicates exist anywhere on the grid
Many published puzzles are also designed to have exactly one valid solution.
Summary
Sudoku is a game of logic and careful observation. Every move must respect row, column, and box constraints. When a mistake creates an invalid placement, correcting it early makes the puzzle much easier to solve. With practice, scanning and simple elimination techniques become second nature.