Sudoku Rules — Complete Guide
Everything you need to know to understand and play Sudoku — from grid basics to valid solutions and common mistakes.
Introduction
Sudoku is a logic-based number puzzle played on a 9×9 grid. The goal is simple: fill every empty cell with a digit from 1 to 9 so that no digit repeats in any row, column, or 3×3 box. No arithmetic is required — only logical deduction.
Despite its simplicity, Sudoku puzzles range from beginner-friendly to fiendishly difficult, making it one of the most popular puzzles in the world. Once you understand the rules, you can start solving puzzles immediately.
The Grid
The Sudoku grid has three levels of structure, each important for the rules:
9 Rows
Horizontal lines running left to right, numbered 1–9 from top to bottom.
9 Columns
Vertical lines running top to bottom, numbered 1–9 from left to right.
9 Boxes (3×3)
Nine non-overlapping 3×3 sub-grids arranged in a 3×3 pattern.
Rows, columns, and boxes are collectively called houses. The 9×9 grid has 27 houses total (9 rows + 9 columns + 9 boxes), and each house must contain every digit from 1 to 9 exactly once.
The Basic Rules
Sudoku has exactly three rules, each applying to one type of house:
Row Rule
Each row (horizontal line of 9 cells) must contain every digit from 1 to 9, with no digit appearing more than once.
Column Rule
Each column (vertical line of 9 cells) must contain every digit from 1 to 9, with no digit appearing more than once.
Box Rule
Each 3×3 box must contain every digit from 1 to 9, with no digit appearing more than once. The nine boxes tile the full 9×9 grid.
Valid Solutions
A Sudoku puzzle has a unique solution — only one arrangement of digits satisfies all three rules simultaneously. Well-crafted puzzles are designed so this solution can always be reached through pure logic, without guessing.
Valid Solution Requirements
- All 81 cells filled
- Every row contains 1–9
- Every column contains 1–9
- Every 3×3 box contains 1–9
- No contradictions anywhere
Invalid (Error Conditions)
- Duplicate digit in a row
- Duplicate digit in a column
- Duplicate digit in a 3×3 box
- Empty cells remaining
- Digit outside 1–9 range
Example Puzzle
Here is a classic Sudoku puzzle. The white digits are the given clues. Your task is to fill in every empty cell (shown as ·) so each row, column, and 3×3 box contains 1–9.
This is the classic Sudoku puzzle used in many textbooks. 3×3 box borders are shown with thicker lines.
Common Mistakes
Ignoring 3×3 box constraints
Always check the box you're filling — not just the row and column. Many beginners focus only on rows and columns and miss box violations.
Placing the same digit twice in a house
Before placing a digit, scan the entire row, column, AND box for existing occurrences of that digit.
Guessing without logical justification
All valid Sudoku puzzles can be solved through logic. If you're guessing, look more carefully — there's always a logical next step.
Not using pencil marks
For medium and hard puzzles, write small candidate digits in cell corners. This prevents losing track of possibilities.
Skipping easier numbers
Always start with digits that appear most often in the grid — they have the fewest remaining locations and are easiest to place.
Ready to Start Playing?
Now that you know the rules, dive in with an easy puzzle and start building your skills.