Sudoku Rules — The Complete Official Rules of Sudoku (2026)
Sudoku is one of the most popular logic puzzles in the world. Despite using numbers, it requires absolutely no math. Here are the complete, official rules of Sudoku.
The Sudoku Grid
A standard Sudoku puzzle uses a 9x9 grid containing 81 cells. This grid is divided into:
- 9 rows — horizontal lines running left to right
- 9 columns — vertical lines running top to bottom
- 9 boxes — 3x3 regions marked by thicker borders
When a puzzle begins, some cells already contain numbers. These are called givens or clues. The number of givens determines the puzzle’s difficulty.
The One Rule of Sudoku
Sudoku has exactly one rule:
Fill in every empty cell so that each row, each column, and each 3x3 box contains the numbers 1 through 9 exactly once.
That is the entire ruleset. There are no exceptions, no special cases, and no additional rules.
What This Means in Practice
- No repeated numbers in any row — If a row already contains a 5, no other cell in that row can be 5
- No repeated numbers in any column — If a column already contains a 3, no other cell in that column can be 3
- No repeated numbers in any box — If a 3x3 box already contains a 7, no other cell in that box can be 7
- Every number 1-9 must appear — Each row, column, and box must contain all nine numbers
Important Properties of Sudoku
No Math Required
Despite using the numbers 1 through 9, Sudoku involves zero arithmetic. You never add, subtract, multiply, or divide. You could replace the numbers with letters, colors, or symbols and the puzzle would work identically. Sudoku is purely a logic and placement puzzle.
Exactly One Solution
A properly constructed Sudoku puzzle has exactly one valid solution. There is never ambiguity — every cell has one and only one correct number. If a puzzle has multiple solutions, it is considered poorly designed.
No Guessing Required
Every Sudoku puzzle can be solved through logical deduction alone. You should never need to guess and check. If you find yourself guessing, there is a logical technique you have not yet applied. This is what makes Sudoku a pure logic exercise.
Sudoku Difficulty Levels
The rules remain the same regardless of difficulty. What changes is:
| Difficulty | Given Numbers | Solving Techniques Required |
|---|---|---|
| Easy | 36-45 | Basic scanning, Naked Singles |
| Medium | 30-35 | Hidden Singles, elimination |
| Hard | 25-29 | Naked Pairs, Pointing Pairs |
| Expert | 22-26 | X-Wing, Swordfish, chains |
Fewer givens means more empty cells, which requires more advanced logical reasoning to solve.
Common Sudoku Variations
While standard 9x9 Sudoku is by far the most popular, variations exist:
- Mini Sudoku (4x4 or 6x6) — Smaller grids for beginners or quick games
- Killer Sudoku — Adds arithmetic constraints with caged regions
- Sudoku X — Diagonals must also contain 1-9
- Hyper Sudoku — Additional overlapping 3x3 regions
Sudoku Spark focuses on the classic 9x9 format — the standard used in competitions, newspapers, and puzzle books worldwide.
Where to Play Sudoku
Sudoku Spark offers free Sudoku puzzles across four difficulty levels with Smart Hints that teach you solving techniques as you play. Available for Android on the Google Play Store.
Want to learn solving techniques? Read our How to Play Sudoku beginner guide or explore advanced strategies.
Ready to Put These Skills to Work?
Download Sudoku Spark and start playing with smart hints, daily challenges, and 4 difficulty levels.
