Sudoku Rules
Sudoku follows a short set of simple rules, but those rules create deep logic challenges. Once you understand the structure of the 9x9 grid, every solving method becomes easier to learn.
Summary
Learn the rules that define classic 9x9 Sudoku and make every puzzle logically solvable without guessing.
Table of Contents
What this technique is
Classic Sudoku is a 9x9 logic puzzle where rows, columns, and 3x3 boxes each contain digits 1 to 9 exactly once.
When to use it
Use these rules from the first move to the last move. Every solving technique is built on this constraint system.
How it works
Validate each placement against row, column, and box constraints. If any unit would repeat a digit, the move is invalid.
Step-by-step
- Scan a row, column, or box for missing digits.
- Eliminate impossible values using existing digits.
- Place only values that satisfy all three constraints.
Example
If a row already contains digit 6, no other cell in that row can be 6, even if the column or box would otherwise allow it.
What Sudoku Is
Classic Sudoku uses a 9x9 grid divided into nine 3x3 boxes. The goal is to fill every empty cell with a number from 1 to 9.
Core Rule Set
Each row must contain digits 1 through 9 exactly once. Each column must contain digits 1 through 9 exactly once. Each 3x3 box must also contain digits 1 through 9 exactly once.
What Is Not Allowed
A number cannot repeat in a row, column, or 3x3 box. Every placement must satisfy all three constraints at the same time.
How to Read the Board
Start by scanning rows, columns, and boxes with many filled cells. Look for missing numbers and eliminate impossible placements before writing a digit.
Common Mistakes
- Checking only the row and forgetting column or box constraints.
- Placing a digit too early without validating all three units.
- Rushing to fill cells instead of using elimination.
Practice Links
FAQ
What are the basic Sudoku rules?
Rows, columns, and 3x3 boxes must each contain digits 1 to 9 exactly once.
Do I need math to solve Sudoku?
No. Sudoku is a logic puzzle, not a calculation puzzle.
Can a Sudoku puzzle be solved without guessing?
A well-formed Sudoku has one valid solution and can be solved with logic.