Sudoku Rules: How to Play & Win
Quick Summary
Sudoku is a logic puzzle played on a 9×9 grid. The goal: fill every cell so each row, column, and 3×3 box contains the numbers 1–9 exactly once. No guessing—just logic!
What is Sudoku?
Sudoku is a logic-based, combinatorial number-placement puzzle. The classic version uses a 9×9 grid, subdivided into nine 3×3 boxes (sometimes called blocks, regions, or subgrids). The objective is to fill the grid so that every row, every column, and every 3×3 box contains all the digits from 1 to 9 exactly once.
- Grid: 9 rows × 9 columns, divided into 3×3 boxes
- Given numbers: Some cells are pre-filled as clues
- Empty cells: Your task is to fill these using logic
The Three Core Rules
- Row Rule: Each row must contain the numbers 1–9, with no repeats.
- Column Rule: Each column must contain the numbers 1–9, with no repeats.
- Box Rule: Each 3×3 box must contain the numbers 1–9, with no repeats.
No guessing is required—every puzzle has a logical solution.
Sudoku Grid Structure
The Sudoku grid has 9 rows, 9 columns, and 9 boxes (3×3 regions). Some cells are pre-filled. Your job: fill the rest using logic.
- Each row must contain the numbers 1–9, with no repeats
- Each column must contain the numbers 1–9, with no repeats
- Each 3×3 box must contain the numbers 1–9, with no repeats
How to Play Sudoku
- 1Scan the grid for obvious placements. If a number is missing from a row, column, or box, and only one cell is possible, fill it in.
- 2Use logic to eliminate impossible numbers for each cell. Pencil marks (small notes) can help track candidates.
- 3Continue filling in cells until the entire grid is complete and all rules are satisfied.
Step-by-Step Example
Here’s a simple example. The highlighted cell is the only empty spot in its row, column, and box. What number goes there?
Solution
Row 1 currently contains: 5, 3, 4, 6, 7, 8, 9, 1
The missing digit is: 2
Winning Conditions
- Every cell is filled
- Each number 1–9 appears exactly once in every row
- Each number 1–9 appears exactly once in every column
- Each number 1–9 appears exactly once in every 3×3 box
Allowed & Illegal Moves
- Allowed: Placing a number in an empty cell if it does not violate any of the three core rules above.
- Illegal: Placing a number that already appears in the same row, column, or box.
- Tip: Use pencil marks to keep track of possible candidates for each cell.
Uniqueness & Solution
- Every classic Sudoku puzzle has exactly one solution.
- If you find a contradiction, backtrack and try a different logical path.
- Guessing is discouraged—use logic and elimination.
Common Mistakes to Avoid
- •Guessing: Always use logic, never guess
- •Missing constraints: Check row, column, and box for every move
- •Duplicate numbers: Never repeat a number in a unit
Tips for Beginners
1. Start with Easy Puzzles
Build confidence by practicing on easy puzzles before moving to harder ones.
2. Use Pencil Marks
Write small numbers in empty cells to keep track of possible candidates.
3. Double-Check Your Work
Review your placements regularly to avoid mistakes.
4. Remember: One Solution
Every valid Sudoku puzzle has a unique solution. If you find a contradiction, backtrack and try again.
Advanced Rule Clarifications
- Only the three core rules apply unless otherwise stated. Diagonals do not have to contain 1–9 unless playing a variant (e.g., "Diagonal Sudoku").
- Numbers cannot be repeated in any row, column, or box—even if the cell is empty elsewhere.
- All clues (given numbers) are correct and do not contradict each other.
- There are no hidden or ambiguous rules in classic Sudoku.
Sudoku Variants
Many variants exist, each with extra rules. Some examples:
- Diagonal Sudoku: Both main diagonals must also contain 1–9 once each.
- Killer Sudoku: Cages with a given sum, no repeats in a cage.
- Irregular Sudoku: Boxes are irregular shapes instead of 3×3 squares.
- Hyper Sudoku: Extra 3×3 regions must also contain 1–9.
Always check the puzzle instructions for variant rules.
Frequently Asked Questions (FAQ)
Q: Can a number appear twice in a row, column, or box?
A: No. Each number 1–9 must appear exactly once in every row, column, and box.
Q: Do diagonals matter in classic Sudoku?
A: No. Diagonals are only relevant in certain variants.
Q: Can there be more than one solution?
A: A well-formed Sudoku puzzle has exactly one solution.
Q: Are pencil marks allowed?
A: Yes! Pencil marks are a standard tool for tracking possible candidates.
Q: Can I guess?
A: Guessing is discouraged. All puzzles can be solved with logic alone.