Sudoku Rules

Overview

Sudoku is a logic-based number puzzle played on a 9×9 grid. The goal is to fill every cell with a number from 1 to 9 so that each row, each column, and each 3×3 box contains all nine numbers exactly once. No math is required — it's pure logic and deduction.

The Grid

The Sudoku grid consists of 81 cells arranged in 9 rows and 9 columns. The grid is further divided into nine 3×3 boxes (also called blocks or regions). At the start, some cells are pre-filled with numbers — these are called 'givens' and cannot be changed.

The Basic Rule

There is only one rule in Sudoku, applied three ways:

• Each row must contain the numbers 1 through 9 exactly once.
• Each column must contain the numbers 1 through 9 exactly once.
• Each 3×3 box must contain the numbers 1 through 9 exactly once.

How to Play

Follow these steps to solve a Sudoku puzzle:

1. Look at the pre-filled numbers (givens) on the grid.
2. Use logic to determine which numbers can go in each empty cell based on the row, column, and box constraints.
3. Fill in cells when you are certain of the answer. Use pencil marks (small candidate numbers) when you've narrowed down the possibilities but aren't sure yet.
4. Continue until every cell is filled correctly. A valid Sudoku puzzle has exactly one solution.

Pencil Marks (Notes)

Pencil marks are small candidate numbers you write in a cell to track which numbers are still possible. As you eliminate candidates through logic, you erase pencil marks until only one remains — that's your answer. This technique is essential for harder puzzles.

Difficulty Levels

Puzzle difficulty is determined by how many givens are provided and which solving techniques are required:

• Easy: 36–40 givens. Solvable with naked singles (cells where only one number fits).
• Medium: 30–35 givens. Requires hidden singles (a number can only go in one place in a row/column/box).
• Hard: 25–29 givens. Requires pointing pairs and box/line reduction techniques.
• Expert: 20–24 givens. Requires advanced techniques like naked pairs, X-wings, and more.

Important Notes

• Every valid Sudoku puzzle has exactly one solution. You should never need to guess.
• If you reach a point where guessing seems necessary, look more carefully — there's always a logical deduction available.
• Start with the easiest cells first (those with the fewest candidates) and work your way to harder deductions.