Sudoku Rules: How to Play in 5 Minutes

Breaking Down the Three Sudoku Rules

The puzzle is played on a standard 9x9 grid, which you can learn more about in our guide to the Sudoku Grid Explained. The grid is also divided by thicker lines into nine 3x3 regions, commonly called 'boxes,' 'blocks,' or 'sub-grids.' Every single cell you fill must satisfy all three rules simultaneously, which is what creates the logical challenge.

To visualize it, pick any empty cell. The digit you write there cannot already appear in the row that runs left to right across the grid. It also cannot appear in the column that runs top to bottom. Finally, it cannot appear anywhere else within the 3x3 box that contains the cell. All three conditions must be true for your answer to be correct.

• Remember: 'Exactly once' means no digit can be missing or duplicated in a row, column, or box.
• The boxes are fixed 3x3 areas. Don't create your own arbitrary squares.

Understanding the Givens

Every Sudoku puzzle starts with some cells already filled in. These are called 'givens' or 'clues.' Their placement is carefully designed so that there is only one logical solution to the entire puzzle. You never need to guess. The number of givens varies, but a proper puzzle will have enough to be solved through deduction alone.

The givens provide the initial constraints. Your job is to use the three rules to figure out what the missing digits must be. You cannot change the givens. They are the immutable foundation from which all other logic flows. For a step-by-step walkthrough of applying these rules from the start, see our How to Play Sudoku Quick Start Guide.

Your First Two Solving Moves

Once you understand the rules, you can start solving. The simplest logical techniques involve looking for cells that can only be one possible number. There are two primary ways to find these.

The first is a **Naked Single**. This happens when an empty cell has only one possible candidate digit left after you consider all the numbers already present in its row, column, and 3x3 box. If all other digits from 1-9 are 'blocked' by the givens and your previous fills, the remaining digit is the answer.

The second is a **Hidden Single**. This is slightly more subtle. It occurs when a digit has only one possible cell left to go within a specific row, column, or box. Even though other digits might also be possible in that cell, that particular digit has no other home in that unit. You find it by scanning a single row, column, or box for a missing number.

Quick-Start Checklist for Your First Puzzle

Ready to play? Follow this simple checklist. First, scan the puzzle for any obvious Naked Singles. Look for empty cells where eight of the nine digits are already seen in the intersecting row, column, and box. Second, if no Naked Singles are visible, scan each row, column, and box for Hidden Singles. Pick one unit (like 'top row') and check which numbers 1-9 are missing. For each missing number, see if only one cell in that unit can legally hold it.

Alternate between these two scans. Each number you place adds new information and creates new opportunities for the next Naked or Hidden Single. This process of applying the three rules repeatedly is the core of basic Sudoku logic. Avoid guessing. If you get stuck, take a step back and scan the grid systematically from a different angle.