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How to Play Sudoku

Sudoku is one of the most popular logic puzzles in the world, and learning to play is straightforward. This guide walks you through everything you need to know, from understanding the grid to developing efficient solving habits. By the end, you will be ready to tackle easy and medium puzzles with confidence.

Getting Started

To begin a Sudoku puzzle, you need a 9x9 grid with some numbers already filled in. These pre-filled numbers are called givens or clues. Your task is to fill in the remaining empty cells so that every row, column, and 3x3 box contains the digits 1 through 9 exactly once. No math is involved; Sudoku is purely a logic puzzle.

Start with an easy puzzle. Easy puzzles have more givens, which means fewer empty cells and a more obvious solving path. As you build confidence and recognize patterns faster, you can move to harder difficulties. There is no time limit, so take as long as you need, especially when learning.

You can play Sudoku with pencil and paper or on a screen. Online Sudoku tools often include features like pencil marks, undo buttons, and error checking that make learning much easier.

Understanding the Grid

The Sudoku grid has 81 cells arranged in 9 rows and 9 columns. Thicker lines divide the grid into nine 3x3 boxes. Every cell belongs to exactly one row, one column, and one box. These three groups are called units, and the core rule is that no unit can contain a duplicate number.

Key Terms

Row:
A horizontal line of 9 cells running left to right.
Column:
A vertical line of 9 cells running top to bottom.
Box:
One of the nine 3x3 sub-grids outlined by thicker borders.
Cell:
A single square in the grid that holds one digit.
Candidates:
The possible digits that could go in an empty cell based on the current state of the puzzle.

When you look at an empty cell, think about all three units it belongs to. Any number already present in the same row, column, or box is ruled out as a possibility for that cell. The remaining possibilities are the cell's candidates.

Basic Strategy

The most natural way to start solving is called scanning. Pick a row, column, or box and look at the numbers that are already there. Then examine the empty cells and see which numbers are missing. Often, only one number can fit in a particular cell.

1

Scan rows and columns

For each empty cell, check which digits already appear in its row and column. Cross off those digits from the possibilities.

2

Check the 3x3 box

Also check which digits are already in the cell's 3x3 box. This third constraint often eliminates more candidates.

3

Place the number

If only one candidate remains, that is the answer. Place it and move on. Each new number you place opens up further deductions.

This approach is known as the naked singles technique. Combined with hidden singles, where a number can only fit in one place within a unit, these two beginner techniques will carry you through most easy and many medium puzzles.

Using Pencil Marks

Pencil marks (also called notes or candidates) are small numbers you write in a cell to track which digits are still possible there. They become essential once you move beyond easy puzzles. Without pencil marks, it is nearly impossible to spot intermediate and advanced techniques.

To set up pencil marks, go through each empty cell and note every digit from 1 to 9 that does not already appear in the cell's row, column, or box. Write those digits as small numbers in the cell. As you solve the puzzle and place digits, update the pencil marks by removing the placed digit from all cells in the same row, column, and box.

Pencil Mark Tips

  • Always update pencil marks immediately after placing a number.
  • If a cell's pencil marks narrow to a single digit, place it right away.
  • Online Sudoku tools handle pencil mark updates automatically, which is ideal for learning.
  • Accurate pencil marks are the foundation for every technique beyond basic scanning.

Common Mistakes to Avoid

Guessing instead of deducing

Every valid Sudoku puzzle can be solved through logic alone. If you feel the need to guess, you have likely missed a deduction. Step back and recheck your pencil marks.

Forgetting to check all three units

New players sometimes check the row and column but forget the 3x3 box. Always verify against all three constraints before placing a number.

Not updating pencil marks

Stale pencil marks lead to errors. Every time you place a number, remove that digit from the pencil marks of all cells in the same row, column, and box.

Jumping to hard puzzles too soon

Build a strong foundation with easy puzzles before moving up. Consistent practice at your current level is more effective than struggling with puzzles that require techniques you have not learned yet.

Tips for Improvement

  1. Practice daily. Solving one puzzle per day builds pattern recognition faster than occasional marathon sessions.
  2. Learn one technique at a time. Master naked singles before moving to hidden singles, then to naked pairs, and so on.
  3. Time yourself. Tracking your solve times gives you a clear measure of progress and motivates you to improve.
  4. Analyze your mistakes. When you make an error, figure out why. Was a pencil mark wrong? Did you miss a constraint? Understanding mistakes prevents them from recurring.
  5. Use an undo button. Online Sudoku tools let you undo moves easily, which encourages experimentation without fear of ruining the puzzle.
  6. Study solving techniques. Visit our techniques page to learn specific strategies for each difficulty level.