How to Play Sudoku: Rules & Winning Tips

Sudoku is a logic puzzle with one simple rule—fill each cell with a number from 1 to 9 without repeating numbers in any row, column, or 3x3 block. The puzzle consists of a 9x9 grid with 81 cells segmented into smaller 3x3 blocks. Each row, column, or block is referred to as a unit.

When you start the puzzle, some cells are pre-filled with numbers, also called givens. You use deduction skills and the process of elimination to solve for the missing numbers. Easy Sudoku puzzles have more givens, while difficult puzzles have fewer, requiring you to use more advanced solving techniques.

We’ll explain the rules and strategies to solve your next puzzle, whether it’s your first time or you’re trying to level up your game.

Sudoku Objective

The object of Sudoku is to fill cells in every row, column, or 3x3 block with the numbers 1 through 9, without repeating numbers in each unit.

Sudoku Tips to Improve Your Game

While Sudoku doesn’t have complex rules, solving puzzles may be challenging. The first step to learning Sudoku is understanding their terminology. Then, follow these tips and guidelines to improve your solving speed and ability to tackle more challenging puzzles.

• Begin by examining rows, columns, or blocks that already contain several numbers. The fewer empty cells there are, the easier the process of elimination is.
• Use pencil marks. Note potential solutions, also known as candidate numbers, for each cell. This helps you avoid redoing work and is important for the techniques we’ll discuss below.
• Focus on one section at a time. A methodical approach will help narrow down your options and notice patterns.
• Use the process of elimination. Answers to empty squares aren’t always obvious. Often, you have to undergo multiple rounds of elimination before you arrive at the correct answer.
• Look for chain reactions. Adding an answer to the Sudoku grid can lead to additional answers. Each time you fill in an empty box, check the surrounding units to see if candidates can be eliminated or added or whether you can put answers in cells based on the updated information.
• Don’t guess the answer. Random guessing can lead to mistakes that make it more difficult to solve the puzzle in the long run.
• Check your answers. If you’re playing Sudoku online, you can check your moves or disallow incorrect moves. It’s helpful to check periodically because one wrong answer can lead to a chain reaction of errors.

Sudoku Techniques and Examples to Help You Win

In addition to tips and best practices, you can utilize tried-and-true Sudoku strategies. Which techniques you’ll use depends on the puzzle’s level of difficulty.

You can typically solve easy puzzles by cross hatching and counting. Intermediate puzzles may require simple candidate elimination techniques like hidden pairs or naked triples. And hard puzzles require advanced elimination techniques like x-wing and swordfish.

If you’re a Sudoku beginner, focus on these strategies and remember you typically need to use multiple strategies to solve the grid.

Crosshatching

Choose a single number, for example 4, that you want to solve for and then draw lines through the columns and rows where that number is present. This helps you eliminate cells that 4 cannot be placed in and show cells where 4 is the answer.

In the example below you’ll see that 4 is the answer for A3, B8, D1, F6, and G9 because a 4 is not present in the adjoining units. One simple technique helped you find five answers!

Counting

Since numbers 1 through 9 can only appear in each row, column, or block once, counting can help you identify solutions. First, look for a unit with a few missing numbers. Then, count one through nine, skipping numbers already present in the unit.

For example, looking at the top right block above, you’ll find the numbers 2, 3, 4, and 5. You can count 1, 6, 7, 8, and 9, looking across rows and columns to see if you can eliminate candidates and find solutions.

Since 9 is present in C2 and F3, you deduce that G1 is 9.

Last Possible Number

Choose a cell and look across its row, column, and block. If eight out of nine numbers are present in all those units, you know that the solution is the “last possible number.” For example,

Row 8 contains 1, 2, 6, 8, 9; Column B contains 2, 3, 5, 8; and the bottom left block contains 2, 3, 7, 9. Between these units, the only number missing is 4, so that’s the solution to B8.

Naked Single and Hidden Single

A naked single is when there’s only one possible candidate number for a cell. In the example below, 2 is the only candidate in C1, so that is the solution. The other candidate 2s can be eliminated.

A hidden single uses the same concept, but the cell has other candidate numbers, making the single “hidden.” For example, B2 is the only cell in which the 3 candidate appears, so candidates 2 and 8 are eliminated and 3 is the answer.

Naked Pair and Hidden Pair

A naked pair consists of two cells within a unit that have the same two candidate numbers and no other candidates. Since those two candidates must be the answer to the two cells, all other instances of the candidates can be eliminated within the unit.

A hidden pair is when two candidates only appear in two specific cells within a unit. However, the cells may have other candidates, too. For example, C1 and C3 have the hidden pair (4, 7). Since we know 4 or 7 must be the answer to those cells, we can eliminate the 1 and 3 candidates in C3.

Play Sudoku Online Today!

Now that you’ve learned Sudoku basics with our tutorial, play online games for free! We offer multiple difficulty levels, from easy to hard Sudoku, and variations including 4x4 Sudoku and 16x16 Sudoku to challenge players at every skill level.