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What Is Sudoku?

Sudoku is a logic-based number puzzle played on a 9×9 grid. The goal is to fill every cell so that each row, column, and 3×3 box contains the digits 1 through 9 exactly once. No math is required - only logic and deduction.

The modern Sudoku puzzle was designed by American architect Howard Garns in 1979 under the name "Number Place." It gained worldwide popularity after being published in Japanese puzzle magazines in 1986. The name Sudoku comes from the Japanese phrase "Sūji wa dokushin ni kagiru," meaning "the digits must remain single."

Today, Sudoku is one of the most popular puzzles in the world. Millions of people play Sudoku online every day, and it remains a staple in newspapers, puzzle books, and apps. Sudoku91 offers unlimited free Sudoku puzzles you can play directly in your browser.

How to Play Sudoku

Learning how to play Sudoku is straightforward. The rules are simple, but mastering the puzzle takes practice. Here is a step-by-step guide to the rules of Sudoku.

The grid

A standard Sudoku puzzle is a 9×9 grid divided into nine 3×3 boxes (also called regions or blocks). Some cells come pre-filled with numbers - these are called "givens" and cannot be changed.

The one rule

Every row, every column, and every 3×3 box must contain the digits 1 through 9 exactly once. No digit can repeat within any row, column, or box.

How to solve

Start by scanning rows, columns, and boxes to find cells where only one number can fit. Use the process of elimination: if a digit already appears in a row, column, or box, it cannot appear there again. Work through the puzzle cell by cell.

Quick start: Open a puzzle, tap an empty cell, and enter a digit from 1-9. Use Notes mode to track candidate numbers. If you get stuck, try scanning a different row or box. Undo is always available.

Sudoku Tips & Strategies

Whether you are a beginner learning how to solve Sudoku or an experienced player looking to get better at Sudoku, these strategies will help you solve puzzles faster.

Scanning and crosshatching

Look across rows and columns to find where a number must go by elimination. Crosshatching means checking a box by scanning its intersecting rows and columns to narrow candidates down to a single cell.

Pencil marks (Notes)

Use the Notes feature to write small candidate numbers in empty cells. As you place digits, eliminate candidates from related rows, columns, and boxes. This is essential for medium and harder puzzles.

Naked singles

If a cell has only one candidate number left after eliminating all others, that number must go there. Always check for naked singles after placing a digit.

Hidden singles

If a number can only go in one cell within a row, column, or box - even if that cell has multiple candidates - it must go there. Scan each group for digits that appear as a candidate in only one cell.

Advanced techniques

For hard and expert Sudoku puzzles, learn naked pairs, pointing pairs, and X-wing patterns. These techniques eliminate candidates across multiple cells and are key to solving difficult puzzles without guessing.

Sudoku Difficulty Levels

Sudoku91 offers five difficulty levels. Each level differs in the number of pre-filled cells (givens) and the solving techniques required.

Easy Sudoku

Easy puzzles provide 36-45 givens. Most cells can be solved using basic scanning and naked singles. Great for beginners or a quick, relaxing game.

Medium Sudoku

Medium puzzles provide 30-35 givens. You will need pencil marks and should watch for hidden singles. A good step up from easy.

Hard Sudoku

Hard puzzles provide 25-29 givens and require advanced elimination techniques like naked pairs and crosshatching. Patience and methodical note-taking are essential.

Expert Sudoku

Expert puzzles provide 22-27 givens and demand advanced strategies including pointing pairs, X-wing, and chain logic. Only for seasoned Sudoku solvers.

Evil Sudoku

Evil puzzles push logic to its limits with minimal givens. Requires mastery of X-wing, swordfish, and chain logic. Only for Sudoku masters who crave the ultimate challenge.

Frequently Asked Questions

How do you play Sudoku?
Fill the 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Start with the pre-filled numbers (givens) and use logic to determine which number goes in each empty cell. No guessing is needed - every Sudoku puzzle on Sudoku91 has exactly one solution.
What do the numbers mean in Sudoku?
The numbers 1-9 in Sudoku are simply symbols used to fill the grid. There is no arithmetic involved - you never add, subtract, or calculate. The challenge is placing each digit so it appears exactly once in every row, column, and 3×3 box.
Is Sudoku good for your brain?
Yes. Research suggests that regularly solving Sudoku and similar logic puzzles can help improve concentration, short-term memory, and logical reasoning skills. It keeps your mind active and engaged, much like a workout for your brain.
Does Sudoku make you smarter?
While Sudoku may not directly raise your IQ, it exercises working memory, pattern recognition, and deductive reasoning. Regular practice can sharpen these cognitive skills and improve your ability to think logically under constraints.
What does Sudoku mean?
Sudoku is short for the Japanese phrase "Sūji wa dokushin ni kagiru," which translates to "the digits must remain single." The puzzle was popularized in Japan in the 1980s, though it was originally created by Howard Garns in the United States in 1979.
How do you solve a hard Sudoku?
Hard Sudoku puzzles require advanced strategies beyond basic scanning. Use pencil marks to track candidates, then apply techniques like naked pairs (two cells in a group sharing the same two candidates), pointing pairs, and X-wing patterns. Work systematically and avoid guessing.
Can you play Sudoku for free online?
Yes! Sudoku91 offers unlimited free Sudoku puzzles in five difficulty levels - Easy, Medium, Hard, Expert, and Evil. No sign-up or download required. Just open the site and start playing instantly in your browser.
What is the best strategy for Sudoku?
Start with scanning - look for rows, columns, and boxes where a digit can only go in one place. Use pencil marks to track candidates. Then apply elimination techniques: naked singles, hidden singles, and for harder puzzles, naked pairs and X-wing. The key is to never guess - every move should be based on logic.