Sudoku has three simple rules. This guide explains each one in detail with examples and clarifies common misunderstandings.
The objective of Sudoku is straightforward: fill every empty cell in the 9x9 grid so that each row, each column, and each 3x3 box contains all of the digits from 1 to 9 exactly once. When every cell is filled correctly - with no duplicates in any row, column, or box - the puzzle is solved.
A Sudoku puzzle begins with a partially completed grid. The pre-filled numbers are called "givens" or "clues." These clues are carefully chosen so that the puzzle has exactly one correct solution. Your task is to use logic and deduction to determine the value of every empty cell based on the clues provided. Learn more about the minimum number of clues needed and why Sudoku has only one solution.
These three rules interact with one another. Every cell in the grid belongs to exactly one row, one column, and one box - together known as its "houses". The intersection of these three constraints is what allows you to deduce the correct number for each cell through pure logic. Ready to start solving? Learn the key techniques in our strategies guide.
Each of the nine horizontal rows must contain every digit from 1 to 9 with no repetition. If the number 5 already appears in a row, no other cell in that row can contain a 5.
Each of the nine vertical columns must contain every digit from 1 to 9 with no repetition. A column functions exactly like a row, but oriented vertically.
Each of the nine 3x3 boxes (the thick-bordered squares) must contain every digit from 1 to 9 with no repetition. Boxes provide an additional layer of constraint that makes the puzzle solvable.
Not every arrangement of numbers in a grid qualifies as a valid Sudoku puzzle. A well-constructed puzzle must meet several important criteria that distinguish it from a random number placement exercise.
A valid Sudoku puzzle must have exactly one solution. Multiple solutions indicate an under-constrained puzzle, while no solution indicates conflicting clues.
The puzzle must be solvable entirely through logical deduction without any guessing. Every step should follow from the given information and previously deduced values.
Classic Sudoku puzzles often feature rotationally symmetric clue patterns - if you rotate the grid 180 degrees, the clue positions remain the same. This is a convention, not a rule.
The number and placement of clues determine difficulty. Easy puzzles typically have 36-45 clues, while expert puzzles may have as few as 22-24. The minimum for a unique solution is 17.
Myth: You need math skills to solve Sudoku
Fact: Sudoku is purely a logic puzzle. The digits 1-9 are just symbols - no arithmetic is involved in solving.
Myth: Harder puzzles have fewer valid solutions
Fact: Every properly constructed puzzle, regardless of difficulty, has exactly one solution. Difficulty comes from requiring more advanced logical techniques.
Myth: Guessing is sometimes necessary
Fact: A valid Sudoku can always be solved through logic alone. If you feel the need to guess, there is a technique or pattern you have not yet identified. See our article on solving without guessing.
Myth: Numbers must add up to a certain total
Fact: In standard Sudoku, there are no sum constraints. Each row, column, and box simply needs all digits 1-9. (Killer Sudoku is a variant that adds sum rules.)
Myth: The puzzle layout must be symmetric
Fact: Rotational symmetry is a common design convention, especially in newspapers, but it is not a rule. Many digital Sudoku generators produce asymmetric puzzles that are perfectly valid.
Standard Sudoku uses the digits 1 through 9. This numbering system was chosen because nine symbols neatly fill a 9x9 grid divided into nine 3x3 boxes. The numbers themselves carry no mathematical weight in the context of the puzzle - they are simply nine distinct symbols used for convenience and familiarity.
In some cultures, Sudoku variants use letters, colors, or other symbols instead of numbers. The logic remains identical regardless of the symbol set used. What matters is that you have exactly nine distinct symbols and that each appears exactly once in every row, column, and box.
The grid uses a coordinate system where cells are often referenced by their row and column position. Row 1 is the top row and Column 1 is the leftmost column. This notation is commonly used in solving guides and strategy discussions, such as "R3C7" meaning the cell in Row 3, Column 7.
Standard Sudoku always follows the same three rules. However, variant puzzles like Killer Sudoku, Diagonal Sudoku, and Irregular Sudoku add extra constraints while keeping the core rules intact.
No. A properly constructed Sudoku puzzle must have exactly one unique solution. If a puzzle has multiple solutions, it is considered poorly designed or invalid.
The minimum number of clues for a valid Sudoku puzzle with a unique solution is 17. This was proven mathematically in 2012 by Gary McGuire and his team at University College Dublin.
Not at all. Pencil marks (small candidate numbers written in cells) are a solving tool, not part of the puzzle rules. They are widely used by solvers of all levels and are considered essential for harder puzzles.
No. The digits 1-9 in Sudoku are simply symbols used for convenience. You could replace them with letters A-I, colors, or any nine distinct symbols and the puzzle logic would remain identical.
Placing a wrong number creates a conflict that will eventually prevent you from completing the puzzle correctly. In paper Sudoku, you would erase the mistake. In digital Sudoku apps, many offer error highlighting or undo features. The best practice is to only place a number when you are logically certain it is correct.
A standard Sudoku grid has 81 cells arranged in 9 rows and 9 columns. The grid is further divided into nine 3x3 boxes (also called blocks or regions). Each row, column, and box is called a 'unit' or 'house,' and the three rules of Sudoku apply independently to each unit.
No. The fundamental rule of Sudoku is that no number can repeat within any row, column, or 3x3 box. Each of the numbers 1 through 9 must appear exactly once in every row, every column, and every box. This constraint is what creates the logical challenge of the puzzle.
For a complete reference of Sudoku terminology used throughout our guides, visit the Sudoku glossary where you will find clear definitions for terms like givens, candidates, houses, and more. Our Sudoku terminology guide and terminology glossary article provide additional context.
Brand-new to the game? Read how to play Sudoku and our Sudoku cheat sheet for a quick reference, then practice with a free Sudoku puzzle pack. Avoid early pitfalls with common Sudoku mistakes and how to avoid common mistakes.
Curious about the origins of these rules? Learn about the people and events that shaped the puzzle in our article on who invented Sudoku and the broader history of Sudoku. To explore how the rules adapt to other formats, see Sudoku variants explained, Sudoku with letters, and grid-size variations like 4x4 Sudoku, 6x6 Sudoku, 9x9 Sudoku, 12x12 Sudoku, and 16x16 Sudoku.
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