The Basic Rules of Sudoku: How to Play in Five Points
Sudoku is a 9x9 logic puzzle where you must place the digits 1 through 9 so that each row, each column, and each 3x3 sub-grid contains every digit exactly once. This simple constraint forms the puzzle's entire structure. Understanding these core rules is the first step to solving any puzzle, from easy to expert. Once mastered, they allow you to apply logic and deduction, using techniques like [Naked Single](/techniques/naked-singles) and [Hidden Single](/techniques/hidden-singles) to find the solution.
Rule 1: The 9x9 Grid is Your Playing Field
Every Sudoku puzzle starts with a standard 9x9 grid, which is further subdivided by thicker lines into nine smaller 3x3 grids, called 'boxes', 'blocks', or 'regions'. The goal is to fill every one of the 81 cells with a number. Some cells are pre-filled with 'givens' or 'clues'. These numbers are permanent and cannot be changed; they provide the logical foundation from which you deduce all other numbers in the grid.
- Look for rows, columns, or boxes with many givens first. They offer the most starting information.
Rule 2: The Row Rule – No Repeats Left to Right
Each of the nine horizontal rows must contain all digits from 1 to 9, with no repetition. This means if you place a '5' in a cell, you cannot place another '5' anywhere else in that same row. Checking rows is a fundamental scanning technique. When you see a row that already has several numbers filled in, you can immediately deduce which numbers are missing and where they might logically fit based on the other rules.
Rule 3: The Column Rule – No Repeats Top to Bottom
Each of the nine vertical columns must also contain all digits from 1 to 9, without any duplicates. The column rule works identically to the row rule. A digit placed in a cell locks that digit out of the eight other cells in the same column. Successful solving requires constantly cross-referencing the implications of a placed digit on its row and its column simultaneously.
Rule 4: The 3x3 Box Rule – No Repeats in the Neighborhood
This rule is what distinguishes Sudoku from other Latin square puzzles. Each of the nine 3x3 boxes, outlined by the thicker lines, must contain every digit from 1 to 9 exactly once. A cell's 'neighborhood' is its home box. When deciding what number can go in a cell, you must check three constraints: what's missing from its row, its column, and its 3x3 box. The intersection of these three missing sets reveals the possible candidates.
Rule 5: Use Pencil Marks to Track Possibilities
While not a strict 'win condition', using pencil marks is a critical practice for all but the easiest puzzles. Pencil marking involves writing small candidate numbers (1-9) in the corners or center of an empty cell to note which digits could possibly go there based on the row, column, and box rules. This visual aid is essential for spotting patterns and applying more advanced solving techniques. For a deep dive on this method, see our guide on Pencil Marking in Sudoku. Systematic pencil marking transforms the puzzle from a guessing game into a logical deduction process.
- Start by pencil marking a single row, column, or box to avoid overwhelm. Look for cells with only one possible candidate—that's a Naked Single.
Key Facts
- ▪Sudoku is played on a 9x9 grid subdivided into nine 3x3 regions called boxes.
- ▪The objective is to fill every cell so each row contains digits 1-9 without repetition.
- ▪Every column must also contain all digits 1-9 exactly once.
- ▪Each 3x3 box must contain all digits 1-9, which is the rule that defines classic Sudoku.
- ▪Pre-filled 'given' numbers provide the immutable starting clues for the puzzle.
- ▪Pencil marking is the practice of noting possible candidate numbers in empty cells to aid logic.
- ▪A cell's valid number is determined by combining the row, column, and box constraints.
- ▪The puzzle is solved when all 81 cells are filled correctly according to the three core rules.
Frequently Asked Questions
Can I change the 'given' numbers in a Sudoku puzzle?
No. The pre-filled clue numbers are permanent and correct. Your task is to deduce all other numbers based on these fixed starting points.
Do I need to use math or arithmetic to solve Sudoku?
No. Sudoku is a pure logic puzzle. You only need the digits 1-9 as symbols. No addition, subtraction, or calculation is involved.
What is the most important rule for beginners to remember?
Every row, column, and 3x3 box must have the numbers 1-9 once. Always check all three constraints before placing a number. Learn more in our guide How to Play Sudoku.
Is guessing allowed in Sudoku?
Proper Sudoku requires logical deduction, not guessing. If you're stuck, use pencil marks to find cells with only one possible candidate, known as a Naked Single.
What happens if I break a rule?
The puzzle becomes invalid. A digit will be duplicated in a row, column, or box, making a correct solution impossible. You must backtrack to the last correct placement.