The rule in one sentence
Fill the empty cells with digits 1 to 9 so every row, every column, and every 3x3 box contains each digit exactly once.
That is the whole rule set. The rest of Sudoku is learning how to apply it without guessing. For a fuller beginner walkthrough, start with How to Play Sudoku.
Rows, columns, and boxes
A row, column, or box is also called a house. A move is legal only when it works in all three houses that touch the cell.
- Each row must contain 1 to 9 with no repeated digit.
- Each column must contain 1 to 9 with no repeated digit.
- Each 3x3 box must contain 1 to 9 with no repeated digit.
What makes a move legal
Choose an empty cell and check its row, column, and box. If the digit already appears in any of those places, it cannot go in that cell.
Given numbers are fixed clues. You solve the remaining cells by placing only digits that satisfy the row, column, and box rules.
Quick examples
If row 6 already contains 8, no other cell in row 6 can be 8.
If a 3x3 box already contains 5, every empty cell in that box must use a different digit.
If a row has eight filled cells and the only missing digit is 7, the empty cell is 7 as long as its column and box also allow it.
What makes a Sudoku puzzle valid
A proper Sudoku puzzle has exactly one solution. The given digits are chosen so that logic alone forces every remaining cell; if a grid allows two different complete solutions, it is not a valid puzzle.
The number of givens varies with difficulty, but there is a hard mathematical floor: a classic 9x9 Sudoku needs at least 17 givens to have a unique solution. Most published puzzles use somewhere between 22 and 32.
Uniqueness is why you never need to guess. Every proper puzzle, including the hardest ones, can be finished with deduction - the harder the puzzle, the more advanced the deductions it asks for.
Common rule misconceptions
The diagonals do not matter. In classic Sudoku only rows, columns, and 3x3 boxes constrain a digit. A digit may repeat freely along a diagonal unless you are playing the Diagonal Sudoku variant.
There is no arithmetic. The digits 1 to 9 are just nine different symbols - nothing is added or summed. A Sudoku with nine letters or nine shapes would play exactly the same.
Givens are fixed. The digits printed at the start are clues and can never be changed or moved; you only fill the empty cells.
Variants with extra rules
Once the classic rules feel natural you may meet variants that add constraints on top of them. Diagonal Sudoku (X-Sudoku) also requires 1 to 9 on both main diagonals. Killer Sudoku adds dashed cages whose digits must sum to a target without repeating. Jigsaw Sudoku replaces the 3x3 boxes with irregular shapes.
Everything on this site, and everything in this learning path, uses the classic 9x9 rules described above - master these first and the variants come easily.
Frequently asked questions
Can a Sudoku have two of the same number in a row?
No. A digit can appear only once in each row.
Can a number repeat if it is in a different box?
Yes, as long as it is not repeated in the same row, column, or box.
Do I need to guess?
No. A valid Sudoku puzzle can be solved with logic. If you are stuck, use simpler scanning, pencil marks, or singles before guessing.
Can a Sudoku puzzle have more than one solution?
A proper puzzle cannot. If a grid can be completed in two different ways, it is considered invalid. Every puzzle on LiveSudoku has exactly one solution.
How many givens does a Sudoku need?
At least 17 givens are required for a unique solution. The number of givens alone does not determine difficulty, though: what matters is which techniques the puzzle forces you to use.
Practice next
Use the rules immediately on an easy puzzle. When the rules feel natural, move to the step-by-step solving guide.