Sudoku rules
In classic Sudoku you fill a 9×9 grid so that every row, every column and every 3×3 box contains each digit from 1 to 9 exactly once. Some cells are given at the start and cannot be changed. Every puzzle here has exactly one solution.
Each variant on this site keeps that core and changes one thing, such as a smaller or larger grid, an extra region that must also hold every digit, or a restriction on which cells may share a digit.
What are the rules of classic Sudoku?
The board is a square grid of 81 cells, divided into nine boxes of 3×3 cells. A digit from 1 to 9 must end up in every cell, subject to three conditions.
- Each row holds the digits 1 to 9 once each.
- Each column holds the digits 1 to 9 once each.
- Each box holds the digits 1 to 9 once each.
A puzzle starts with some cells already filled. These are the givens, and they are locked. You fill in the rest. Every puzzle on this site has exactly one solution, so a correct deduction always exists and you never need to guess. How that is checked is explained in how puzzles are generated and verified unique.
What can a cell contain?
An empty cell can hold a value or a set of pencil marks, never both. A value is your answer for the cell. Pencil marks, also called notes, are small candidate digits that you write to track what could still go there. Entering a value clears the cell’s notes. Marks are only a memory aid and have no effect on whether the puzzle is solved. The pencil marks guide explains how to use them.
When is the puzzle solved?
The puzzle is solved when every cell is filled and no house contains a repeated digit. A house is any group that must contain each digit once: a row, a column, a box, or, in a variant, an extra region. When you finish, the game shows your time, the hints you used and your mistakes.
What counts as a mistake?
The game distinguishes two things.
- A conflict is a digit that repeats within a house, or breaks the variant’s extra rule.
- A wrong digit is a value that differs from the puzzle’s single solution. It may not conflict with anything yet.
With the default feedback setting, both are flagged as you play. With feedback off, nothing is flagged until you ask the game to check. With the strict setting, three mistakes end the game. The controls page lists where these options are.
What does each variant change?
Each variant has its own page with a playable board. The summary below gives the one rule it adds or changes. Everything else is as in classic Sudoku.
Sudoku 4×4 and 6×6
The 4×4 board has 16 cells, the digits 1 to 4 and four 2×2 boxes. The 6×6 board has 36 cells, the digits 1 to 6 and six boxes that are each two rows tall and three columns wide. In both, each row, column and box holds every digit once.
Sudoku 16×16
The 16×16 board has 256 cells and sixteen 4×4 boxes. The symbols are 1 to 9 and then the letters A to G, and each row, column and box holds each of the sixteen symbols once.
Killer Sudoku
In killer Sudoku the board is divided into cages, groups of adjacent cells drawn with a target sum. The digits in a cage must add up to its sum, and a digit may not repeat within a cage. The usual row, column and box rules still apply. A cage of three cells with a sum of 15 can hold 4, 5 and 6, or 1, 5 and 9, but not 5, 5 and 5. A cage of one cell reveals its digit directly. Killer puzzles here start with few or no given digits, and the cage sums carry the information.
Diagonal Sudoku
In diagonal Sudoku, both main diagonals must also contain each digit from 1 to 9 once: the one from the top left to the bottom right, and the one from the top right to the bottom left. They cross at the center cell.
Irregular Sudoku
In irregular Sudoku the nine 3×3 boxes are replaced by nine jigsaw regions. Each region is a connected group of nine cells and must contain every digit once. Rows and columns work as usual. The region layout is different from puzzle to puzzle.
Hyper Sudoku
Hyper Sudoku adds four extra 3×3 windows, and each must contain every digit once. On this site the windows cover rows 2 to 4 and 6 to 8, in columns 2 to 4 and 6 to 8. They overlap the ordinary boxes. In the grid below the letters mark which window each cell belongs to.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
|---|---|---|---|---|---|---|---|---|---|
| 1 | |||||||||
| 2 | A | A | A | B | B | B | |||
| 3 | A | A | A | B | B | B | |||
| 4 | A | A | A | B | B | B | |||
| 5 | |||||||||
| 6 | C | C | C | D | D | D | |||
| 7 | C | C | C | D | D | D | |||
| 8 | C | C | C | D | D | D | |||
| 9 |
Anti-knight Sudoku
In anti-knight Sudoku, two cells a chess knight’s move apart may not hold the same digit. A knight’s move is two cells in one direction and one cell at a right angle, so a cell in the middle of the board has eight such neighbors. This adds to the row, column and box rules. In the diagram, no K cell may hold the same digit as X.
| K | K | |||
| K | K | |||
| X | ||||
| K | K | |||
| K | K |
Daily Sudoku
Daily Sudoku follows the classic rules. What changes is the puzzle choice: there is one shared puzzle for each UTC day, the same for every player, and the level depends on the day of the week. Sunday is Evil, Monday Easy, Tuesday and Wednesday Medium, Thursday Hard, and Friday and Saturday Expert. Completing the daily puzzle on consecutive days builds a streak, which is stored in your browser.
Expert mode
Expert mode uses the classic rules with stricter conditions. Only Expert and Evil puzzles are offered. There are no hints, no auto-notes and no mistake feedback. You have one attempt per puzzle, and the result is recorded: if you fill the grid and it is not correct, the attempt counts as a loss.
What do the levels mean?
Easy, Medium, Hard, Expert and Evil describe the techniques needed to solve a puzzle, not the number of givens. That is this site’s own grading, explained in how Sudoku difficulty is graded. Not every board has all five levels: the 4×4 board has one, the 6×6 and 16×16 boards two, and killer three. Diagonal, hyper and anti-knight have four, and have no Evil level.
Where to go next
If you are new, the guided first puzzle shows the rules in action. The glossary defines the vocabulary used here, Sudoku variants through history traces where the extra rules came from, the FAQ answers common questions, and the games hub lists every board.