SUDOKU v21
Play free Classic or Killer Sudoku online. Five difficulty levels, notes, hints, saving, leaderboards and uniquely solvable puzzles.
Free online Sudoku: Classic and Killer
Play Sudoku for free in your browser with no download. Choose Classic Sudoku or Killer Sudoku, progress from easy to master, and use notes, hints, saving and leaderboards to track your progress.
Play Sudoku nowComplete guide: rules and strategiesHow to play Sudoku
Understand the goal
Fill all 81 cells with digits 1 through 9. Each digit may appear only once in every row, column and 3 × 3 box.
Enter a digit
Select an empty cell, then tap or click a number. The related row, column and box are highlighted to help you read the grid.
Use the tools
Notes let you record small candidate digits. You can also undo a move, erase a cell or request a hint when you are stuck.
Choose a variant
Classic Sudoku uses the given digits. Killer Sudoku adds cages with target sums, and a digit cannot repeat inside the same cage.
A short history of Sudoku
Modern Sudoku appeared in the United States in 1979 as Number Place in a puzzle magazine and is generally credited to architect Howard Garns. Japanese publisher Nikoli introduced it in Japan in 1984, popularized it and shortened its name to Sudoku. It later became a worldwide fixture in newspapers, books, apps and online games.
Fun facts about Sudoku
- Sudoku uses digits, but no arithmetic is required: it is a logic puzzle.
- There are exactly 6,670,903,752,021,072,936,960 valid completed 9 × 9 Classic Sudoku grids.
- A uniquely solvable Classic Sudoku needs at least 17 starting clues; no valid 16-clue puzzle exists.
- Replacing digits 1 through 9 with letters, colors or pictures does not change the logic.
How Galaximum puzzles are generated
Galaximum does not simply select a puzzle from a fixed list. The game builds and checks puzzles in your browser to provide fresh challenges.
- In Classic mode, every puzzle comes from a bank validated by a human-technique solver. Levels match the most advanced required technique—singles, pairs, X-Wing or Swordfish—then mathematical transformations create many variants while preserving uniqueness and difficulty.
- In Killer mode, connected cages are built from a valid solution. The game checks target sums, prevents repeated digits inside a cage and tests uniqueness; difficulty changes with the number of revealed digits.
- Games in progress are saved locally. Signing in to Galaximum also enables cloud syncing and score publishing to the leaderboards.
Sources and verification
The historical and mathematical facts in this guide are supported by the references below. The generator description is based directly on how this game works.
- Nikoli — Sudoku history and rules
- Crown Hill Heritage Foundation — Howard S. Garns
- McGuire, Tugemann & Civario — There is no 16-Clue Sudoku
- Felgenhauer & Jarvis — Sudoku grid enumeration
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Frequently asked questions
Is Galaximum Sudoku free?
Yes. You can play for free in your browser on mobile, tablet or desktop without installing an app.
What is the difference between Classic and Killer Sudoku?
Classic Sudoku uses the prefilled digits as clues. Killer Sudoku adds cages whose digits must reach a target sum without repeating inside a cage.
Do the puzzles have a unique solution?
Yes. The generator checks the constraints to create puzzles designed to have one solution.
What are notes for?
Notes let you place several possible digits in a cell before deciding on the final answer. They are especially useful on harder levels.
Is my game saved?
Yes. Games in progress are saved in your browser. When you sign in to Galaximum, cloud syncing can also let you resume on compatible devices.
How can I improve at Sudoku?
Start by finding missing digits in each row, column and box. Use notes, look for single candidates and avoid guessing while a logical deduction is available.