Frequently asked questions
Playing the game
Is Sudoku Online free to play?
Yes. You can play every board without paying or signing up. There is no account system at present, so there is nothing to register for.
Which versions of Sudoku can I play?
Classic 9×9 is on the home page, where you choose a level. The games hub lists ten more boards: 4×4, 6×6, 16×16, killer, diagonal, irregular, hyper, anti-knight, the daily puzzle and expert mode. The rules page explains the one rule each one adds.
How do I write pencil marks?
Switch on Notes with the button or the N key, select a cell and tap digits to add or remove them as notes. The Auto-notes button fills every empty cell with its possible digits. See the pencil marks guide for when and how to use them.
How do hints work?
Pressing Hint three times on the same board shows three levels: where to look, which technique applies, and the move itself. Hints are worked out from the digits you have placed and ignore your pencil marks. Expert mode has no hints. The controls page has the details.
What is the difference between a conflict and a wrong digit?
A conflict is a digit that repeats inside a row, column, box or other region. A wrong digit is a value that does not match the puzzle’s single solution, even if it does not conflict with anything yet. With mistake feedback on, the game flags both.
Can I play with friends or against other players?
No. There is no multiplayer or leaderboard. When you finish a puzzle you can copy a short text summary of your result to share it.
Can I play offline?
Not at the moment. The game loads a file of puzzles when you choose a level, so it needs a connection at that point. There is no offline mode.
Can I enter a puzzle from a newspaper or app?
No. The game plays only the puzzles stored on this site, and it has no way to type in your own. If you want to see how a solver works, the TypeScript solver tutorial shows how to write one.
Puzzles and difficulty
Does every puzzle have only one solution?
Yes. Each stored puzzle is checked by a solution counter, and the check is repeated for every stored puzzle before each release. A puzzle with no solution, or with more than one, fails the build. How puzzles are generated and verified unique explains the process.
How is the difficulty level decided?
By the techniques a solver needs, not by the number of given digits. Easy needs only naked and hidden singles, and each higher level adds harder techniques. This is the site’s own grading and not an official or industry standard, so another site’s “Hard” may not match this one’s. See how Sudoku difficulty is graded.
Why does a puzzle feel easier or harder than its level?
The level records the hardest technique a puzzle needs. It does not measure how many steps there are, how easy a pattern is to spot, or how long you take. A Medium puzzle with a hidden pair that you miss can feel much harder than one where the same technique is obvious. The strategy guide shows what to learn for each level.
Why do some boards have fewer levels?
Not every board produced puzzles at every level. The 4×4 board has one level, the 6×6 and 16×16 boards have two, and killer has three. Diagonal, hyper and anti-knight have four and no Evil level. The site does not label a puzzle Evil unless it needs a unique rectangle or something beyond.
Where do the puzzles come from?
They are generated by the site’s own program, not copied from other sites, books or apps. The program builds a grid, removes clues while the puzzle stays uniquely solvable, then grades the result.
How does the daily puzzle work?
There is one puzzle for each UTC day, the same for every player, and its level depends on the weekday: Sunday is Evil, Monday Easy, Tuesday and Wednesday Medium, Thursday Hard, and Friday and Saturday Expert. Finishing it on consecutive days builds a streak that is stored in your browser. See the daily Sudoku page.
Saving, privacy and your data
Is my progress saved?
The game saves your board in your browser when you leave the page or switch tabs or apps, and your statistics are kept there too. Progress is not tied to an account, so it does not move between devices. If your browser blocks storage, for example in some private modes, the data lasts only until you close the page.
Why did the game pause by itself?
The game pauses when the tab is hidden or the window loses focus, so the timer does not keep running while you are away. Press Resume to carry on. The grid is hidden while paused, so a pause cannot be used to study the board.
How do I clear my statistics?
Statistics live in your browser’s storage for this site, so clearing the site’s data in your browser settings removes them.
What data does the site collect?
The site counts page and game events with its own first-party analytics, which uses no cookies and holds no personal information. Where the law requires consent, including the EEA, the UK and Switzerland, the site asks before any third-party advertising or analytics scripts are loaded. The privacy policy and cookie policy give the details.
Accessibility and the site
Can I play with only a keyboard or a screen reader?
Yes, all play is possible from the keyboard, and the grid announces each cell and each move to screen readers. The game has not yet been tested with a wide range of real assistive devices, so reports of problems are welcome. See the controls page for the key map and settings.
Which languages does the site support?
English only for now. The text of the game is kept in separate message files so that other languages can be added later.
What do terms like “hidden single” or “peer” mean?
The glossary defines them, and each term links to the page that uses it.
How do I report a problem or a wrong puzzle?
Use the contact page. Please say which board and level you were playing and what happened.