Sudoku Frequently Asked Questions
Clear answers about Sudoku rules, unique solutions, difficulty levels, pencil marks, Killer Sudoku, daily puzzles, saved progress, accounts, and puzzle-quality checks.
Is Sudoku a math game?
No. Sudoku uses the digits 1–9 as symbols, but normal Classic Sudoku does not require arithmetic. The puzzle is solved by logic, candidate elimination, and the rule that each row, column, and 3×3 box contains each digit once.
Should a Sudoku puzzle have only one solution?
A standard playable Sudoku should have exactly one valid solution. The puzzle catalogue can count solutions and quarantine puzzles that are invalid or non-unique.
How are Sudoku difficulty levels determined?
Difficulty is not based on clue count alone. The site uses a human-oriented grader that records the techniques needed, including singles, locked candidates, pairs, X-Wing, and any remaining search work. Difficulty is still an estimate because players have different experience levels.
Which Sudoku difficulty should a beginner choose?
Start with Easy Sudoku until scanning rows, columns, boxes, naked singles, and hidden singles feel comfortable. Move to Medium when you can finish Easy puzzles consistently without relying heavily on hints.
What are pencil marks or candidates?
Candidates are the digits that could still fit an empty cell without breaking Sudoku rules. Pencil mode lets you record those possibilities and remove them as new placements create logical eliminations.
What does Auto candidates do?
Auto candidates can maintain possible digits for empty cells as the board changes. It is a convenience tool; you can still use manual pencil notes if you prefer to practice candidate tracking yourself.
What is the difference between Check and Hint?
Check is used to validate the current board state or detect mistakes according to the active game rules. A Hint is intended to help you make progress. Using assistance can affect how you choose to treat a solve for personal practice or competition.
How is Killer Sudoku different from Classic Sudoku?
Killer Sudoku keeps the normal row, column, and 3×3 box rules, then adds cages with target sums. Digits cannot repeat inside the same cage, so cage combinations become an additional source of logic.
Can I play a new Sudoku every day?
Yes. Daily Challenge provides date-based Sudoku play and tracks results such as completion and streak-related progress where the feature is enabled. Classic and Killer daily modes are kept distinct.
Will my Sudoku game be saved if I reload the page?
The browser can store an unfinished game and gameplay preferences locally. If you use an account, supported progress, history, statistics, or competition-related data can also be associated with that account.
Do I need an account to play Sudoku?
No account is required for the core browser-based Sudoku experience. Accounts are optional and are used for features such as identity, saved progress, statistics, and competition data where available.
How does Sudoku Online test puzzle quality?
The theme includes checks for clue/solution consistency, unique solutions, difficulty grading, Killer cage logic, duplicate groups, and catalogue quality. The How We Test Sudoku Puzzles page explains what is automated and what remains subjective.
What should I do if I find an invalid puzzle or incorrect guide?
Send the exact page URL and a short explanation through the Contact page. Puzzle, logic, translation, and editorial issues can be reviewed under the site’s Content Correction Policy.