Sudoku FAQ

Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. The digits printed at the start (the givens) stay as they are.

A proper sudoku has exactly one solution. Every puzzle on this site was checked by a computer solver to have exactly one.

Seventeen. In 2012 McGuire, Tugemann and Civario completed an exhaustive computer search showing that no 16-clue sudoku has a unique solution; the result was published in Experimental Mathematics in 2014.

You can guess, but you never have to. A good puzzle can be solved by logic alone, and every puzzle here is graded so that it never needs trial and error.

No. There is no arithmetic in regular sudoku; the digits could be letters or colours. It is a logic puzzle. Killer sudoku is the variant that adds sums.

Small notes in a cell listing the digits that are still possible there, also called candidates. Press Notes (or N) to write them here, or use Auto-notes to fill them all in.

They are the same puzzle at different sizes. A 4×4 grid uses digits 1–4 and 2×2 boxes, a 6×6 grid uses 1–6 and 2×3 boxes, and the classic 9×9 uses 1–9 and 3×3 boxes. Smaller grids are often used to teach children. This site currently offers 9×9 only.

The techniques it requires, not the number of givens. A hard puzzle reaches points where no simple move is available and you must spot a pattern such as an X-Wing to continue.

Our solver works through each puzzle the way a person would and records the hardest technique it needs. Easy needs only singles; medium adds pairs, pointing pairs and box/line reduction; hard adds triples, X-Wing and XY-Wing; expert adds Swordfish, Jellyfish, XYZ-Wing, unique rectangles and coloring.

There is no right time. It depends on the level, the particular puzzle, whether you use pencil marks, and how familiar the techniques are. We do not publish average times because we do not collect them. Your own times are kept in your stats so you can compare yourself with yourself.

A pattern where a digit can go in only two cells in each of two rows, and those cells line up in the same two columns. The digit must sit in two opposite corners of that rectangle, so it can be removed from the rest of both columns. It is one of the techniques our hard puzzles can require.

A naked single is a cell that can hold only one digit. A hidden single is a digit that has only one possible cell in a row, column or box.

If you paste a puzzle that our logical techniques cannot finish, the solver still finds the solution by searching, and says so clearly instead of pretending it used logic.

At midnight in your own time zone. The day is your device’s local calendar date, so everyone gets the same puzzle for the same date, but it arrives at local midnight wherever you are.

Yes. Every daily puzzle has its own page, linked from the archive and from the previous/next links on each daily page.

Yes. Use the Print button under the board (or your browser’s print command); the print layout shows just the grid. For several puzzles with an answer key, use the printable page.

Yes. The daily puzzles come from a fixed bank in an order that depends only on the date, so anyone playing the same date gets the same four puzzles.

In your browser. Solving the daily puzzle (any level) on its own date adds that day to your streak. Nothing is stored on a server, so streaks do not carry over to another device or browser.

No. There are no accounts or sign-ups. Your saved games, settings and stats live in your browser only, and you can delete them at any time.

Yes. Use the button in the top corner of every page. The site starts in your device’s preferred theme and remembers your choice.

There is no offline mode or installable app. If your connection drops after a puzzle has loaded, you can usually keep playing it, but opening new pages needs a connection.

You can type or paste any puzzle into the solver for your own use. It checks the puzzle, explains each step and links to the technique guides. We never publish puzzles from other sources; all puzzles on the site are our own.

Yes, the easy puzzles suit beginners of any age, and the how-to-play guide explains the rules in plain words. There is no separate children’s section yet.

Yes. NineByNine's own generator made every puzzle, and each one was checked for a unique solution and graded by our solver before it was added to the bank.

Yes. Undo and redo cover every move, including notes, and the mistake highlighter can show digits that do not match the solution if you switch it on.

Killer sudoku adds cages: groups of cells whose digits must add up to a given sum and cannot repeat. It usually starts with an empty grid, so the sums are the only clues.

The modern puzzle first appeared in the United States: Dell published it as “Number Place” in 1979, and it is usually credited to the puzzle constructor Howard Garns. Nikoli introduced it in Japan in 1984, and it became a worldwide craze after The Times started printing it in November 2004.

It is a shortened form of the Japanese name Nikoli gave the puzzle in 1984, “Suuji wa dokushin ni kagiru”, roughly “the digits must be single”, meaning each digit appears only once.

Many people enjoy it as a mental workout, but we do not make health claims. Whether puzzles protect long-term brain health is not settled, and studies in this area mostly show associations rather than cause and effect.