How to play Sudoku: rules, notes and your first moves

The three Sudoku rules

A standard Sudoku board has 81 squares: nine rows, nine columns and nine smaller 3×3 boxes. Complete the board so that:

• Every row contains 1–9 exactly once.
• Every column contains 1–9 exactly once.
• Every outlined 3×3 box contains 1–9 exactly once.

Each square belongs to all three groups at the same time. A number that fits a row may still be blocked by its column or box. The starting clues cannot be changed. Ordinary Sudoku has no additional rule for diagonals.

Thinking of the digits as nine different symbols can help: their numerical size does not matter, and rows do not need to appear in counting order.

Find one square with only one answer

Begin with a nearly complete row, column or box. The fewer empty squares it has, the fewer missing digits you need to inspect.

1. List the digits missing from that group.
2. Choose one empty square and inspect its other two groups.
3. Cross out every missing digit already present in its row, column or box.
4. If one candidate remains, enter it and check the groups that the new number touches.

Here is a small teaching example: one row is missing only its last number.

The row already contains 1, 3, 4, 5, 6, 7, 8 and 9. Only 2 is missing. On a correctly filled board, its column and box must allow 2 as well. If they do not, inspect your earlier entries before continuing.

A square with just one remaining candidate is called a naked single. The square does not have to be the last blank in its row: the combination of its row, column and box can eliminate eight digits.

Use pencil notes to keep possibilities visible

When a square could hold more than one number, use Notes. Select the square, turn Notes on, and enter the candidates you want to remember. A small 2 and 7 means “2 or 7,” not that either answer has been confirmed.

Start with one promising box or a nearly finished row. Check all three groups for each square before adding candidates. Whenever you place a number, review related notes and remove candidates that no longer fit. Turn Notes off before entering a full-size answer.

Notes are your working hypotheses. A pencil mark is not evidence by itself: an omitted candidate can make a move look forced when it is not. If a step stops making sense, recompute candidates from the large numbers on the board.

Look for a number with only one possible home

A hidden single asks a different question: where can this particular digit go within one row, column or box?

Imagine a row is missing 2, 4 and 7. After checking the relevant columns and boxes, you have these complete candidate lists for its three empty squares:

No square has only one candidate, but 4 appears only in B. The row must contain a 4, so B must be 4. A and C are still unresolved: each could be 2 or 7 until another clue distinguishes them.

Try scanning one digit through all nine boxes. Existing copies in the surrounding rows and columns can rule out positions until only one square remains. This is especially helpful when a full board of notes feels overwhelming.

Use a repeatable routine when you get stuck

Alternate between squares and digits: first look for a square with one candidate, then look for a digit with one available position in a group. Each confirmed answer can unlock another move.

• Recheck recent entries. Undo lets you return through moves and note changes. A duplicate-free entry is not necessarily the correct answer.
• Inspect a smaller area. Choose the row, column or box with the most filled squares instead of repeatedly scanning the whole board.
• Read an explained hint. Focus on why the number belongs there. Try applying that reasoning to the next move yourself.
• Take a break. Pause or close the page and return in the same browser. The puzzle does not need to be completed in one sitting.

Check and hints count as assistance in this game. They are useful learning tools; you can try solving a later board without them when you feel ready.

Choose Easy while learning naked singles. Medium introduces hidden singles. Hard uses further eliminations such as locked candidates and pairs: use the in-game explanation to understand the affected squares before entering a number. Fewer starting clues alone do not reliably tell you which puzzle will feel harder.

Play on a phone or use the keyboard

On a phone or tablet, tap the square first, then a digit on the number pad. Use Notes for pencil marks and Erase to clear an entry. Keep an eye on the Notes state when switching between candidates and answers.

The buttons work with a mouse, too. You can hide the timer to focus on the logic, and browser zoom stays available.

Keep a puzzle, or send the same board to a friend

The game keeps up to 24 recent puzzles locally, including their entries and notes. Use Saved puzzles to return to one. These saves belong to the current browser; changing devices or clearing site data does not transfer them.

Pause hides the board and stops timed play. Opening a dialog or hiding the browser page also suspends the timer. If you want a recurring puzzle, Daily uses one Medium board for each UTC calendar date.

A shared link identifies the starting puzzle and difficulty, not your filled answers or elapsed time. A friend confirms before opening it and works on their own copy. Daily links preserve their original date. There is no live shared board or global leaderboard.

For the game’s modes and save details, visit the Sudoku overview.