How to play Sudoku

The three rules

• Every horizontal row must contain the digits 1 to 9, with no repeats.
• Every vertical column must also contain the digits 1 to 9, with no repeats.
• Every small 3x3 box, outlined with heavier lines, must contain the same nine digits once each.

Some digits are printed at the start. These are fixed clues and cannot be erased. The empty cells are yours to fill. A placement has to satisfy all three rules at the same time, so a digit that is missing from a row may still be excluded by its column or box. Every Sudoku in our daily collection and free-play packs has exactly one solution, checked before it is published.

Your first few moves

Choose a row, column or box that already has many digits. Work out which digits it still needs, then check the crossing lines for each empty cell. If eight digits are present in a row, its ninth is fixed. If a box needs a 4 and all but one of its empty cells sit in rows or columns that already contain a 4, you have found its only home. Make that certain placement and scan again: a single new digit often unlocks several more.

Playing with touch or a keyboard

• Tap an empty cell, then tap a digit on the number pad below the board. The small count under each number shows how many more of that digit the grid needs.
• On a computer, the arrow keys move the selection and the number keys enter a digit. Press 0, Backspace or Delete to erase the selected cell.
• Press N or choose Notes to switch between final digits and pencil marks. Small notes appear in a consistent 3x3 arrangement inside each cell.
• Undo, or Z, restores the previous board and notes. Redo, or Y, restores an undone move. Ctrl+Z, Ctrl+Y and Shift+Ctrl+Z also work.
• Press H or choose Hint when you need another way to look at the puzzle. Helpers opens the assistance settings and, in free play, your local statistics.

Keeping useful pencil notes

A candidate is a digit that does not yet break a row, column or box rule. Notes are a record of those possibilities, not promises that every written number will be used. Fill notes writes all currently legal candidates into empty cells. You can then remove candidates as you make deductions. Automatic note removal clears a placed digit from its peers; switch it off in Helpers if you prefer to manage every mark yourself. Filling notes again recalculates them from the placed digits, so it can restore candidates that you previously removed through a more advanced deduction.

Easy: naked and hidden singles

A naked single is a cell with just one candidate left. A hidden single is a digit with only one possible cell in a row, column or box, even if that cell has other notes. These are the two most useful habits for a beginner: inspect the possibilities of a cell, then inspect the possible homes of a digit. Easy puzzles can be completed using these steps alone.

Medium: pairs and locked candidates

A naked pair is two cells in one unit containing the same two candidates. Those digits must occupy those cells in some order, so remove them from other cells in that unit. A hidden pair reverses the view: two digits have only the same two homes, allowing other notes in those homes to be removed. Pointing pairs and box-line reduction use an overlap. When a box confines a digit to one row, that digit is removed from the rest of the row. When a row confines a digit to one box, remove it from the rest of that box. Columns work in the same way.

Hard: triples and wings

Triples extend the pair idea to three digits and three cells. An X-Wing looks across two rows or two columns: if a digit is confined to the same two crossing lines, its places elsewhere in those crossing lines can be removed. An XY-Wing uses three two-candidate cells. Its pivot sees both wings; whichever value the pivot takes, one wing must use their shared outside digit. Any other cell that sees both wings cannot contain that digit. Look for these shapes only after checking the simpler steps again.

Expert: larger patterns and chains

Swordfish extends the X-Wing idea to three base lines and three crossing lines. An XYZ-Wing uses a pivot with three candidates and two wings with two each. Simple colouring and X-chains follow relationships between the possible homes of a single digit. A W-Wing links matching two-candidate cells through a strong relationship elsewhere. These techniques remove candidates through certainty rather than speculation. Our grader records the hardest technique it actually needs to complete a puzzle; a low clue count on its own does not decide the level.

Daily schedule and scoring

A new daily arrives at midnight UK time, following GMT in winter and BST in summer. Monday and Tuesday are Easy, Wednesday and Thursday Medium, Friday and Saturday Hard, and Sunday Expert. Press Start when you are ready: the server clock keeps running if you reload or leave the tab. Your board and notes resume on the same device. The daily begins at 1,000 points. Each second costs 1 point on Easy, 0.6 on Medium, 0.4 on Hard and 0.3 on Expert. Each hint costs another 50 points. Any completed grid scores at least 100; giving up scores 0. Digit placements are recorded as moves for comparison and do not deduct points.

Hints that explain the next step

There are up to 12 hints per daily. A hint first checks for a wrong entry and points to the first one it finds. Otherwise it names the easiest available technique and highlights the relevant cells, units and candidate removals. A candidate-removal hint may include the resulting placement, with the intervening reasoning, so you can follow the deduction through. Choose Fill it in to apply the help, or close the hint and work it out yourself. Asking for the hint counts whether you apply it or not.

Free play and your statistics

Choose Free play at the top, or New random game on the start screen, for extra puzzles at any of the four levels. Each pack contains 500 puzzles and never uses a daily grid. Hints are unlimited and play is unranked. Your device keeps games played, wins, win rate, current and best winning streaks, best time, fewest moves and average finishing time for each level. Starting another puzzle after making moves abandons that attempt and ends its winning streak. Clearing browser storage clears these local records. Free play never changes your daily result.

You can revisit recent daily puzzles from the calendar for practice. Once a daily is closed, its solution belongs in the Sudoku answers archive. Today's answer stays private until its play window has ended. For a different daily challenge, explore all cyber-jack dailies.

Frequently asked questions

Does Sudoku involve maths?

No. The digits are distinct symbols. You check where each symbol can appear, with no addition, multiplication or calculation.

Can every puzzle be solved without guessing?

Yes. Every published puzzle has one exact solution and is also completed by our separate technique solver without guesses. Puzzles that the technique solver cannot finish are discarded.

How do I clear pencil notes?

Select the cell and use Erase, 0, Backspace or Delete. Undo restores notes as well as placed digits. Fill notes recalculates legal candidates across the whole board.

What makes a Sudoku Expert?

The hardest deduction our human technique solver needs determines its tier. Expert uses techniques such as Swordfish, XYZ-Wing, colouring, X-chains or W-Wing, rather than a fixed number of empty cells.

Are hints free?

Free-play hints are unlimited and have no score cost. A daily allows 12 hints, each costing 50 points. Undo, notes and the helper switches are always free.

Can I share my result without spoiling the puzzle?

Yes. The result share text contains your time, tier, hints and points, with a link to Sudoku. It contains no digits or cell positions.

Do I need an account?

No. Play in a browser on your phone or computer. The daily and the four free-play packs are free, and your local progress stays on your device.