Sudoku9×9 grid

Goal of the game

Fill the 9 × 9 grid with the digits 1 to 9 so that each digit appears exactly once in every row, once in every column and once in every 3 × 3 box.

A well-made grid has only one solution, and that solution can always be reached by reasoning. If you feel forced to guess, a deduction has almost certainly slipped past you.

The grid in three words

• Row: the 9 horizontal cells. Each digit from 1 to 9 appears in it once.
• Column: the 9 vertical cells. Same rule.
• Box (or block, or region): one of the nine 3 × 3 squares outlined by the thick lines. Same rule.
• Every cell therefore belongs to three groups at once. Every deduction comes from that overlap.

The six techniques that unlock a grid

• The naked single — a cell where only one digit is still possible, because the other eight already appear in its row, column or box. It is the basic deduction, and it is enough to finish an easy grid.
• The hidden single — the reverse: in a box, a digit can only go in one cell, even if that cell would accept other digits too. Many players forget it, and it is the one that unlocks most medium grids.
• Cross-hatching — take one digit, say 7. Scan the three boxes of a horizontal band: if two of them already have their 7, its row is often forced in the third.
• The naked pair — two cells of the same group only accept the same two digits, for example 3 and 8. Whatever their order, those digits belong to them, and you can remove them from every other cell of that group.
• The hidden pair — two digits that, within a group, can only fit in the same two cells. Those cells then belong to them alone: every other candidate there can go.
• Pointing — in a box, every possible spot for a digit lines up on a single row. That digit will occupy that row within that box, so it disappears from the rest of the row elsewhere.

Pencil marks, and how to use them

• Notes mode (the pencil) writes small candidate digits in a cell without confirming it.
• Do not mark everything straight away: a fully annotated grid becomes unreadable and makes you miss the simple deductions. Annotate a box or a row when it resists.
• On hard levels the opposite is true: methodical annotation is the only way to see the pairs appear.
• A wrong note spreads. When a grid goes off the rails, it is almost always a note left behind after placing a digit.

The four levels

• Easy — naked singles are enough. No notes needed.
• Medium — you alternate naked and hidden singles, and scan digit by digit.
• Hard — naked and hidden pairs become essential, and so do notes.
• Expert — long chains, pointing, and often a single way in for the whole grid. Count the time in tens of minutes.

The mistakes that cost the most

• Guessing. A well-made grid never requires a bet. A digit placed “on a hunch” is paid for ten minutes later, when the contradiction shows up too far away to trace back.
• Only looking at rows and columns. Boxes are the most fertile group, and the most neglected.
• Searching cell by cell. Much slower than searching digit by digit: take a 4 and look for all its places in the grid.
• Forgetting to update your notes after confirming a digit.

Controls

Keyboard and mouse

• Arrow keys move the selection, digits 1 to 9 place a value.
• Backspace or Delete clears the selected cell.
• The N key (or the pencil button) switches between normal entry and notes.

Touch screen

• Tap a cell to select it, then a digit on the keypad at the bottom.
• Long-press a digit to write it as a note.
• The keypad greys out digits already placed nine times: a handy cue near the end of a grid.

Frequently asked questions

Do you need to be good at maths to play Sudoku?

No. There is no addition involved. The digits could be nine colours or nine symbols: all that matters is that each appears once per row, column and box.

Does a Sudoku grid always have a single solution?

Yes, that is the very definition of a valid grid. A grid with two solutions is considered badly built. Every grid offered here is checked before it is served.

Where should I start a grid?

With the digit that appears most often in the starting grid. The more it is already present, the more it constrains the rest, and the faster you will place its last copies.

What should I do when I am stuck?

Change your angle rather than pushing on: switch from reasoning cell by cell to reasoning digit by digit, or annotate the one box that resists. Being stuck almost always comes from a hidden single you did not look for.

Does the game work without an internet connection?

Yes. Level Farm Sudoku plays entirely offline, with no account and no ads. The optional Pass only syncs your progress across devices and puts you on the leaderboard.

More classics, free as well

• Sumito — fourteen marbles, push your opponent off the board
• Yams — five dice, thirteen combinations, three rolls
• Air hockey — same screen, two paddles, a real physics puck