Sudoku

The board, and how you win

The grid is 9×9, divided into nine 3×3 boxes. A cell holds a given number that cannot be changed, a number you entered, or nothing. Any cell can also hold pencil notes.

You win when all 81 cells are filled and every row, every column and every box contains 1–9 exactly once. That is the same thing as "no duplicates anywhere and no empty cells", which is how the game checks it.

Every puzzle is generated with exactly one solution. That is what makes "this number differs from the answer" a fact the game can state rather than a guess.

Controls

• Tap a cell to select it. Selecting a cell highlights every other cell holding the same number — including the given numbers.
• Tap 1–9 on the pad to place a number, or tap it again to clear the cell.
• Switch to notes mode and the same pad adds and removes pencil marks instead.
• Placing a number clears that cell’s notes, and removes that number from the notes of the same row, column and box. It saves the bookkeeping; it does not change the rules.
• The pad shows how many of each number are still missing, and a number is disabled once all nine are on the board.

Mistakes, and the help you get

There is no mistake limit and no game over. Three wrong numbers do not end anything.

• Show mistakes marks a number that differs from the puzzle’s solution. It is on by default and can be turned off in Settings.
• Duplicate highlighting is always on: two of the same number in a row, column or box are flagged. That is pointing out a broken rule, not checking your answer.
• Undo is unlimited and free. It steps back placements, clears and note changes. It does not rewind the mistake count — a mistake that happened, happened.
• Hint is unlimited and free. It points at one cell that logic can settle right now and highlights the row, column or box that proves it. It never fills the cell in for you.

In the app a hint is phrased in plain words ("in this row, 5 only fits here") rather than by technique name. The names live on this page instead, so the game stays readable in fourteen languages.

What Easy, Medium and Hard actually mean

Difficulty is defined by two things at once: which solving techniques are enough to finish, and how few given numbers you start with. Both are guaranteed when the puzzle is generated.

• Each tier includes the techniques of the ones above it, and starts you with fewer numbers. Harder means both "more tools allowed" and "less to go on".
• Every tier is solvable without guessing. Trial and error is never required at any difficulty.
• Techniques beyond X-Wing (XY-Wing, Swordfish, chains) are not used. A dig that would require them is rolled back during generation.

A tier promises what can be guaranteed — which techniques suffice, and how deep the puzzle is dug — not "the hardest technique you will be forced to use". We measured the latter: only about one maximally-dug board in twenty genuinely requires the pair/triple techniques, so calling a puzzle Hard on that basis would have been a claim we could not keep.

Levels and the Daily puzzle

• Levels run 1–999 and unlock in order. Any level you have unlocked can be replayed, and it is always the same board.
• The mix shifts as you climb: the first twenty levels are all Easy, Hard starts appearing around level 151, and from 701 it is mostly Hard — but never entirely, because 300 identical hard puzzles is a chore rather than a game.
• The Daily puzzle is one per day and is always Medium. Weekday-based difficulty would create lucky and unlucky days, so there is none.
• You can go back and play past days. The days you finish are recorded — but consecutive days are not counted, and no streak is shown anywhere.
• Every board is generated on your device from a seed. Nothing is downloaded, and the same level is the same puzzle for everyone.

Tips

• Start by hunting Hidden Singles: pick a number, and look for a row, column or box where it has only one cell left.
• From Medium onwards, write notes early. Most of the techniques below are about spotting a pattern in the notes.
• Locked Candidates: if inside one box a number can only go in a single row, then it cannot be anywhere else in that row outside the box — clear it from those notes.
• Naked Pair: two cells in the same unit holding exactly the same two candidates. Those two numbers belong to those two cells, so remove them from every other cell in that unit.
• X-Wing: a number that fits in only two columns of one row, and only the same two columns of another row. It must take one of each, so it can be removed from the rest of those two columns.
• When you are stuck, use a Hint. It costs nothing, it is not limited, and it shows you the reasoning rather than the answer.

Questions people ask