Sudoku

Sudoku — how to play and tips

Classic 9×9 sudoku. Fill the grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. There's a new daily puzzle in each of four difficulties, plus unlimited practice puzzles generated on demand.

How to play

Tap a cell to select it, then tap a number on the on-screen pad — or type 1–9 directly on your keyboard, with the arrow keys moving the selection around the grid. Selecting a cell highlights its row, column and box, and dims every other copy of the same digit already on the board, which makes it much faster to spot where a number can and can't go without manually re-scanning three lines every time. The number pad itself tracks how many of each digit remain: once all nine copies of a digit are placed correctly, its button greys out and stops showing a remaining count, which is a quick way to see which digits are effectively finished.

Use Notes (or press N) to switch into pencil-mark mode and add small candidate digits to a cell instead of a final answer. Notes are entirely your own bookkeeping and are never checked against the solution. What is checked automatically is cleanup: the moment you place a real number in a cell, that digit is removed from the pencil marks of every other cell in the same row, column and box, so your notes never go stale just because you filled something in elsewhere. Erase (or Backspace) clears a given cell's entry and notes together. Undo (or Ctrl+Z) steps back through your last 200 moves one at a time, restoring both the digit and the exact pencil marks that cell had before, and Hint reveals a correct digit — it fills your currently selected cell if it's empty or wrong, or otherwise finds and reveals the first incorrect empty cell on the board.

Every puzzle has exactly one solution

This matters more than it sounds. Puzzles are built by first generating a complete, fully valid 9×9 grid using randomised backtracking, and then removing clues from it one at a time in random order. After each removal, the puzzle is re-solved with a solution counter that aborts the instant it finds a second valid answer — if pulling a clue would let two different completions both satisfy the puzzle, that clue is put straight back and the generator tries a different cell instead.

Generators that skip this uniqueness check routinely produce grids with more than one legal completion, which means a player can fill every single cell following the rules perfectly and still be told they're wrong, because the game happened to be checking against the other valid answer. That failure mode is structurally impossible here: if your finished grid is legal — every row, column and box correct — it is, by construction, the one and only solution, and every puzzle can always be finished by logic alone without ever needing to guess.

Difficulty

Easy starts each puzzle with about 45 given clues, Medium with 36, Hard with 30 and Expert with just 26 — out of 81 cells total. Fewer starting clues means longer, more indirect chains of elimination have to be worked through before any single digit can be placed with certainty, which is what actually drives the difficulty rather than any change in puzzle size or rules. Best completion times are tracked and saved separately for each of the four difficulties rather than as one combined record, and only a clean solve — one where the Hint button was never used — counts toward that best time, so a fast but hint-assisted run won't overwrite a genuine personal best.

Daily mode

The daily puzzle is generated from a random-number seed built out of the calendar date in UTC, so every player anywhere in the world is solving the exact same grid on any given day, with no server round-trip needed to guarantee that. Each of the four difficulties gets its own distinct daily puzzle rather than four different views of a single shared grid, so choosing Hard for the daily is a genuinely different challenge from choosing Easy, not just an Easy grid with extra clues hidden. After you finish, a countdown shows exactly how long remains until the next daily puzzle unlocks, and a Share Result button lets you send your time, difficulty and mistake count to a friend, either through the device's native share sheet or by copying the result text to your clipboard.

Mistakes and finishing a run

You get three mistakes per puzzle before the run ends. A mistake is only counted the moment you place a full digit that contradicts the solution — pencil marks never trigger it, so you can jot down every candidate you're considering without any risk. Practice mode has no time or mistake pressure beyond that three-strike limit; there's no par time and no penalty for pausing to think, so it works equally well as a quick five-minute puzzle or a slow, careful solve.

Tips

Start by scanning for digits that already appear several times on the board — a digit that shows up in five or six rows already constrains the remaining boxes far more than one that has only just been placed. When you stall, switch your approach from "where does this digit go in this box" to "what digits can still go in this specific cell"; the two directions of scanning unlock different deductions, and alternating between them will carry you through almost any Hard grid without needing to guess. Lean on pencil marks once a puzzle gets busy — they exist specifically so you don't have to re-derive the same row/column/box logic every single time your eye lands back on a cell, and because placing a real digit automatically strips it from every relevant pencil mark nearby, keeping notes up to date costs you nothing extra. Finally, don't treat Hint as a last resort only for cells you're stuck on — using it once early to clear a genuinely ambiguous cell can be worth it, just remember it costs you eligibility for that difficulty's best time on this run.