How to Play Sudoku
Sudoku is a single-player number puzzle played on a 9×9 grid, and unlike most games in this guide series, there’s no opponent to outwit — just the puzzle itself, and the discipline to fill it in through pure logic rather than guesswork. This guide covers the full rule set as it plays out in our version, including the two extra variants we offer beyond the classic form.
Overview & Goal
Your goal is to fill every cell of a 9×9 grid with a digit from 1 to 9, so that each row, each column, and each of the nine 3×3 boxes contains every digit exactly once. A puzzle starts with some cells already filled in — these are called “givens” and can never be changed — and the rest are empty for you to complete. Every puzzle we serve has exactly one valid solution and can always be reached through logical deduction alone, so if you ever find yourself guessing, there’s a cleaner deduction available that you haven’t spotted yet.
Components
- A 9×9 grid, divided by heavier lines into nine 3×3 boxes.
- Given cells, shown in a bold, locked style — these are your starting clues and can’t be edited.
- A number pad with digits 1 through 9 and an erase control, each digit showing how many of that number are still left to place.
- Pencil marks (notes), small candidate numbers you can jot into an empty cell as your own private working memory. Notes are never checked for correctness and never count toward finishing the puzzle — they’re purely a tool for your own reasoning.
- A timer tracking your solve time, and, in modes that use one, a mistake counter.
- In Killer puzzles: dashed outlines called cages, each with a small number in the corner showing what its cells must add up to.
- In X-Diagonal puzzles: the two main diagonals of the grid are lightly highlighted as an extra constraint region.
Setup
Pick a variant — Classic, Killer, or X-Diagonal — and a difficulty level from 1 to 10. The puzzle then loads with its givens already placed and every other cell empty. Killer puzzles are a special case: because the cage sums themselves carry so much information, a Killer grid can start with very few, or even zero, given digits — the cages do the work that givens do elsewhere. In X-Diagonal, the board looks and plays exactly like Classic except that the two diagonals running corner to corner are now constraint lines in their own right. The timer doesn’t start ticking until you interact with the board, so simply studying the puzzle before you begin costs you nothing.
A Turn, Step by Step
Sudoku doesn’t have turns in the traditional sense — you work at your own pace, in any order, taking as long as you like on each cell. A typical solving loop looks like this:
- Select a cell. Tapping any cell highlights its row, column, and box, plus every other cell holding the same digit — handy for tracking where a number can and can’t go.
- Place a digit, by selecting an empty cell and tapping a number on the pad. If your settings have conflict-highlighting on, a digit that clashes with another in its row, column, or box will be flagged.
- Or toggle a pencil mark instead, if you’re not ready to commit — switch to Notes mode and tap a candidate digit to add or remove it from that cell’s list of possibilities.
- Erase a cell’s value or notes if you change your mind.
- Use Undo or Redo to step backward or forward through your recent moves.
Worked example: suppose a particular row already has the digits 1, 2, 4, 5, 6, 8, and 9 placed, and your selected cell is one of the two remaining empty cells in that row. Only 3 and 7 are left for the row overall. If you check the cell’s column and see it already contains a 7, then this cell can’t be 7 — logically, it must be 3. That’s the simplest kind of deduction in Sudoku (a “naked single,” in solver terms): a cell where every possibility except one has been ruled out by its row, column, or box.
After every action, the app automatically checks whether the grid is complete and valid, updates the “remaining” badges on the number pad, and — if you have the setting on — clears that same digit from the pencil marks of related cells nearby, since a placed digit can no longer be a candidate for its neighbors.
Special Rules & Edge Cases
Killer cages add two extra rules on top of the normal row/column/box constraints: the digits inside a cage must add up to the cage’s printed sum, and no digit may repeat within a single cage, even if the cage crosses multiple rows or boxes.
X-Diagonal adds one extra rule: each of the two main diagonals (from corner to corner) must also contain every digit 1–9 exactly once, just like a row, column, or box. The very center cell of the grid belongs to both diagonals at once.
Mistake handling is a setting, not a fixed rule — you can choose how the game reacts to incorrect entries:
- Highlight conflicts flags a digit that clashes with another in its row, column, or box, without penalty.
- Strict checks your entry against the actual solution and counts a wrong digit as a mistake; reach the mistake limit (typically three) and that puzzle ends as a loss.
- Zen gives no feedback at all until you attempt to finish, at which point it tells you whether the grid is correct.
Undo does not erase a counted mistake in Strict mode — once a wrong digit has been counted, undoing it restores the board visually but the mistake still stands, so you can’t undo your way around the mistake limit.
Hints come in three forms: revealing a cell’s correct value outright, explaining the next logical deduction by name (for example, “Hidden single: only cell in this box that can be 7”), or simply checking the board and highlighting anything currently wrong. Using a reveal or accepting a technique hint still lets you finish the puzzle, but it disqualifies that particular solve from counting as a personal best.
How the Game Ends & Scoring
You win the moment all 81 cells are filled and every constraint for your variant is satisfied — every row, column, and box (plus cages for Killer, plus diagonals for X-Diagonal) contains each digit exactly once. Your solve time is recorded, and if it’s your fastest time for that variant and difficulty, it’s saved as a personal best (unless you used a reveal hint along the way).
You can fail a puzzle only in Strict mode, by reaching the mistake limit — the board locks and you can retry the same puzzle or start a new one.
Leaving mid-solve simply pauses your progress; single-player puzzles are auto-saved and can always be resumed later from where you left off, with no win or loss recorded.
There’s no numeric score to chase beyond your solve time — Sudoku’s only real measures are whether you solved it, how long it took, and how clean the solve was.
| Situation | Result |
|---|---|
| Grid complete and fully valid | Solved (win); time recorded |
| Mistake limit reached (Strict mode) | Puzzle failed; retry or start new |
| Quit before finishing | Saved and resumable, no result recorded |
| Used a reveal or technique hint | Still counts as completed, but not eligible for a personal best |
Variants
- Classic 9×9 — the standard game described above: rows, columns, and boxes only. This is the default, and the only variant available in two-player Race mode.
- Killer — adds cage sums and the no-repeat-within-a-cage rule, often with very few starting givens since the cages carry most of the information.
- X-Diagonal — adds the two main diagonals as extra constraint lines, on top of the standard rules.
We also offer a Daily Challenge, a single Classic puzzle shared by everyone each day at a medium difficulty, and a casual Race mode where two players solve an identical Classic puzzle on their own private grids — nobody can see the other’s board, only their progress, and the first correct, complete solve wins.
Quick-Reference Summary
- Fill every cell 1–9 so each row, column, and 3×3 box contains every digit exactly once.
- Killer adds cage sums with no repeats inside a cage; X-Diagonal adds the two main diagonals as constraint lines.
- Given cells are locked; everything else is yours to fill in, using placed digits or private pencil-mark notes.
- Every puzzle has exactly one solution and is solvable by logic alone — no guessing required.
- Mistake handling (Highlight / Strict / Zen) is a setting you choose, not a fixed rule.
- You win the instant the grid is completely and correctly filled.