How to Play Sudoku
What is Sudoku?
Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. Some cells start filled in. Your job is to fill the rest so that every row, every column and every box contains the digits 1 to 9 exactly once. No arithmetic is involved, only deduction.
Rules
- Each digit 1 to 9 appears exactly once in every row.
- Each digit appears exactly once in every column.
- Each digit appears exactly once in every 3×3 box.
- Given digits are fixed and cannot be changed.
- Every puzzle here has one solution and can be solved without guessing.
Controls
- Click or tap a cell to select it. Selecting a cell highlights its row, column and box, and every cell holding the same digit. You can select a given to highlight all copies of that digit. Tapping the selected cell again deselects it.
- Number keys 1 to 9 or the on-screen number pad enter a digit.
- Pencil marks: toggle pencil mode with the pencil button or the N key, then enter digits as notes. Shift + digit toggles a note in either mode. Placing a digit clears that note from the cells it rules out.
- Arrow keys move between cells.
- Delete, Backspace or 0 clears the selected cell.
- Ctrl/Cmd + Z or the Undo button takes back the last entry, including notes and hints.
- Hint fills the selected empty cell (or the first empty cell) with its correct digit.
Difficulty Levels
Difficulty is graded by the techniques a puzzle needs, not just how many givens it has.
- Easy — Solvable with singles only: a cell with one possible digit, or a digit with one possible cell in a row, column or box. 36 to 45 givens.
- Medium — Needs pairs and box-line reasoning: naked and hidden pairs, pointing pairs and box-line reduction. 30 to 35 givens.
- Hard — Needs triples or an X-wing somewhere along the way. 25 to 29 givens.
Techniques
- Naked single: a cell where only one digit remains possible.
- Hidden single: a digit that fits in only one cell of a row, column or box.
- Naked pair: two cells in a unit that share the same two candidates; those digits can be removed from the rest of the unit.
- Pointing pair: a digit confined to one row or column within a box can be removed from that line outside the box.
- Box-line reduction: a digit confined to one box within a row or column can be removed from the rest of that box.
- X-wing: a digit that appears in exactly two cells of two rows, in the same two columns, can be removed from those columns elsewhere.
Tips
- Scan for digits that already appear eight times; the ninth is forced.
- Work box by box: look for a digit missing from a box and see which cells are not ruled out by its row and column.
- Use pencil marks on Medium and Hard; pairs and X-wings are hard to see without them.
- Conflicts are highlighted immediately, so you never build on a mistake for long.