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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.