Hard Sudoku

These grids start sparse and will not fall to scanning alone. Expect to keep careful pencil marks and hunt for advanced patterns.

How to play Hard Sudoku

• Fill the 9×9 grid so that every row, every column and every bold-outlined 3×3 box contains each digit from 1 to 9 exactly once.
• Some digits are given at the start and cannot be changed. Every puzzle has exactly one solution, and you never need to guess.
• Tap a square, then tap a number (or type it). Tap the same number again, or use Erase, to clear it.
• Switch on Notes to pencil in small candidate digits. Placing a digit removes it from the notes in the same row, column and box automatically.

Controls

• Arrow keys: move   1–9: enter a digit   Shift + 1–9: toggle a note
• N: notes mode   Backspace / 0: erase   Z or Ctrl+Z: undo   H: hint

Tips

• Fill in all candidates early; advanced patterns only show up when your notes are complete.
• X-wing: if a digit fits in exactly two squares in each of two rows, and those squares share two columns, remove it from the rest of those columns.
• XY-wing: a two-candidate square that sees two other two-candidate squares can rule out their shared digit wherever both of them can see.

FAQ

What makes a Sudoku hard?

Not the number of clues alone. We grade each grid by the toughest technique it needs: a hard Sudoku cannot be finished with singles, pairs and locked candidates, and needs at least one triple, X-wing or XY-wing.

Will I ever need to guess?

No. Every Hard grid has a single solution that can be reached by logic. Our grader confirms it needs at least one hard technique.

Do hints count against me?

Hints are always available. A solve without hints is the only kind that sets a new best time.