Sudoku

Hard Sudoku — Play Free Online

Free hard Sudoku puzzles for players who already know the basics. Around 28 clues, and grids that reach positions where no individual square can be resolved on its own — the next move has to come from what two squares own between them.

Clues to start
about 28
Par time
17 min
Cost
free, no sign-up

Why these are hard

Not because there is less information. A hard puzzle has exactly as much information as an easy one — a single solution, reachable without a guess. What it lacks is easy access to it. The deductions are still simple; they are just two steps deep instead of one.

Practically, that means pencil notes stop being optional. A hard grid is solved on the candidate marks: two squares in a row showing the same two digits let you erase those digits everywhere else in the row, and a digit confined to one row within a box lets you erase it from the rest of that row. Neither pattern is visible without notes.

What you get on the board

  • Pencil notes — press N or tap the pencil, and the pad writes candidates into the corner of a square instead of committing an answer.
  • Undo steps back through everything, including the note that turned out to be wrong.
  • Three hints per puzzle, each costing points, for when you would rather keep moving than grind.
  • Autosave — a hard grid is not always a one-sitting puzzle, so it is kept on your own device and is still there when you come back.

Mistakes matter more here

Three wrong digits end the game, and at this level a wrong digit is expensive in a second way: every note you write afterwards is built on it. If you find yourself about to try a digit and see what happens, that is the moment to re-check one unit's notes instead. The hard guide lists the techniques in the order worth trying them, and the full techniques reference explains each one properly.

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