sudoku.games
Today's puzzle

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.

Scoring, and the real cost of a mistake

A hard finish is worth 500 points, with 2 points per second under the 17-minute par. A mistake costs 50 — about 25 seconds of speed bonus — and a hint 75.

But the points are the smaller half of what a mistake costs here. A wrong digit entered at this level invalidates every candidate you write afterwards, and the position can look perfectly consistent for another ten minutes before it contradicts itself. The expensive part is the time spent solving a grid that no longer has a solution.

Working the notes efficiently

Do not annotate the whole grid. A full set of candidates for 81 squares goes stale faster than you can maintain it — every digit placed invalidates several marks — and a stale note is worse than no note, because you will trust it.

Annotate the unit you are attacking. Pick the row, column or box with the fewest empty squares, write its candidates, and look for a pair. If nothing comes, move to the next densest unit rather than widening the one you are on. Most hard grids are a sequence of four or five such local attacks, each unlocking a couple of placements that make the next one easier.

Common questions

What is the difference between hard and medium Sudoku?

A medium puzzle always has a square you can resolve on its own. A hard one does not. Hard grids reach positions where the only progress available is eliminating candidates rather than placing digits — you remove possibilities from several squares, and it is the accumulation that eventually forces one. That is why pencil marks stop being optional here.

Do I have to use pencil notes for hard Sudoku?

In practice yes. The techniques this level needs — naked pairs, pointing pairs, box/line reduction — are all patterns in the candidates, and there is nothing to spot if the candidates are not written down. You do not need to annotate the whole grid at once, but you do need marks on whichever unit you are working.

How long does a hard Sudoku take?

Seventeen minutes is par for the speed bonus, and a careful solve often runs longer. A hard grid is not always a one-sitting puzzle, which is why the board saves itself to your own device and is still there when you come back.

Why do I keep making mistakes on hard puzzles?

Usually because a wrong digit went in early and every note written afterwards was built on it. Three mistakes end the game, so the useful habit is to stop before entering anything you are not certain of and re-check one unit's candidates instead. If you find yourself thinking you will try a digit and see what happens, that is the moment to go back to the notes.

What should I do when a hard Sudoku will not move?

Work through the techniques in order rather than staring: naked singles, hidden singles, then naked pairs in each unit, then pointing pairs from each box, then box/line reduction. The move is nearly always one of those and nearly always in a unit you have stopped looking at. Three hints are available if you would rather keep going than grind.

Other levels