What a Hard puzzle asks for
Our grader solves each puzzle the way a careful person would, and it labels the puzzle by the hardest move it needed. A puzzle is Hard when singles alone stall and one of these is required to carry on: a pointing pair, a box-line reduction, a naked or hidden pair, or a naked triple. None of them involve guessing. Each one removes candidates, and removing candidates is what turns a stuck grid back into one where singles work again.
Hard is about technique, not about how bare the grid looks. Some Hard puzzles start with more numbers than an Expert one, because a single tricky spot is what decides the level.
Start with good notes
Before you reach for patterns, fill in the obvious numbers, then switch Notes on and pencil every possible digit into the empty cells of one box at a time. Leave Auto-clear notes switched on in Settings: each number you place removes itself from the notes in its row, column and box, so your candidates stay honest. Most mistakes on Hard come from a stale note, not from a hard step.
Four moves to know
- Pointing pair. Inside one box, a digit's possible cells all sit on the same row. That digit must land in this box on that row, so delete it from the rest of the row. Columns work the same way.
- Box-line reduction. The mirror of pointing: in one row, a digit can only go in cells that share a box. Remove it from the other cells of that box.
- Naked pair. Two cells of a row, column or box both hold exactly the same two candidates. Those two digits are taken, so clear them from every other cell of that unit.
- Hidden pair. Two digits can each appear in only the same two cells of a unit. Those cells belong to them, so wipe any other notes from those two cells.
After every removal, look for singles again. Often a single pattern is all it takes for the grid to open up again.
Hard sudoku on AllFun
Hint places one number and names the move behind it, so when it says "pointing pair" you know which pattern to look for next time. Three hints a puzzle are free. Keep mistakes on Classic for a real test, or Relaxed to practise without losing the grid. Your Hard record, best time and streak are counted apart from the other levels. Step back to Easy sudoku for a warm-up, or go on to Expert sudoku when Hard stops resisting. Every technique has a worked description in our sudoku guide, and more brain games are on the puzzle games page.
Questions
Do hard sudoku puzzles need guessing?
No. Each one was solved by our grader with logic alone, so there is always a next step to find.
Is Hard harder than Medium only because it has fewer numbers?
No. Medium needs only singles; Hard needs at least one of the techniques above, whatever the number of givens.
When is the daily sudoku Hard?
On Thursday and Saturday. Everyone gets the same grid that day.
Ready for a game? AllFun Sudoku is free and opens right here in your browser.
Play Hard SudokuOpens with Hard chosen
