What changes at hard
Scanning cells one at a time will not finish a hard grid. Every hard puzzle here has points where no cell has an obvious single and the way on is a hidden single — a digit with only one possible home in a row, column or box. In about one grid in five you will also need locked candidates: a digit trapped on one line inside a box. Naked pairs and hidden pairs turn up only occasionally.
A method that works
- Scan first. Even a hard grid gives up six to twelve cells to plain scanning.
- Mark candidates for one box or line at a time, starting with the unit that has the fewest empty cells.
- Look for two cells in a unit holding the same two candidates. Those two digits belong to those two cells, and can be struck out everywhere else in the unit.
- Check whether a digit is confined to one line inside a box before you reach for anything more advanced — locked candidates are cheap and they often unlock the rest.
- Keep your marks honest. One stale candidate will send you down a false path and cost you more time than the puzzle.
No guessing, ever
A hard puzzle here is still a logical puzzle. The generator refuses any grid it cannot solve with named techniques, so when you are stuck there is a real move to find. Press hint and the board will name it and point at it.
If you are finishing these comfortably, expert gives you fewer clues and asks for locked candidates in most grids, and now and then for a wing.
What these puzzles actually need
We generated 300 puzzles at this level with the same code that builds the board above, solved each one along its logical path, and counted. A technique counts if the path uses it at least once.
| Measured across the sample | Hard |
|---|---|
| Starting numbers, fewest to most | 23–35 |
| Starting numbers, average | 27.6 |
| Moves to solve, average | 53 |
| Solved by naked singles alone | 0% |
| Need a hidden single | 100% |
| Need locked candidates | 18% |
| Need a naked or hidden pair or triple | 2% |
| Need an X-Wing, XY-Wing or Swordfish | 0% |
The sample uses fixed seeds and is measured again whenever the generator changes, so these figures describe the puzzles this page actually serves.
Common questions
How many clues does a hard Sudoku have?
Usually 24 to 30. The difficulty comes from the techniques required and the length of the solving path, not only from the count.
Which techniques do I need for hard puzzles?
Hidden singles in every grid, and locked candidates in roughly one in five. Pairs are rare at this level, and none of the 300 hard grids we measured needed a wing or a fish.
How long does a hard Sudoku take?
Fifteen to forty minutes is normal. Experienced solvers who mark candidates efficiently are often under ten.
Is there always a logical next move?
Yes. Every puzzle is verified before publication as solvable by technique alone, so you never have to guess and backtrack.