Hard Sudoku

Hard sudoku is where full pencil marks stop being optional. The puzzles on this page are verified to require hidden pairs, triples or an X-wing at their hardest point, and those patterns live in the candidates, not the placed digits. Plan to work marked-up.

Here, “hard” means something you can check.

What hard means on ClearSudoku

Every puzzle we publish is graded by a solver that only uses human techniques, in order from cheapest to most advanced. The hardest technique it needs becomes the puzzle’s tier. Hard puzzles need at least one move from this page’s toolkit, and none of them need the XY-wings or swordfish that mark expert territory.

The other guarantee matters just as much: no guessing, ever. Each puzzle has exactly one solution and a complete logical path to it. Stuck means a pattern is sitting on the board unfound, which is a better place to be than wondering whether the puzzle is broken.

Before starting, tap Auto notes under the New Game button. Hard techniques read the candidate picture, and they need it complete.

Hidden pairs

A naked pair shows itself: two cells with identical two-digit notes. A hidden pair hides behind clutter. If two digits can only live in the same two cells of a unit, those cells belong to them, and every other candidate written there is noise you can erase.

5

1

4

3

159

2

6

8

45

149

7

45

9

To find them, count homes per digit inside a unit. Any digit with exactly two homes is half of a possible hidden pair; check whether another digit shares both cells.

Triples

Three cells in a unit whose candidates draw from the same three digits form a naked triple. The cells need not each hold all three. Below, the column’s three highlighted cells hold 2 and 5, 5 and 8, and 2 and 8. Between them they own 2, 5 and 8, so those digits die everywhere else in the column.

1

8

256

25

9

6

4

9

2

58

3

6

7

6

589

2

5

28

9

Hidden triples exist too, three digits confined to three cells, and the same counting method uncovers them. Triples are the tier’s grind: not hard to verify, easy to walk past.

The X-wing

The first pattern that spans the whole board. Find one candidate that appears in exactly two cells of one row, and in exactly two cells of another row, with both pairs sharing the same two columns. The four cells form a rectangle. However the digit resolves, it occupies one cell in each column of the rectangle, so that candidate disappears from both columns everywhere else.

4

7

2

36

9

1

5

38

68

4

36

46

6

5

1

69

3

4

29

7

26

8

6

26

9

Hunt for X-wings one digit at a time. Select any placed copy of a digit to light up all of its instances, then look for rows carrying exactly two.

Working a hard puzzle

Expect 20 to 40 minutes. The order that serves most solvers: exhaust singles, run locked candidates and pairs, then go pattern hunting in the candidates. When you crack the key move, the puzzle usually folds quickly, and that collapse is the tier’s signature feeling.

Keep mistake checking on solution mode while you learn; a wrong digit early poisons every deduction after it, and instant flagging keeps the damage contained. Undo rewinds anything, including a hint you regret. The full technique reference lives on the how to play page whenever you want the whole ladder in one place, and expert sudoku is there when rectangles start feeling small.

Frequently asked questions

What is the difference between hard and expert sudoku?

Our hard puzzles need hidden pairs, triples or an X-wing at their hardest point. Expert puzzles need techniques a step beyond that: XY-wings, swordfish, skyscrapers or coloring. The boundary is which technique the puzzle forces, not how few clues it starts with.

What is an X-wing in sudoku?

A pattern where one candidate appears in exactly two columns across two rows, forming a rectangle. Whichever way it resolves, that digit is used up in those two columns, so you can erase it from the rest of both columns. The guide below shows one on a board.

Do hard sudoku puzzles ever require guessing?

Not here. Every hard puzzle on this page is verified by our solver to fall to logic alone, and each has exactly one solution. If you are stuck, a technique you have not applied yet is waiting somewhere on the board.

How long should a hard sudoku take?

Around 20 to 40 minutes for most players. Full pencil marks speed this up rather than slow it down. The Auto notes button fills every legal candidate in one tap and saves you the bookkeeping.

Other difficulties

• Easy sudokuSolvable with scanning and singles. The right place to learn the habits.
• Medium sudokuIntroduces pencil marks, locked candidates and naked pairs.
• Expert sudokuXY-wings, swordfish and coloring. Logic all the way, no guessing.
• Master sudokuForcing chains. The hardest puzzles we generate.