Hard and expert: graded by technique, not just by clue count
Taking more numbers out of a grid does not always make it harder. Some 24-clue puzzles fall to singles alone. That is why every puzzle here is solved by a logic solver before you see it:
- Hard puzzles cannot be finished with singles. Each one needs at least one intermediate technique: pointing pairs, box/line reduction, naked pairs or triples, or hidden pairs or triples. They can be finished with those techniques, with no guessing.
- Expert puzzles cannot be finished even with those techniques. Expect X-wings, swordfish, chains or very careful case analysis. Pick Expert in the generator above when hard puzzles stop being a challenge.
Both levels have exactly one solution, checked by a solver that counts every possible answer.
Techniques you will need for hard sudoku
Pointing pairs and box/line reduction
If the only places for a 4 inside a box are in the same row, the 4 for that row must be in that box. You can remove 4 from the rest of the row. Box/line reduction is the reverse: if a row's only places for a 4 sit in one box, remove 4 from the rest of that box.
Naked pairs and triples
Two cells in a row that can each only be 3 or 8 must hold the 3 and the 8 between them. No other cell in that row can be 3 or 8. The same idea works with three cells and three numbers.
Hidden pairs and triples
If 2 and 6 can only go in the same two cells of a box, those two cells must be 2 and 6. You can rub out every other candidate from them.
How to approach a hard printable sudoku
- Fill in every single you can find first. Hard puzzles still have plenty of them.
- Pencil in candidates for every empty cell, small and neat. Hard puzzles are much easier to read with full candidates.
- Look for pairs and pointing patterns, remove the candidates they rule out, then go back to looking for singles.
- Print one or two per page so there is room for pencil marks.
Frequently asked questions
What is the difference between hard and expert sudoku?
Hard puzzles need at least one intermediate technique, such as pointing pairs or naked pairs, but can be finished with them. Expert puzzles cannot be finished with those techniques and need advanced patterns such as X-wings or chains.
How many clues does a hard sudoku have?
Usually 22 to 29. Difficulty is set by the techniques needed, not only by the number of clues.
Do hard puzzles need guessing?
No. Every hard puzzle is checked by a solver that uses only singles, locked candidates and pairs or triples, and it must be solvable that way. Expert puzzles may need more advanced logic.
Is there an answer key for the hard puzzles?
Yes. With “Answer key on page 2” ticked, page 2 shows every solution in the same order. Starting numbers are bold and solved numbers are grey.
What layout is best for hard sudoku?
One or two puzzles per page. The bigger grid leaves room for pencil marks, which you will need for hard and expert puzzles.