Hard sudoku puzzles online
Hard sudoku is for players who have outgrown singles. Each board starts sparse, and you will need full pencil notes plus naked pairs, hidden pairs and X-Wings to move forward. Every puzzle still has exactly one solution, and none of them needs a guess.
Play unlimited hard sudoku for free, with notes and undo. When hard puzzles stop slowing you down, expert sudoku is the next step.
How to play Hard Sudoku
- Fill every empty cell with a digit from 1 to 9.
- Each row, each column and each 3×3 box must contain every digit exactly once.
- The starting digits are fixed. Only the empty cells are yours to fill.
- Hard boards have a single solution. They need advanced techniques, but never trial and error.
Hard Sudoku tips
Find naked pairs and triples
If two cells in a unit can only hold the same two digits, those digits belong to them. Remove both digits from every other cell in that row, column or box.
Look for X-Wings
When a digit has exactly two possible cells in each of two rows, and those cells share the same two columns, the digit can be removed from the rest of those columns.
Re-check your notes before a long stare
On hard boards one wrong or missing candidate hides every next step. When progress stops, rebuild the notes of one box from scratch.
Frequently asked questions
What makes a sudoku hard?
A hard sudoku has few starting digits and can't be finished with singles alone. You need techniques like naked pairs, hidden pairs and X-Wings.
Is hard sudoku solvable without guessing?
Yes. Every hard puzzle here has one solution that follows from logic. Guessing is never required.
How long does a hard sudoku take?
Anywhere from 20 minutes to over an hour. Time drops quickly once pair and X-Wing patterns become familiar.
