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.
What makes a sudoku hard?
Hard puzzles get stuck even after all the singles are found. To make progress, you need to spot patterns among the pencil marks that remove candidates, which then reveal new singles.
The techniques you need
- Naked pairs and triples. If two cells in a unit both have only the candidates 4 and 7, those digits must go in those two cells, so remove 4 and 7 from the rest of the unit. The same works with three cells and three digits.
- Hidden pairs. If two digits can only go in the same two cells of a unit, those cells hold those digits, and all their other candidates can be removed.
- X-Wing. If a digit can only go in two cells in each of two rows, and those cells line up in the same two columns, the digit is locked into those four corners. Remove it from the rest of both columns.
- Pointing pairs and box-line reduction, from medium puzzles, still do a lot of work.
A good routine
Write full pencil marks for every cell before looking for patterns; partial notes hide pairs and X-Wings. Scan unit by unit for pairs first, since they are the most common, then look for X-Wings on single digits. Each elimination usually unlocks a single, so go back to scanning after every step.
How to play here
Tap a cell and type a digit, or use the number pad on a phone. Right-click a cell on a computer for pencil marks, or use Mark mode on a phone. Ready for the toughest grids? Try expert sudoku.
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.
