Hard Sudoku Online
Hard sudoku is for you if you have mastered the basic techniques and want real resistance. The puzzles have few cells filled in from the start, and the easy moves run out quickly. To reach the end, you need to recognize larger patterns that stretch across several rows and columns. All puzzles are free, and you can play as many as you like.
Even the hardest puzzles at this level are guaranteed to be solvable with pure logic. Our solving engine has verified that every puzzle has exactly one solution that can be reached without guessing.
The techniques that open up hard puzzles
The best-known pattern at this level is the X-Wing: four cells that form a rectangle and lock a digit into two rows or columns, so the digit can be eliminated elsewhere. The Swordfish is the same idea extended to three lines. Box/line reduction and hidden pairs also show up often as key moves.
Complete notes are a prerequisite. Without candidate notes in every empty cell, the patterns are almost impossible to spot. Write the notes first, and then look for patterns digit by digit.
How to work systematically
Experienced players work in a fixed order: first all the singles, then pairs and locked candidates, and finally fish patterns such as X-Wing and Swordfish. Every time you place a digit, you go back to the start of the sequence, because new singles have often appeared.
If you are completely stuck, you can enter the puzzle into our sudoku solver. It finds the next logical step and explains the technique behind it, so you learn the pattern for next time instead of just getting the answer.
Techniques you need at this level
These techniques come up again and again on hard puzzles. Each technique has its own page with interactive examples that show you the pattern step by step.