Hard Sudoku Online
Play a free advanced 9x9 Sudoku puzzle. Compare candidates, use notes, and solve logically without guessing.
Select an empty cell, then enter a number.
Naked Pairs Basics
If two cells in the same row, column, or box contain the same two candidates and no others, those digits must belong to those two cells. Remove them from the rest of that unit.
Box-Line Reduction
When every candidate for a digit inside a box lies on one row or column, remove that candidate from the rest of the matching line.
Check Fundamentals Before X-Wing
Before looking for complex patterns, clean up single candidates, hidden singles, and pairs. A hard puzzle often becomes simpler after candidate cleanup.
Follow Candidate Chains Carefully
Think through what happens when one candidate is removed and which cells become singles. Remove only candidates that are ruled out by clear logic.
Why Guessing Hurts
A single guess can create several wrong entries. When stuck, rebuild your notes and revisit the basic steps in the Sudoku strategy guide.
Hard Sudoku FAQ
Do I need to guess in hard Sudoku?
No. The puzzles on this site are designed for logical solving, so candidate removal and pattern checks are safer than guessing.
When should I look for naked pairs?
Look for them after single candidates become harder to find and several cells have exactly two candidates.
Where should I practice before hard puzzles?
Use medium Sudoku to practice hidden singles and locked candidates before returning to hard puzzles.