- Difficulty
- Hard
- Time
- 00:00
- Checks
- 0
Select a square and place a number.
hard Sudoku
Let candidate relationships do the work.
Hard Sudoku uses the same three rules as every standard grid: each row, column, and 3 × 3 box must contain 1 through 9 exactly once. The difference is the length and depth of the logical path, not a change in the rules.
The puzzle shown here is designed for about 15 minutes. Its verified solution takes 52 supported steps and uses Hidden Single, Locked Candidates, and Naked Single. It has exactly one solution, so no branch or random guess is required.
Keep notes current and look for candidates confined to one row or column inside a box. Locked candidates remove possibilities without choosing a value prematurely.
Use Notes to keep candidates visible, Check when you want to audit the current grid, and Hint for the next logical idea rather than a filled answer. Progress is stored on this device, so a longer solve can be paused and resumed without an account.
Common questions
What makes this Sudoku hard?
The rating reflects the verified solving path: 52 steps using Hidden Single, Locked Candidates, and Naked Single. Difficulty is based on deductions, not on guesswork or simply counting the givens.
How should I approach a hard Sudoku?
Keep notes current and look for candidates confined to one row or column inside a box. Locked candidates remove possibilities without choosing a value prematurely.
What should I do when I get stuck?
Audit your notes, then search for a box-line interaction or pair. After any elimination, return to singles before looking for another advanced pattern.
Can I pause and continue later?
Yes. Your entries, notes, elapsed time, and completion state are saved locally in this browser and restored when you return.