Hard Sudoku
A 9 × 9 Sudoku with only about 25 numbers given. Spotting singles won’t be enough: you’ll need notes and more advanced techniques.
How to play
- Tap or click a square, then tap a number or type it on your keyboard. Arrow keys move around the grid.
- Fill the grid so every row, every column and every box contains each number from 1 to 9 exactly once.
- Turn on Notes to pencil in numbers you’re considering. If a number clashes with another in its row, column or box, it turns red.
- Stuck? Undo a move, or press Hint to fill in one correct square. Your progress is saved if you leave the page.
What it practises
- Advanced logical reasoning
- Naked and hidden pairs
- Keeping careful notes
Level: Teens and adults
Tips for hard Sudoku
Fill in notes for every empty square first. Hard puzzles are much easier to crack with a full set of pencil marks.
Look for pairs: if two squares in a row can only be 3 or 7, no other square in that row can be 3 or 7.
Questions and answers
What makes a Sudoku hard?
Hard puzzles start with fewer numbers (about 25 of 81 here), and at some point you can’t make progress just by finding the only place for a number. You need techniques like pairs.
Do hard Sudoku puzzles need guessing?
No. Every puzzle has exactly one solution, so it can always be solved by logic. The logic is just harder to spot.
What is a naked pair?
When two squares in the same row, column or box can each hold only the same two numbers, those numbers must go in those two squares. You can remove them from the notes of every other square in that group.