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Hard Sudoku
Hard Sudoku puzzles strip the starting grid down as far as possible while keeping exactly one solution, usually leaving somewhere in the mid-20s of the 81 squares filled. With so few givens, simple scanning stalls quickly and you will need candidate notes and techniques such as naked pairs, hidden singles, and pointing pairs to make progress.
Hard Sudoku Rules
Fill the 9x9 grid so that every row, every column, and every 3x3 box contains each number from 1 to 9 exactly once.
The starting numbers are fixed. Enter numbers in empty squares, and use pencil mode to record candidates for each square.
Every puzzle has a single unique solution, so it can always be solved by logic alone, without guessing.
Hard Sudoku Strategy & Tips
Fill in candidates first
On hard grids it pays to pencil in every possible number for each empty square early. Patterns that are invisible in your head become obvious on paper.
Hunt for hidden singles
Look for a number that can only go in one square of a row, column, or box, even if that square still has other candidates. It must go there.
Use naked pairs
If two squares in the same group can only be the same two numbers, those numbers are taken. Remove them from every other square in that group.
Look for pointing pairs
When a number's only candidates in a box lie in one row or column, that number can't appear elsewhere in that row or column outside the box.
Frequently Asked Questions
How many clues does a hard Sudoku have?
Our hard puzzles remove as many numbers as possible while keeping one unique solution, usually leaving roughly 22 to 28 givens. The fewest clues any valid Sudoku can have is 17.
Do hard Sudoku puzzles require guessing?
No. Each puzzle has exactly one solution and can be solved with logical techniques. If you feel stuck, look for pairs and hidden singles in your candidate notes.
What's the difference between hard and expert Sudoku?
Labels vary between sites. Here, hard means fewer starting numbers and a need for candidate-based techniques, rather than exotic patterns like X-Wings in every puzzle.
Is there an easier version?
Yes. Try Easy Sudoku for beginner puzzles with about 45 starting numbers, or the regular Sudoku page for medium difficulty.