How to Tell Whether a Sudoku Is Valid

Does any row, column, or box contain a duplicate clue?

This is the fastest way to spot an invalid grid. A valid starting puzzle must never have the same digit appear twice in the same unit. Examine each of the 9 rows, 9 columns, and 9 boxes individually. For example, if you see two '5's in the same row among the given numbers, the puzzle is immediately invalid. Our hint engine finds that many errors come from misreading a handwritten puzzle or a typo in an online generator. A quick scan can save you from attempting to solve an impossible grid.

• Check boxes carefully, as duplicates across box boundaries are easy to miss.
• Use pencil marks or a highlighter to track each digit 1-9 as you scan a unit.

Does the puzzle have at least one solution?

A puzzle with no duplicates can still be unsolvable if the clues logically contradict each other. This is less obvious and requires testing the logic. You can use basic techniques like finding Naked Singles. If you reach a point where a cell has no possible candidates that don't break a rule, the puzzle has zero solutions. For instance, if an empty cell sees digits 1 through 8 in its row, column, and box, the only candidate is 9. If placing that 9 later causes another cell to have zero candidates, the puzzle is invalid. You can learn more about this scenario in our article on Sudoku with no solution.

Does the puzzle have more than one solution?

A proper, well-constructed Sudoku must have exactly one solution. Multiple solutions mean the puzzle is poorly designed or 'broken'. This often happens when there are too few givens or the givens are placed in a way that doesn't force a single logical path. You cannot detect multiple solutions by hand without almost solving it twice; it requires algorithmic checking. If you find two different ways to complete the grid that both satisfy all Sudoku rules, you have a multi-solution puzzle. Understanding puzzle uniqueness is key for puzzle creators and advanced solvers.

How can I check for multiple solutions?

For a human, checking is impractical. You must rely on software or advanced solving techniques that prove uniqueness, such as examining all possible branches of logic. Some solvers use 'brute force' backtracking to count all possible completions. As a solver, if you are forced to make a pure guess between two equally likely digits with no logical deduction to choose one, it may be a sign the puzzle has multiple solutions. However, some very hard puzzles use advanced techniques like Unique Rectangles to avoid this guessing scenario and maintain a single solution. Explore the concept of how many solutions a Sudoku can have for a deeper dive.

Is the puzzle merely hard, or is it actually broken?

Distinguishing between difficulty and invalidity is crucial. A hard puzzle will always offer a logical next step, even if it requires advanced techniques like X-Wings or Swordfish. A broken puzzle will lead to a contradiction (no solution) or an ambiguous choice (multiple solutions). If you are stuck, try using a hint system that explains the logic. If the hint system cannot find a logical deduction and suggests guessing, the puzzle might be flawed. Most players get stuck on hard puzzles because they haven't yet learned the necessary technique, not because the puzzle is invalid.

• If you're stuck, research techniques for hard Sudoku before assuming the puzzle is broken.
• Use the 'check solution' feature on reputable Sudoku sites to verify your progress without spoilers.

What should I do with an invalid puzzle?

If you confirm a puzzle has duplicate clues or no solution, discard it. There is no point in continuing. If you find a puzzle has multiple solutions, it is considered defective for standard Sudoku play, though it can be an interesting curiosity. Report it to the source if it's from a website or app. Always source puzzles from reputable generators or publications that guarantee a single, logical solution to ensure a satisfying solving experience.