- A valid Sudoku has exactly one solution — that is part of the definition
- A puzzle with multiple solutions is considered "broken" or "unsound"
- The most common multi-solution flaw: a "deadly pattern" where two digits can swap in four cells
- If you reach a point with two apparently valid options and both seem to work, the puzzle is likely flawed
- Puzzles generated by reputable apps and sites are always verified to have exactly one solution
The One-Solution Requirement
Every well-formed Sudoku puzzle is designed to have exactly one valid solution. This is not just a convention — it is what makes the puzzle logically solvable without guessing. If two different completed grids both satisfy all constraints, then no amount of logic can determine which one is "correct." The puzzle becomes a guessing game, not a logic puzzle.
What Causes Multiple Solutions?
A puzzle has multiple solutions when it does not have enough given digits, or when the given digits are arranged in a way that leaves some cells undetermined. The most common structural flaw is the deadly pattern: four cells arranged in a rectangle (two rows, two columns, two boxes) where two digits can be swapped without violating any constraint.
If cells R1C1, R1C5, R5C1, and R5C5 each contain either digit 3 or 7, and swapping all the 3s and 7s in those four cells produces a second valid grid, the puzzle has at least two solutions.
How to Tell if Your Puzzle Is Flawed
During solving, the first sign is reaching a bifurcation point where two options both produce a consistent grid with no contradictions, even after tracing the consequences many steps forward. On a valid puzzle, one of the two options will eventually lead to a contradiction — a cell with no candidates, or a unit where a digit has no valid cell.
If neither option produces a contradiction and both seem to reach a complete, valid grid, the puzzle likely has multiple solutions.
Does This Affect Your Solving Strategy?
For puzzles from reputable sources — published books, established apps, and sudoku.by — multiple-solution puzzles do not occur. Every puzzle on this site is generated and verified computationally. If you are solving a user-submitted puzzle or one from an unknown source, be aware that quality control may vary.
The Daily Challenge and all difficulty levels on this site are single-solution guaranteed. Learn more about how puzzles are made in our article on how Sudoku puzzles are generated and rated.