What a Sudoku solver does
A Sudoku solver takes a standard 9x9 Sudoku grid and calculates a complete answer that obeys the three core rules: every row, every column, and every 3x3 box must contain the digits 1 to 9 exactly once.
That makes a solver useful for more than one kind of player. It helps beginners who need a way out of a dead end, experienced solvers who want to verify a hard puzzle, and puzzle creators who need to test whether a clue set is valid and well formed.
- Check whether a puzzle is solvable.
- Reveal a full completed grid.
- Step through one correct cell at a time.
- Export and share puzzles in a standard string format.
How to use this Sudoku solver
You can enter the puzzle manually by clicking a cell and pressing a number, or you can paste a full 81-character string into the import box. The import format is popular across Sudoku communities because it is compact, easy to share, and easy to store.
Once the clues are in place, choose Solve to complete the whole board or Step to reveal the next correct move. Step mode is especially useful when you want help without spoiling the entire puzzle.
- Enter the givens in the grid or paste a puzzle string.
- Use Solve for the complete answer.
- Use Step for one correct move at a time.
- Use Export to copy the current grid as a string.
- Use Clear to start over.
Puzzle strings, validity, and uniqueness
A Sudoku string contains 81 characters, read left to right and top to bottom. Filled cells use digits 1 to 9. Empty cells use 0 or a dot. This format is ideal for online sharing because it is small and unambiguous.
Validity comes first. If a row, column, or 3x3 box already contains a duplicate digit, the grid is not a valid Sudoku state and no trustworthy solve should continue from there. After the grid passes validation, the solver can search for a solution and, when practical, test whether more than one answer exists.
- Valid Sudoku input cannot contain duplicate digits in a row.
- Valid Sudoku input cannot contain duplicate digits in a column.
- Valid Sudoku input cannot contain duplicate digits in a 3x3 box.
- A strong published puzzle should normally lead to one unique solution.
Why step-by-step solving matters
Many solver pages only dump the finished answer, which is useful for checking but not great for learning. A step-by-step tool is better when you want to stay in the puzzle and understand where the next confirmed digit belongs.
That makes this kind of page valuable for searchers looking for more than just a quick solve. It supports learning intent, not only answer intent, which is why good Sudoku solver content should explain both the tool and the reasoning around it.
How the solver works behind the scenes
This tool uses a backtracking search with a minimum remaining values style choice: it looks for the emptiest cell with the fewest legal candidates, tries the best options first, and backs out when a contradiction appears. That keeps solving fast even on hard grids.
For uniqueness checking, the solver searches for more than one valid completion. If it finds a second answer, the clue set is not unique. If the grid is extremely open, the page tells you that the puzzle solved but the uniqueness check was too open-ended to confirm quickly.
How to solve Sudoku by hand
You can solve most Sudoku without any solver — the puzzle is built for pure logic. Begin by scanning one digit at a time: pick a number, look along every row, column and 3x3 box where it already appears, and find the empty cell in a nearby box that is the only one left for it. That cross-hatching alone places a surprising number of digits.
When scanning stalls, pencil in candidates — the digits still possible in each empty cell. A cell with only one candidate is a naked single; a digit that fits only one cell in a unit is a hidden single. From there, naked pairs and pointing pairs remove more candidates. The solver here is best used to confirm your answer or to reveal the next forced step when you are stuck.
- Cross-hatch one digit at a time to find the easy placements.
- Pencil candidates into empty cells when scanning stalls.
- A cell with one candidate is a naked single.
- A digit that fits only one cell in a unit is a hidden single.
- Use the solver to check your grid or reveal the next step.
How many clues does a Sudoku need?
A proper Sudoku has exactly one solution, which raises a famous question: how few given numbers can a puzzle have and still be unique? The answer is 17. In 2012 an exhaustive computer search proved that no Sudoku with only 16 clues can have a single solution, so 17 is the hard minimum.
Most printed puzzles give you more than that — often 22 to 30 — because extra clues make the logic gentler and the difficulty easier to tune. Paste any grid into the solver and it tells you whether your clues pin down one answer or leave the puzzle ambiguous, which is exactly the check you need when trimming clues to raise the difficulty.
Sudoku solver vs Sudoku helper
This site has two Sudoku tools, and they answer different needs. The solver takes a full grid and returns the complete answer, or tells you the puzzle is invalid or ambiguous. Reach for it when you want to check a finished grid, settle a disagreement, or confirm that a puzzle you are designing has exactly one solution.
The Sudoku helper is gentler. Instead of revealing everything, it shows candidate notes and offers a single hint at a time, so you keep solving yourself while being nudged past a sticking point. Use the solver when you want the answer, and the helper when you want to learn — the two work well side by side.
- Solver: enter a grid, get the full solution.
- Solver: checks validity and whether the answer is unique.
- Helper: shows candidate notes and one hint at a time.
- Helper: keeps you solving instead of revealing everything.
- Use the solver to check, the helper to learn.