Tool

Sudoku Solver

A Sudoku solver fills a valid 9x9 puzzle by checking rows, columns, and 3x3 boxes until every empty cell has a legal number.

Sudoku Duck solving a puzzle with a magnifying glass

At a glance

What the solver does

Solver messages and meaning
MessageMeaningWhat to do
Invalid rowA number repeats in one horizontal row.Compare that row with your original puzzle.
Invalid columnA number repeats in one vertical column.Check copied givens in that column.
Invalid boxA number repeats inside a 3x3 box.Review the nine cells inside the highlighted box.
No solutionThe givens do not lead to a complete legal grid.Look for a copied clue or an impossible custom puzzle.
Multiple possibilitiesThe current clues may not force one unique answer.Add missing givens if you are testing a puzzle design.

Backtracking in simple steps

  1. Find an empty cell.
  2. Try numbers that do not break the row, column, or box rules.
  3. Continue to the next empty cell.
  4. Backtrack if a later cell has no legal number.
  5. Return the solved grid when every cell is filled legally.

Validate and solve

Enter your puzzle

Type the starting numbers into the grid. Leave blank squares empty, then run the solver.

Enter givens or load the example puzzle.

How the Sudoku solver works

The solver starts with validation. It checks each row, column, and 3x3 box to make sure the same number does not appear twice. If there is a conflict, the puzzle cannot be solved as entered because it already breaks the rules of Sudoku. Fix the clue or clear the square before solving.

After validation, the solver uses a backtracking method. It looks for an empty square, finds the numbers allowed by the row, column, and box, places one candidate, and continues. If a later square has no allowed number, the solver backs up and tries a different candidate. This search continues until the grid is solved or every path has failed.

Using the solver for learning

A solver is most helpful when it explains where your puzzle went wrong. If you copied a grid from paper, validation can catch a duplicated number before you spend time on an impossible board. If you are stuck on a real puzzle, solve it here and compare the answer with your notes.

Try not to paste every puzzle into the solver at the first pause. Work by hand first. When you finally check the solution, look for the square that would have opened the board. Was there a hidden single in a box? Did a row remove the last possible number from a candidate list? That review turns a solved answer into practice.

What the solver does not do

The tool does not send your puzzle to a server. It runs in the browser and does not require an account. It also does not judge your solving style. Some people use solvers for learning, some for checking newspaper puzzles, and some for making sure a handmade puzzle is valid. All of those uses are fine when the goal is understanding the grid.

If you enter a puzzle from a book, newspaper, or worksheet, check the givens slowly before pressing Solve puzzle. A single copied digit in the wrong square can turn a normal puzzle into an impossible one. When validation highlights a conflict, compare the row, column, and box with your original source before changing other numbers.

For practice, solve as much as you can on paper or on the online board first. Then enter the stuck position in the solver. If the solver can finish it, compare its answer with your notes and look for the first square where your candidate list was too broad or too narrow.

Questions

FAQ

What is a Sudoku solver?

A Sudoku solver is a tool that fills a valid Sudoku puzzle by checking the row, column, and 3x3 box rules. You enter the given numbers, leave unknown cells blank, and the solver validates the grid before completing it. It is useful for checking mistakes, studying answers, or testing copied puzzles.

How do I enter a puzzle into the solver?

Enter the starting numbers from your puzzle into the matching squares and leave empty squares blank. Work carefully from left to right and top to bottom so copied clues stay in the correct positions. After entering the givens, use Validate or Solve puzzle to check the grid.

What makes a Sudoku puzzle valid?

A Sudoku puzzle is valid when the givens do not already break the row, column, or 3x3 box rules and the grid can lead to a legal solution. A valid partial puzzle may still have many blanks. A solved puzzle has all 81 squares filled correctly.

Why does the solver say my puzzle has an invalid row?

The solver says a row is invalid when the same number appears twice in one horizontal row. Compare that row with your original puzzle and check whether one clue was copied into the wrong cell. A repeated given must be corrected before the solver can finish the grid.

Why does the solver say my puzzle has an invalid column?

The solver says a column is invalid when the same number appears twice in one vertical column. Look down the highlighted or suspected column and compare each entered digit with the source puzzle. One wrong entry can make an otherwise normal Sudoku puzzle impossible.

Why does the solver say my puzzle has an invalid 3x3 box?

The solver says a 3x3 box is invalid when a number repeats inside one thick-lined box. Review the nine cells in that box and check the givens against your original puzzle. A duplicate inside a box breaks Sudoku rules even if the row and column look fine.

What does "no solution" mean?

No solution means the entered givens cannot lead to a complete legal Sudoku grid. This often happens because a clue was copied incorrectly, a number was placed in the wrong square, or a custom puzzle has a contradiction that is not obvious from duplicates alone.

Can a Sudoku puzzle have more than one solution?

Yes, a Sudoku puzzle can have more than one solution if the clues do not force a single completed grid. A well-formed classic Sudoku puzzle should have one solution. If multiple solutions are possible, the puzzle may be incomplete or not designed as a standard single-answer Sudoku.

Does the solver use guessing?

The solver uses an algorithmic search called backtracking, which can try allowed numbers and back up when a path fails. That is different from human guessing for practice. Human solvers usually learn more by using logic first, then using the solver to compare or confirm the finished grid.

Is using a Sudoku solver cheating?

Using a Sudoku solver is not cheating when your goal is learning, checking, or testing a puzzle. It can feel like a shortcut if you use it before trying the grid yourself. For practice, solve as far as you can, then use the solver to understand what you missed.

How can I use the solver to learn instead of just getting answers?

Use the solver to learn by entering a puzzle only after you are genuinely stuck, then comparing the completed grid with your notes. Look for the first square where your candidate list was too broad or too narrow. The current tool gives the completed grid, so the lesson comes from your review.

Should I enter zeros, blanks, or dots for empty cells?

You can leave empty cells blank in the solver grid. If you paste a puzzle string, zeros or dots can represent empty cells when supported by the paste format. The important part is that the actual given numbers stay in the correct positions.

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