What is Sudoku?

Sudoku is a logic-based number puzzle that originated in Japan. The game is played on a 9×9 grid divided into nine 3×3 subgrids called "boxes" or "regions." The objective is to fill every cell with a digit from 1 to 9, ensuring that each number appears exactly once in every row, column, and 3×3 box.

The puzzle begins with some cells already filled in—these are called "givens" or "clues." A well-constructed Sudoku puzzle has only one valid solution, and it can be solved purely through logical deduction without guessing.

How does Sudoku solving work?

Sudoku puzzles are solved using various logical techniques, ranging from simple to advanced:

Basic techniques:

  • Naked Singles: When a cell has only one possible candidate number
  • Hidden Singles: When a number can only go in one cell within a row, column, or box

Intermediate techniques:

  • Naked Pairs/Triples: When two or three cells in a unit share the same candidates
  • Pointing Pairs: When candidates in a box are restricted to a single row or column
  • Box/Line Reduction: Eliminating candidates based on box-line interactions

Advanced techniques:

  • X-Wing: Pattern involving four cells in a rectangle formation
  • Swordfish: Extended version of X-Wing with three rows and columns
  • XY-Wing: Chain-based elimination technique

Computer algorithms typically use backtracking—a systematic trial-and-error approach that efficiently explores possible solutions and backtracks when contradictions are found.

Why use a Sudoku solver?

While solving Sudoku manually is an enjoyable mental exercise, a solver tool serves several practical purposes:

  • Learning aid: See the correct solution to understand where you went wrong
  • Verification: Confirm that a puzzle you created has a valid, unique solution
  • Time-saving: Quickly solve puzzles when you're stuck or short on time
  • Puzzle creation: Test generated puzzles for solvability

Tool description

This Sudoku Solver is an interactive web-based tool that allows you to input any standard 9×9 Sudoku puzzle and find its solution instantly. The tool features a visual grid interface where you can enter numbers directly, with real-time error highlighting to help identify invalid entries. Once you've entered your puzzle, the solver uses an efficient algorithm to find the solution and display it visually.

The interface separates the input puzzle from the solution output, making it easy to compare your progress with the complete answer. You can also print the solved puzzle for offline reference.

Features

  • Interactive 9×9 grid input with keyboard navigation between cells
  • Real-time error highlighting that identifies duplicate numbers in rows, columns, or boxes
  • Instant solving using an optimized backtracking algorithm
  • Visual solution display with clear distinction between given numbers and solved cells
  • Print functionality to export the solved puzzle for offline use
  • Cell statistics showing filled and empty cell counts
  • Clear function to reset the grid and start fresh

Use cases

  1. Stuck on a newspaper puzzle: Enter the remaining numbers from a partially completed Sudoku to see the solution and understand where you went wrong

  2. Verifying puzzle validity: Puzzle creators can use the solver to confirm their handcrafted Sudoku has exactly one solution

  3. Learning Sudoku strategies: Compare your solving approach with the correct solution to identify which logical deductions you missed

  4. Speed competitions: Quickly verify solutions during Sudoku competitions or timed challenges

  5. Teaching tool: Educators can demonstrate Sudoku logic by showing how puzzles progress from partial to complete solutions