Free Sudoku Solver

Sudoku Solver: Solve Any Puzzle Instantly

Our free Sudoku solver lets you enter any standard 9×9 Sudoku puzzle and get the complete solution in seconds. Whether you're stuck on a newspaper puzzle, want to verify your own solution, or need to check if a puzzle has a unique answer — this tool does it all, right in your browser with no downloads or sign-ups needed.

📋 How to Use the Sudoku Solver

1. Enter your puzzle — Click a cell and type a number (1–9) to fill in the givens. Use the numpad buttons on mobile.
2. Or import a string — Paste an 81-character string into the import box (use 0 or . for empty cells) and click Import.
3. Click Solve — The solver fills in all remaining cells instantly and highlights them in green.
4. Use Step — Click Step to solve one cell at a time to see the solution unfold gradually.
5. Check uniqueness — After solving, the solver tells you whether the puzzle has a unique solution or multiple.
6. Export — Click Export to copy the current grid as an 81-character string you can share.

🧠 Understanding Sudoku Solving Algorithms

Behind every Sudoku solver is an algorithm — a set of rules the computer follows to find the answer. Here are the main approaches:

• Brute-force backtracking — The simplest approach. Try a number, move to the next cell, backtrack if a conflict arises. Works for any valid puzzle but can be slow on hard grids without optimisation.
• Constraint propagation — Each time you place a digit, eliminate it as a candidate from all peers (same row, column, box). This dramatically reduces the search space.
• MRV heuristic — Always pick the empty cell with the fewest remaining candidates to try first. This our solver uses — it finds contradictions faster and prunes dead-end branches early.
• Dancing Links (DLX) — Donald Knuth's Algorithm X with dancing links treats Sudoku as an exact cover problem. Extremely fast for standard 9×9 puzzles.

✅ What Makes a Valid Sudoku Puzzle?

A valid Sudoku puzzle must follow these rules:

• No digit appears more than once in any row, column, or 3×3 box.
• The puzzle has exactly one solution — this is what separates a proper puzzle from a random arrangement of clues.
• The minimum number of givens for a unique-solution puzzle is 17 — proven mathematically in 2012.

Our solver validates your input before attempting to solve, catching duplicates and impossible configurations immediately.

🎮 When Should You Use a Sudoku Solver?

• Verify your solution — Finished a hard puzzle but not sure it's correct? Enter it here to double-check.
• Get unstuck — Use the Step button to reveal just one cell and continue solving on your own.
• Check puzzle quality — Creating your own Sudoku? The uniqueness check tells you if your puzzle has exactly one solution.
• Learn solving techniques — Watch the solver work cell by cell and reverse-engineer the logic.

📊 Sudoku Solver Import Format

The import string is 81 characters representing the grid read left-to-right, top-to-bottom. Use digits 1–9 for filled cells and 0 or . for empty ones.

Example: 530070000600195000098000060800060003400803001700020006060000280000419005000080079

This format is widely used by Sudoku communities, apps, and puzzle databases, making it easy to share and solve puzzles from any source.

🎮 More Sudoku Puzzles to Explore

• Classic 9×9 Sudoku — Play the original puzzle with four difficulty levels.
• 4×4 Sudoku — Mini grids perfect for kids and beginners.
• 6×6 Sudoku — A medium-sized stepping stone.
• Killer Sudoku — Cage sums replace the standard givens.
• Jigsaw Sudoku — Irregular regions for a visual twist.