Free Sudoku solver step 1: Find an empty cell
The solver scans your 9×9 grid left-to-right, top-to-bottom to locate the first empty cell that needs a number.
Free Online Sudoku Solver
Solve any Sudoku puzzle instantly — for free.
Enter your 9×9 Sudoku grid below and our free Sudoku solver will find the unique solution using a backtracking algorithm. Works on mobile and desktop with no account needed.
Free Sudoku Solver — Enter your puzzle
Click any empty cell and type a number (1–9). Use 0 or leave blank for empty cells. Pre-filled numbers appear in blue.
How the free Sudoku solver works
Our free online Sudoku solver uses a proven backtracking algorithm — the same technique used in most professional Sudoku solvers.
Free Sudoku solver step 2: Try digits 1–9
For each empty cell, the solver tries placing digits 1 through 9, checking that each candidate doesn't violate the row, column, or 3×3 box rules.
Free Sudoku solver step 3: Backtrack if stuck
If no valid digit works for a cell, the solver backtracks to the previous decision and tries the next digit — repeating until the entire grid is filled correctly.
Sudoku solver tips for faster results
These tips help you enter puzzles accurately and get the most from our free Sudoku solver.
01
Use 0 for empty cells when pasting
If you copy a Sudoku from another source, replace blank spaces or dots with 0. Our free Sudoku solver treats 0 as an empty cell that needs to be filled.
- Dots and spaces are also accepted
- Leading zeros in rows are fine
02
Check for duplicate numbers before solving
Our Sudoku solver will catch most errors, but double-check your input if you get 'No solution'. A single wrong number in the given clues means no valid solution exists.
- Scan each row for repeated digits
- Check each 3×3 box visually
03
Use the sample puzzle to test the Sudoku solver
New to SolverBox? Click 'Load a sample puzzle' to fill in a pre-set Sudoku and watch the free solver find the solution in milliseconds.
The sample puzzles range from easy to expert difficulty — great for exploring how the backtracking algorithm behaves on hard grids.
Free Sudoku solver FAQ
Common questions about our free online Sudoku solver.
How does the free Sudoku solver find the answer?
Our free Sudoku solver uses a backtracking algorithm. It finds each empty cell, tries placing digits 1–9, validates against all constraints (row, column, and 3×3 box), and backtracks when a conflict is found. This guaranteed approach solves any valid Sudoku puzzle in milliseconds.
What if the free Sudoku solver says 'No solution'?
This means the puzzle as entered has no valid solution. Either a given clue is wrong, or you mistyped a number. Check that no row, column, or 3×3 box has duplicate numbers among the pre-filled cells.
Can the free Sudoku solver handle expert-level puzzles?
Yes. The backtracking algorithm our free Sudoku solver uses works on puzzles of any difficulty level — easy, medium, hard, or expert. It typically solves even the hardest Sudoku puzzles in under 50 milliseconds.
Does the free Sudoku solver work without internet?
Once the page loads, the entire Sudoku solver runs locally in your browser using JavaScript. You can solve puzzles even if your internet connection drops after loading the page.
Does the free Sudoku solver show the solution steps?
The solver highlights which cells it filled in (shown in green) vs. the original clues (shown in blue). A step-by-step mode showing each backtracking decision is planned for a future update.
Can I use the free Sudoku solver on my phone?
Yes. The Sudoku grid, number input, and result display are all fully responsive. The grid adapts to smaller screens so you can tap each cell and solve puzzles comfortably on any device.
Need more free puzzle solvers?
SolverBox has free online solvers for word games, anagrams, crosswords, and more.
All tools are free, browser-based, and require no account.
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