Free Sudoku solver step 1: Find the tightest spot
The solver looks for the empty cell with the fewest possible digits, or a digit that has the fewest places left in a row, column, or 3×3 box.
Free Online Sudoku Solver
Solve any Sudoku puzzle instantly — for free.
Enter your 9×9 Sudoku grid below and our free Sudoku solver fills in the solution in milliseconds, and warns you if the puzzle has more than one. Works on mobile and desktop with no account needed.
Number Puzzles Free Browser-based
Free Sudoku Solver — Enter your puzzle
Click any empty cell and type a number (1–9), or paste a whole puzzle. Use 0 or leave blank for empty cells. Solved numbers appear in blue italics. Stuck? Next hint explains one move at a time.
Solving steps
R3C5 means row 3, column 5. Boxes are numbered 1–9 from the top-left, row by row. Small grey digits are candidates.
Keyboard
- Move←→↑↓
- Fill1–9
- Clear0⌫
- SolveEnter
From blank grid to solved Sudoku
The solver is built around the same flow you use by hand: enter the clues, catch contradictions, then fill the remaining cells with valid numbers.
Enter only the clues you know
Tap any cell in the 9×9 grid and add the fixed numbers from your puzzle. Empty cells can stay blank, so you do not need to reformat the puzzle before solving.
Check every row, column, and box
Before solving, the tool checks your clues against Sudoku rules and marks any duplicate numbers in red, so you can fix the mistake instead of chasing an impossible puzzle.
Get the completed grid instantly
The solver fills the remaining cells in your browser, usually in a few milliseconds, and highlights the solved values separately from your original clues.
How the free Sudoku solver works
Our free online Sudoku solver uses backtracking search, guided so it always works on the most constrained part of the grid first.
Free Sudoku solver step 2: Try only legal options
It places one of the remaining options and updates the row, column, and box constraints. If any cell or digit is left with no option at all, that branch is dropped immediately.
Free Sudoku solver step 3: Backtrack and double-check
When a branch fails, the solver steps back and tries the next option. After finding a solution it keeps searching for a second one, so it can warn you if the puzzle is not unique.
Sudoku solver tips for faster results
These tips help you enter puzzles accurately and get the most from our free Sudoku solver.
Paste a whole puzzle at once
Copied a Sudoku from a website or text file? Click any cell and paste. 81 or more characters replace the whole grid; a shorter string fills from the selected cell onward. Use 0 or a dot for empty cells.
- Spaces, line breaks, and grid lines like | - + are ignored
- Both 530070000… and 53..7.... work
Check for duplicate numbers before solving
Our Sudoku solver catches duplicate numbers right away, but double-check your input if you get 'No solution' or a multiple-solution warning. A single wrong or missing clue is the usual cause.
- Scan each row for repeated digits
- Check each 3×3 box visually
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 three samples go from easy to hard. The hard one has only 17 clues and an empty top row, built to slow down simple backtracking solvers.
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 search. At every step it picks the empty cell or the row, column, or box digit with the fewest options left, tries those options, and backtracks as soon as anything has no option. It then keeps looking for a second solution so it can tell you whether the answer is unique.
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.
Why does the free Sudoku solver say the puzzle has more than one solution?
A proper Sudoku has exactly one answer, and it needs at least 17 clues to have one. If the solver finds two different valid grids, it shows one and warns you. That almost always means a clue is missing or was typed into the wrong cell, so compare your grid with the original.
Can the free Sudoku solver handle expert-level puzzles?
Yes. Easy, medium, hard, or expert makes little difference to the solver. In our tests, puzzles designed to defeat simple backtracking, such as 17-clue grids, are solved in a few milliseconds, and the slowest cases we found still finish well under a tenth of a second.
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?
Yes. Next hint explains one move at a time, and Show all steps lists the whole solution path. It uses the techniques people solve with: hidden and naked singles, locked candidates, and naked or hidden pairs and triples, with the cells and candidates involved highlighted on the grid. If a puzzle needs a harder technique such as X-Wing or chains, the hint says so and reveals one cell so you can keep going.
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?
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All tools are free, browser-based, and require no account.
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