An online sudoku solver that explains the next move
Paste a puzzle or type it into the grid and this sudoku solver fills it in a fraction of a second. It also tells you something most solvers skip: whether the puzzle has exactly one solution, several, or none at all — the three cases that separate a well-set puzzle from a misprint or a typo.
If you would rather keep solving yourself, use Hint — next cell. It places one digit and says why: “the only digit that fits this cell” (a naked single) or “the only cell in its row that can take a 7” (a hidden single). Only when no single exists does it fall back to the full solution and say so.
The solver works for the classic 9×9 grid and for 6×6 grids with 2×3 boxes, accepts puzzle strings from the Sudoku Generator, and runs entirely in your browser.
How to use the Sudoku Solver
- Type or paste the puzzle into the text box as 81 characters (one row after another; use 0, . or - for empty cells), or click into the grid and type the digits — the cursor moves on automatically and the arrow keys move around.
- Red cells mean two equal digits share a row, column or box; fix those before solving.
- Press Solve. Solved digits appear in blue, with the number of logical placements and guesses used and a verdict: unique, multiple or no solution.
- Prefer to work it out? Press Hint — next cell as often as you like; each press adds one digit with its reason.
- Tick 6 × 6 grid for a kids’ sudoku (a 36-character string switches automatically). Copy the solution text to save it.
Sudoku Solver features
Uniqueness check
The solver keeps searching after the first solution and reports “multiple solutions” when a second one exists — the sign of a puzzle with too few givens.
Hints with reasons
Naked singles and hidden singles are found by the same constraint propagation the solver uses, so every hint is a move you could have made.
Conflict detection
Duplicate digits in a row, column or box are highlighted immediately as you type, before you waste a solve on a typo.
Step counts
See how many cells were placed by pure logic and how many needed a guess — a rough measure of how hard the puzzle is.
Paste-friendly input
Any 81-character string works: newspaper apps, puzzle sites and the generator all use the digits-and-dots format.
9×9 and 6×6
Classic grids and 2×3-box kids’ grids share the same solver and the same hint logic.
How the solver works
Each empty cell starts with the candidates 1–9 minus whatever already appears in its row, column and 3×3 box. The solver first applies singles repeatedly: a cell with one candidate (naked single) or a digit that fits only one cell of a unit (hidden single). Most easy and medium puzzles finish here with zero guesses.
When singles run out, it picks the empty cell with the fewest candidates, tries each in turn and recurses — depth-first backtracking with the “minimum remaining values” heuristic. Because the search continues until a second solution is found or the tree is exhausted, it can prove uniqueness, which is why the verdicts are reliable even for very sparse grids.
Reading the result
| Verdict | Meaning | What to do |
|---|---|---|
| Unique solution | Exactly one completion exists — a proper sudoku. | Use the solution, or go back and work through the hints. |
| Multiple solutions | Two or more completions fit the givens. | A digit is probably missing from your entry; compare with the printed puzzle. |
| No solution | The givens contradict each other, even without an obvious duplicate. | Look for a mistyped digit — the row and column of the error is usually near a cell with no candidates. |
Tips for entering puzzles
- Read the puzzle row by row, left to right, top to bottom. 81 characters means a 9×9 grid; 36 means 6×6.
- In the grid, typing a digit jumps to the next cell, so you can key in a whole row without touching the mouse. Backspace clears a cell.
- The generator’s “Open in the solver” link fills the grid automatically through the address bar (
?p=…). - For word puzzles rather than numbers, try the Anagram Solver or the Wordle Helper.
Sudoku Solver FAQ
Can this sudoku solver show the steps?
It reports how many placements were logical singles and how many were guesses, and the hint button walks through the puzzle one justified move at a time. It does not name advanced patterns such as X-wings.
Why does it say my puzzle has multiple solutions?
Because more than one valid filled grid matches the digits you entered. Published puzzles have one solution, so a given is almost certainly missing or wrong in your entry.
What format does the text box accept?
Any string of digits with 0, ., - or * for blanks, with or without spaces and line breaks. Only the characters count: 81 for 9×9, 36 for 6×6.
Does it solve 6×6 sudoku?
Yes — tick “6 × 6 grid” or paste a 36-character string. Boxes are 2 rows by 3 columns, the standard kids’ layout.
How fast is it?
Typical newspaper puzzles solve in a few milliseconds; the hardest known 17-clue puzzles take well under a second in a modern browser.
Is my puzzle uploaded?
No. All solving happens in JavaScript in your browser and nothing is sent to a server.