Sudoku solver
Stuck? Get the whole answer, or just the next step and why.
Click a square and type 1–9, or paste an 81-digit puzzle (0 or . for blanks).
Filled: 0/81Hints used: 0
Nothing leaves your device
This tool runs entirely in your browser. What you type, play or record here stays on your device: nothing is uploaded or saved on a server.
Example
The “world’s hardest sudoku” hint by hint, and what Check says about a wrong 3
The 2012 puzzle often called the hardest sudoku has only 21 givens. Pasted as 81 digits, rows left to right from the top with 0 for blanks, it is 800000000003600000070090200050007000000045700000100030001000068008500010090000400. It is also the third puzzle behind the Example button.
- The first Hint is Row 8, column 7: 9, with “No simple step left: this number comes from the full solution.”
- At the start no square has just one possible number and no number just one place, so the hint takes the empty square with the fewest options, here 3 or 9.
- After that the hints mix logic with numbers from the solution, each with its reason. Done this way the puzzle takes 60 hints, 10 of them from the full solution.
- Had you written 3 in that square yourself, Check would answer “1 wrong” and mark it red: with a 3 there the puzzle has no solution left.
| Hint | Square | Number | Reason shown |
|---|---|---|---|
| 1 | Row 8, column 7 | 9 | from the full solution |
| 2 | Row 7, column 7 | 3 | from the full solution |
| 3 | Row 7, column 1 | 5 | 5 can only go here in this row. |
| 4 | Row 7, column 2 | 2 | from the full solution |
| 5 | Row 7, column 5 | 7 | This cell can only be 7: every other number is already in its row, column or box. |
| 6 | Row 1, column 4 | 7 | 7 can only go here in this box. |
| 7 | Row 2, column 2 | 4 | from the full solution |
| 8 | Row 8, column 1 | 4 | 4 can only go here in this box. |
Solve fills all 60 empty squares at once; Hint shows the way in, and where only the solution helps, it says so. The top row of the answer is 8 1 2 7 5 3 6 4 9.
How it works
- Type the puzzle: click a square and type its number (or use the number pad), or paste the 81 digits with 0 or dots for blanks.
- Press Hint to get the next number with the reason — the cell and its row, column or box light up — or Solve for the whole answer.
- Filling it in yourself? Press Check to see which of your numbers are wrong.
Frequently asked questions
- How do the hints work?
- Each hint finds a cell that can be filled by logic: a cell with only one possible number, or a number that fits in only one place in a row, column or box — with the reason shown. When no such simple step is left, it gives a number from the full solution and says so.
- Can it solve the hardest sudokus?
- Yes — any valid 9×9 puzzle is solved in a fraction of a second, including the ones famous for being the hardest.
- What if the puzzle is wrong?
- The same number twice in a row, column or box is marked in red; a puzzle with no solution, or with more than one, is reported — a real sudoku has exactly one.
- Where can I get new puzzles?
- The sudoku maker prints puzzles at several levels, for adults and children, with answers.
- How do I enter a puzzle from a newspaper or a book?
- Click a square and type its number, or paste all 81 digits at once: rows left to right from the top, with 0 or a dot for each blank. Once the puzzle is in, numbers you type into empty squares count as your answers rather than givens, and those are what Check looks at.
- What does Check tell me?
- Whether the numbers you have filled in can still lead to the solution. If they can, it says “Everything so far is correct.” If not, it counts the wrong ones and marks them red, without showing the right numbers.
- Is anything sent anywhere?
- No. The puzzle is solved in your browser.
Related tools
∷ PDF tools Sudoku generator New puzzles every time, each with exactly one answer. ⊠ PDF tools Calcudoku puzzles Sudoku with sums: every row and column once, every cage its target. ▦ PDF tools Printable nonograms Fill the squares, find the picture — by logic alone. ⊿ PDF tools Maze generator One way in, one way out, and a page of wrong turns.