How to use the sudoku solver
- Click a cell and type the digits you know, from 1 to 9. The cursor moves along as you type, and the arrow keys move around the grid. Backspace or 0 clears a cell.
- To use a puzzle from elsewhere, press Load an example to see the layout, or open Paste a puzzle as text and paste 81 characters with a dot or 0 for each empty cell.
- Press Solve. The empty cells fill in with the solution, shown in italic type, and a message tells you whether the solution is the only one.
- Press Copy the solution to take the finished grid as text, or Clear the grid to start over.
The example puzzle
The example puzzle gives 30 digits and leaves 51 cells empty. Its first row starts 5, 3, then two empty cells, then 7. The solver fills the first row with 5 3 4 6 7 8 9 1 2, and the whole grid has exactly one solution, so the message says so.
If you type a second 5 into the first row, the two fives are outlined and the message says that cells repeat a digit, so no solving is attempted until you fix them. A completely empty grid has many solutions, so the solver shows one of them and says the puzzle is not unique.
How the solver works
A sudoku is solved when every row, every column and every 3 by 3 box holds the digits 1 to 9 once. For each empty cell the solver works out which digits are still possible, picks the cell with the fewest possibilities, tries each one, and backs up when a choice leads to a dead end. This is called backtracking, and choosing the most constrained cell first keeps it fast, even on puzzles that people find very hard.
The solver looks for up to two solutions. Finding exactly one means the puzzle is well formed. Finding two means it has more than one answer, and finding none means the digits you gave cannot be completed. If the search ever runs for an unusually long time it stops and tells you.
Using it to learn
If you are stuck, solve the puzzle and look at only one cell, then go back and try to see why that digit must go there. To check your own work, enter your filled-in grid and press Solve: a valid finished grid is returned as its own unique solution, and a mistake is outlined when digits repeat. To make a fresh puzzle to solve by hand, use the sudoku generator. Other puzzle tools include the word search maker and the Scrabble score calculator.
Assumptions and limits
- The grid is the standard 9 by 9 sudoku with 3 by 3 boxes. Variants such as diagonal, killer or irregular sudoku are not supported.
- Digits you type are treated as givens. Solved cells are shown differently and are removed again as soon as you edit the grid.
- The solver does not explain its steps. It reports the answer, not a human solving technique.
- Nothing you type leaves your browser.
Frequently asked questions
How do I solve a sudoku online?
Type the given digits into the grid, leave the empty cells blank and press Solve. The remaining cells fill in with the solution, and a message tells you if there is only one answer.
What does it mean when a puzzle has more than one solution?
A well-formed sudoku has exactly one answer. If the solver finds two, some digits are missing or were entered wrongly, and the grid shows only one of the possible solutions.
Why does the solver say my puzzle has no solution?
No digit repeats in a row, column or box, but the digits you gave cannot be completed. Check them against the printed puzzle, because a single wrong digit can make the whole puzzle impossible.
Can I paste a puzzle from a website or app?
Yes. Open the paste box and enter 81 characters in reading order, using the digits 1 to 9 and a dot or 0 for empty cells. Spaces, line breaks and grid lines are ignored.