Sudoku Solver

How to enter a puzzle

1. Select a cell and press a digit below the board, or type it. Erase, Backspace, Delete or 0 empties the cell, and so does pressing the same digit again. Digits that repeat in a row, column or box are marked at once.
2. Or paste the whole grid into Paste a puzzle and press Load: 81 characters row by row, 1 to 9 for given digits and 0 or a dot for empty cells.
3. Or press Load an example to try the solver on a hard practice puzzle of this site.
4. Press Solve step by step. Use Next step and Previous step to walk through the solution, and Edit the grid to change the puzzle.

The board has a single stop in the tab order; the arrow keys move between cells, as in the daily Sudoku.

Reading the steps

Cells are named by row and column: R4C7 is row 4, column 7. Boxes are numbered 1 to 9 from left to right and top to bottom, so box 5 is the centre box.

The board shows the position just before the step, with the candidates of every empty cell. The cell that the step fills has a dashed frame, the other cells it reasons from have a thick inner frame (the selected cell has a frame too), and candidates it removes are struck through on a tinted cell.

Candidates are worked out from the digits in the grid, not typed in. Further down, All steps lists every step as text.

The techniques

The level column is how the daily puzzles of this site are rated: a puzzle gets the level of the hardest technique it needs.

• Each technique explained with an example

How the solver works

First it checks that no digit repeats. Then a search tries digits in the empty cells, always in the cell with the fewest options, and stops as soon as it finds a second solution. This shows whether the grid has no solution, exactly one, or several.

The steps come from a second program that works like a person: it keeps the candidates of every cell, always tries the simplest technique that makes progress, and records what it did. It is the same program that rates the daily puzzles, so its steps match the levels there.

Limits

• Only the seven techniques above are explained. When a grid needs more, such as chains or swordfish, the solver says how far the techniques got and offers the solution found by the search, without steps for the rest.
• A grid with more than one solution is not a proper Sudoku. The solver says so and shows the steps that are certain anyway.
• A proper Sudoku needs at least 17 given digits; a grid with fewer never has exactly one solution.

• There is no 16-clue Sudoku (McGuire, Tugemann, Civario)

Example: the puzzle behind Load an example

Load an example puts a hard practice puzzle with 28 given digits on the board. Solve step by step reports: solved by logic in 56 steps, hardest technique naked triple, rated Hard.

The first eleven steps are singles. Step 12 is a naked pair: R6C5 and R6C9 in row 6 together have only the candidates 3 and 4, so no other cell of row 6 can hold these digits, and 3 is removed from R6C4. Step 13 is the naked triple that makes the puzzle Hard: in column 1, R4C1 (6, 8), R6C1 (1, 8) and R9C1 (1, 6) together hold only 1, 6 and 8, so 8 is removed from R3C1 and 1 from R8C1.