Sudoku Solver

Stuck on a sudoku, or want to check a puzzle before printing it? Paste the grid and this solver fills it in instantly. It first checks the givens for clashes in a row, column or box, then searches for every solution up to two, so it can tell you whether the puzzle is proper (exactly one solution). It also estimates the difficulty by trying to solve the grid with the two basic techniques alone.

How to use this tool

1. Type or paste the puzzle: 9 lines of 9 digits, with 0 or . for empty cells.
2. The solver checks the givens for repeated digits.
3. Read the solution; filled-in digits are shown in brackets in the table.
4. Check "Unique solution" and the difficulty estimate.

Formula

Backtracking search: fill the empty cell with the fewest possible digits, try each, and undo on a dead end

Possible digits are tracked as 9-bit masks for every row, column and box, and the cell with the fewest candidates is chosen first (minimum remaining values), which solves even very hard puzzles in milliseconds. The difficulty estimate uses naked singles (a cell with one candidate) and hidden singles (a digit with one place in a row, column or box).

Example

The classic puzzle beginning 53..7.... has 30 givens and a unique solution starting 534678912. It falls to singles alone, so it is rated easy to medium. Arto Inkala's 2012 "hardest sudoku" (21 givens, starting 8........) also has a unique solution, but singles stall early, so it is rated hard.