Choose one of the more than 50 Sudoku solvers via the menu button. Enter the known numbers and click the Solve button or Solve Cell button if you only need 1 number.
If there is more than one solution, up to 10 solutions will be shown, and you can navigate between them using the Previous and Next links. The Unsolve button removes the answers so the input can be changed. The Reset button sets the board to its initial state. Sudokuspoiler will remember your changes to the board, so you can close the browser and continue a next time.