Use the buttons along the left to select numbers, then select
locations in the grid to place the numbers. You can replace a
number by simply clicking on that location again; if you
replace with the same number the space is cleared. If you prefer
keyboard, you can use the arrows and TAB key to navigate,
Backspace to clear a box,
and the other number keys to assign numbers.
If at any point the solver encounters a contradiction (which means the Sudoku has an error), the "valid" checkbox at the bottom will be cleared. It will become checked again as soon as you remove the contradiction. Certain contradictions are not detected, so "valid" being checked is not a guarantee the Sudoku is solvable.
When "resolve" is checked, the puzzle will be solved as values are entered. Computed values are displayed in blue, and are displayed even if there is a contradiction.
When "show redundant clues" is activated, values that were entered but could instead be calculated from other known values are displayed in red. Use the clear button to erase some of the numbers highlighted in red to make the puzzle harder.
When "show options" is checked, unresolved cells display possible remaining values rather than remaining empty.
The checkboxes along the right enable various algorithms. Disabling algorithms will reduce the number of program-solvable Sudokus.
You can print complete or incomplete Sudoku using your browser's print function.