What is Sudoku?

Sudoku is a number puzzle where numbers must be placed in a grid. Modern Sudoku was developed by an American, but it became famous in Japan in the 1980s. The standard sudoku grid has 9 rows and 9 columns. There are also other variations, for example with 16 rows and 16 columns.

What is the goal in Sudoku?

The goal is to insert the numbers from 1 to 9 into every row, column and 3x3 block (hereafter simply referred to as block).

Sudoku grid with rows, columns and 3x3 blocks

Each row, column and block can contain each number from 1 to 9 only exactly once. A Sudoku should only have one solution. If you look at the empty Sudoku grid, there are countless ways to put the numbers from 1 to 9 into the grid without any conflict. A Sudoku like this would not make sense and would contradict the meaning of a number puzzle. For this reason, in each Sudoku a number of fields are already filled in.

Starting Sudoku Grid

Using various techniques, the empty fields are now filled in until the Sudoku is complete and there are no conflicts. The (unique) solution of the above Sudoku then looks like this.

Solved Sudoku Grid

Usually (but not necessarily always), the more cells that are already filled in, the easier the Sudoku is to solve. The developer has to make sure that there is only one solution. This can be done by a solving algorithm that counts the number of different solutions (and dismissing the grid if there is more than one solution) or by simply solving the sudoku on paper and looking for conflicts. For example, the next Sudoku is almost fully solved, but it has two solutions.

Sudoku with two solutions

Both column 1 and column 9 (or row 4 and row 5) are missing the digits 8 and 9. You can now enter the 8 and the 9 in two combinations, both of which are correct (both do not result in any conflicts with the rest of the numbers). So this example is not a valid Sudoku. The puzzles on this website have all been checked for only one solution.

How to play Sudoku on this site?

You can play Sudoku on this site both on a computer and on a phone or tablet. However, it is optimized for computers. If you want to play Sudoku on your phone or tablet, feel free to check out my Sudoku App for Android devices. The graphic and gameplay works better than on this website.The problem is that you enter the numbers with the keyboard. On the small screen of the smartphone, the on-screen keyboard then covers the Sudoku grid.

Below the Sudoku grid you have the following six buttons:

Buttons below the Sudoku grid

The function of these buttons is as follows: