Can someone tell me how to calculate this? If you don’t know the rules:

You start with a 9x9 grid of boxes. Some numbers are pre-entered as clues. The object is to fill in all 81 boxes with the digits 1 to 9.

Each row of nine must include all nine digits, no repeats or omissions.

Each column of nine must include all nine digits, no repeats or omissions.

The large grid is further subdivided into 9 3x3 grid boxes. Each 3x3 box must also include all nine digits, no repeats or omissions.

Here is an online version of the game.

I’m reasonably sure there are many more puzzles possible than solutions, as it would seem you could choose a very large set of starting positions that would result in the same solution. While I haven’t read anything authoritative on this, it seems self-evident.