About

This web site offers 18,653,109,051,064,320 free Sudoku puzzles for you to solve (yes, over 18½ quadrillion puzzles):

4,008,675,278,684,160 easy puzzles
8,029,540,538,449,920 medium puzzles
4,411,554,153,431,040 hard puzzles
2,203,339,080,499,200 very hard puzzles

How can the web site generate so many puzzles? Well, it starts with 30,604 initial puzzles and then uses some properties of Sudoku puzzles to generate one of 609,499,054,080 different variations of each puzzle:

9! (362,880) variations by exchanging the digits 1-9 in the puzzle
6 variations by exchanging rows A/B/C (and 6 for D/E/F, and 6 for G/H/I)
6 variations by exchanging columns J/K/L (and 6 for M/N/O, and 6 for P/Q/R)
6 variations by exchanging column groups S/T/U
6 variations by exchanging row groups V/W/X

All together that is 362,880 x 6 x 6 x 6 x 6 x 6 x 6 x 6 x 6 = 609,499,054,080 possible variations of a single puzzle!