More about Sudoku
How our Sudoku puzzles are made
A Sudoku is only as good as its checks. Here is what we verify for every puzzle before you see it.
Every puzzle is checked by a program to confirm it has exactly one solution and that logic alone can solve it, without guessing. Each puzzle is graded by the hardest technique it needs, from singles on Easy up to XY-Wing and coloring on Extreme.
The three checks
- Uniqueness: a solver searches every possibility and confirms exactly one solution. More than one, and the puzzle is rejected. See why uniqueness matters.
- Logic only: a second solver, which only uses human techniques, must finish the puzzle. If it gets stuck, the puzzle is rejected. So you never have to guess.
- Grading: the logic solver always uses the easiest technique available. The hardest technique it needed decides the level: see the levels.
How many puzzles
The app holds 5,000 puzzles at each of the five levels, 2,000 Killer puzzles and a daily puzzle for every day of the year. All of them passed these checks. This website offers a sample of the puzzles for the online game and printable pages.
The same engine powers the solver
The step-by-step solver on this site uses the same technique order. When it names a technique, it is the one the grader would use.
What we do not claim
Our grading follows our own twelve techniques. Other sites may use more techniques or different names, so levels do not match exactly across sites.
Frequently asked questions
Are your Sudoku puzzles guaranteed to have one solution?
Yes. Every puzzle is verified by an exhaustive search.
Can I always solve them without guessing?
Yes. Each puzzle is solved by a program that uses only human techniques.
How is difficulty decided?
By the hardest technique the puzzle needs, not by the number of given digits.