playr.co.uk/sudoku

The Rating System

Each puzzle in this database is rated by difficulty, which is calculated by the number of different logic techniques that were used to solve the puzzle. The logic techniques are divided into five different levels, ranging from the most basic logic to complicated techniques that are best done by computers.

If a puzzle is rated 2.3.1, it required 2 level 1 techniques, three different level 2 techniques and one level 3 technique to solve. A 2.1 requires two level 1 techniques, and one level 2 technique to solve. This should make the puzzle easier to solve, but it is not always the case. Longer rating numbers therefore mean a puzzle should be harder to solve.

Level 1 techniques

These techniques are used for filling in cells.

Cell only has one number in it
Number only appears as candidate once in a row, column or block

Level 2 techniques

These techniques are used for removing candidate numbers from cells.

Number appears in a specific row or column within a block
Number only appears in one block in a row or column
Number appears in two blocks, twice, in the same row or column
Number pairs
Number chains
Hidden number pairs
Hidden number chains

Level 3 techniques

These techniques are used for removing candidate numbers from cells but the techniques themselves are more complicated than level 2 techniques.

X-Wing
Y-Wing
Swordfish
Fishy Cycles
Colouring

Level 4 techniques

These candidate removal techniques are very powerful, and can solve complicated puzzles with very few clues, but they are quite hard for human solvers to handle.

Forcing chains
Nishio

Level 5 techniques

These techniques are extremely complex for humans to attempt, but that is not to say that puzzles with these ratings are impossible, but they may be very time consuming for human solvers.

Tabling
Templating
Brute force trial and error

Puzzle classes

The puzzles are then divided into several classes of puzzle. Some ratings are simliar and mean that puzzles will be roughly equal in difficulty. Because there are many possible ratings, they have been grouped into puzzle classes.

Easy	Medium	Hard	Very hard	Fiendish	Extreme
1	2	1.0.1 1.0.2 1.1 1.1.1 1.1.2 1.2 1.2.1 1.2.2 1.3 1.3.1 1.3.2 1.4 1.4.1 1.4.2 1.5 1.5.1 1.5.2 1.6 1.6.1 1.6.2 2.0.1 2.0.2 2.1 2.1.1 2.1.2 2.2 2.2.1 2.2.2 2.3 2.3.1 2.3.2 2.4 2.4.1 2.4.2 2.5 2.5.1 2.5.2 2.6 2.6.1 2.6.2	1.0.0.1 1.0.1.1 1.0.3 1.0.4 1.0.5 1.0.6 1.1.0.1 1.1.1.1 1.1.2.1 1.1.3 1.1.4 1.1.5 1.1.6 1.2.0.1 1.2.0.2 1.2.1.1 1.2.2.1 1.2.3 1.2.4 1.2.5 1.2.6 1.3.0.1 1.3.1.1 1.3.3 1.3.4 1.3.5 1.3.6 1.4.0.1 1.4.1.1 1.4.2.1 1.4.3 1.4.3.1 1.4.4 1.4.5 1.4.6 1.5.0.1 1.5.1.1 1.5.2.1 1.5.3 1.5.4 1.5.4.1 1.5.5 1.5.6 1.6.3 1.6.4 1.6.5 1.6.6 2.0.0.1 2.0.0.2 2.0.1.1 2.0.1.2 2.0.2.1 2.0.2.2 2.0.3 2.0.3.1 2.0.4 2.0.5 2.0.6 2.1.0.1 2.1.0.2 2.1.1.1 2.1.1.2 2.1.2.1 2.1.2.2 2.1.3 2.1.3.1 2.1.4 2.1.5 2.1.6 2.2.3 2.2.4 2.2.5 2.2.6 2.3.3 2.3.4 2.3.5 2.3.6 2.4.3 2.4.4 2.4.5 2.4.6 2.5.3 2.5.4 2.5.5 2.5.6 2.6.3 2.6.4 2.6.5 2.6.6	1.1.0.2 1.3.0.2 1.4.0.2 1.4.1.2 1.4.2.2 1.5.0.2 1.5.1.2 1.5.2.2 2.1.3.2 2.2.0.1 2.2.0.2 2.2.1.1 2.2.1.2 2.2.2.1 2.2.2.2 2.2.3.1 2.2.3.2 2.2.4.1 2.2.4.2 2.3.0.1 2.3.0.2 2.3.1.1 2.3.1.2 2.3.2.1 2.3.2.2 2.3.3.1 2.3.3.2 2.3.4.1 2.4.0.1 2.4.0.2 2.4.1.1 2.4.1.2 2.4.2.1 2.4.2.2 2.4.3.1 2.4.3.2 2.4.4.1 2.4.4.2 2.5.0.1 2.5.0.2 2.5.1.1 2.5.1.2 2.5.2.1 2.5.2.2 2.5.3.1 2.5.3.2 2.6.0.1 2.6.0.2 2.6.1.1 2.6.1.2 2.6.2.1 2.6.2.2 2.6.3.1	0.0.1.1.1 0.1.0.1.1 0.2.0.0.1 0.3.1.1.1 1.0.0.0.1 1.0.0.1.1 1.0.0.2.1 1.0.1.0.1 1.0.1.1.1 1.0.1.2.1 1.0.2.0.1 1.0.2.2.1 1.1.0.0.1 1.1.0.1.1 1.1.0.1.2 1.1.0.2.1 1.1.0.2.2 1.1.1.0.1 1.1.1.1.1 1.1.1.2.1 1.1.2.0.1 1.1.2.1.1 1.1.2.2.1 1.2.0.0.1 1.2.0.0.2 1.2.0.1.1 1.2.0.1.2 1.2.0.2.1 1.2.0.2.2 1.2.1.0.1 1.2.1.1.1 1.2.1.2.1 1.2.2.1.1 1.3.0.0.1 1.3.0.0.2 1.3.0.1.1 1.3.0.1.2 1.3.0.2.1 1.3.1.0.1 1.3.1.1.1 1.3.1.1.2 1.3.1.2.1 1.3.2.0.1 1.3.2.1.1 1.3.3.2.1 1.4.0.0.1 1.4.0.1.1 1.4.0.1.2 1.4.0.2.1 1.4.0.2.2 1.4.1.0.1 1.4.1.1.1 1.4.1.1.2 1.4.1.2.1 1.4.2.0.1 1.5.0.0.1 1.5.0.1.1 1.5.0.2.1 1.5.1.0.1 1.5.1.1.1 1.6.0.2.1 2.0.0.0.1 2.0.0.1.1 2.0.0.1.2 2.0.0.2.1 2.0.1.0.1 2.0.1.1.1 2.0.1.2.1 2.0.2.0.1 2.0.2.1.1 2.0.2.2.1 2.1.0.0.1 2.1.0.0.2 2.1.0.1.1 2.1.0.1.2 2.1.0.2.1 2.1.0.2.2 2.1.1.0.1 2.1.1.0.2 2.1.1.1.1 2.1.1.1.2 2.1.1.2.1 2.1.1.2.2 2.1.2.0.1 2.1.2.1.1 2.1.2.2.1 2.1.3.1.1 2.1.3.2.1 2.2.0.0.1 2.2.0.0.2 2.2.0.1.1 2.2.0.1.2 2.2.0.2.1 2.2.0.2.2 2.2.1.0.1 2.2.1.0.2 2.2.1.1.1 2.2.1.1.2 2.2.1.2.1 2.2.1.2.2 2.2.2.0.1 2.2.2.1.1 2.2.2.1.2 2.2.2.2.1 2.2.2.2.2 2.2.3.0.1 2.2.3.2.1 2.3.0.0.1 2.3.0.0.2 2.3.0.1.1 2.3.0.1.2 2.3.0.2.1 2.3.0.2.2 2.3.1.0.1 2.3.1.0.2 2.3.1.1.1 2.3.1.1.2 2.3.1.2.1 2.3.1.2.2 2.3.2.0.1 2.3.2.1.1 2.3.2.2.1 2.3.3.1.1 2.3.3.2.1 2.4.0.0.1 2.4.0.0.2 2.4.0.1.1 2.4.0.1.2 2.4.0.2.1 2.4.0.2.2 2.4.1.0.1 2.4.1.1.1 2.4.1.2.1 2.4.1.2.2 2.4.2.0.1 2.4.2.1.1 2.4.2.2.1 2.4.2.2.2 2.4.3.0.1 2.4.3.1.1 2.4.3.2.1 2.5.0.0.1 2.5.0.1.1 2.5.0.2.1 2.5.0.2.2 2.5.1.0.1 2.5.1.1.1 2.5.1.2.1 2.5.1.2.2 2.5.2.1.1 2.5.2.2.1 2.5.3.1.1 2.5.3.2.1 2.6.0.0.1 2.6.0.1.1 2.6.0.2.1 2.6.1.1.1 2.6.1.2.1