Sudoku Difficulty Levels Explained: Easy to Expert

Clues do not set difficulty

It is tempting to think that fewer given digits means a harder puzzle. That is only partly true. A puzzle with many clues can still demand an advanced technique, and a puzzle with few clues can fall apart using simple moves. What matters is which techniques you need to make progress.

Easy and medium

Easy puzzles need only naked singles and hidden singles. Every step can be found by scanning a row, column or box. Medium puzzles use the same two techniques, but they start with fewer clues, so there are more empty cells to fill and you will need more passes and more careful scanning.

Hard

Hard puzzles ask for more than singles. At some point you will need a naked pair, or a pointing pair or box-line reduction. These are elimination techniques: they do not place a digit directly, they remove candidates until a single appears.

Two techniques in plain words

A naked pair is two cells in the same row, column or box that share the same two candidates, say 3 and 7. Those two digits must go in those two cells, so you can erase 3 and 7 from every other cell in that unit.

A pointing pair happens when a digit's candidates inside one box all line up in a single row or column. That digit must land in that line within the box, so you can erase it from the rest of the line outside the box. Box-line reduction works the other way around, from a line to a box.

Expert

Expert puzzles need techniques beyond these. Expect to hold more candidates in mind and to chain several eliminations together before a digit can be placed.

Moving up a level

Move up when you finish a level without a hint and without feeling rushed. Try one puzzle from the next level and use pencil marks freely. If you get stuck, go back a level for a few days. Mixing levels in the same week keeps the practice varied.