A sudoku's difficulty depends mostly on which techniques you need to solve it, and only partly on how many digits are given. An Easy puzzle can be finished by finding singles: cells with one possible digit, or digits with one possible place. A Hard or Expert puzzle reaches a point where no single exists, and you need notes and patterns such as pairs to go on.
Why fewer clues isn't the whole story
A puzzle with 25 givens can be easier than one with 30, if its 25 digits happen to leave a chain of singles to follow. What makes a puzzle hard is a bottleneck: a position where every remaining cell has two or more candidates and every digit has two or more places. To get past it, you need a technique that removes candidates rather than placing digits.
For the record, the smallest number of givens a sudoku can have and still have one solution is 17. That was proved by an exhaustive computer search published in 2012 (McGuire, Tugemann and Civario). Puzzles that sparse exist, but they aren't automatically the hardest.
Sudofold's four levels
| Level | Given digits | What you need |
|---|---|---|
| Easy | about 40 | Hidden and naked singles only. Notes are optional. |
| Medium | about 32 | Singles only, but fewer givens mean more scanning. |
| Hard | about 27 | Usually more than singles: notes, pairs and pointing pairs. |
| Expert | about 23 | Always more than singles. Full notes from the start. |
Every puzzle is generated on your phone and checked to have exactly one solution. For Easy and Medium, Sudofold also checks that a solver using only singles can finish it, so you never hit a wall on those levels. Hard and Expert are checked the other way round: Expert puzzles are kept only if singles alone can't solve them.
The Daily puzzle is a Medium puzzle, and it's the same for everyone on a given day.
Which level should you play?
- New to sudoku? Start on Easy and finish a few without hints. You're learning to scan.
- Easy feels quick? Medium uses the same techniques with fewer clues. Most players spend most of their time here.
- Comfortable with notes? Move to Hard. You'll meet your first pairs.
- Want a long think? Expert. Expect full notes and several bottlenecks per puzzle.
Your Statistics in Sudofold show games played, win rate, best and average time for each level, so you can see when you're ready to move up. A good sign is finishing a level's puzzles without mistakes or hints: Sudofold counts those as perfect games.
Getting better
The techniques that separate the levels are explained in sudoku strategies for beginners, and how to use pencil marks covers the note-keeping Hard and Expert need. If you're just starting, read how to play sudoku first.
