What Makes a Sudoku Puzzle Hard?
Most people guess it’s the number of starting clues. It mostly isn’t. A puzzle is as hard as the hardest move it forces you to find.
The short version
Difficulty comes from the techniques a puzzle needs. Easy puzzles always have a single you can spot by looking. Expert puzzles go long stretches where the only way forward is a pattern across the whole board.
It isn’t the number of clues
Clues help, but only a little. A puzzle with 30 clues can be easy, and another with 30 clues can be brutal. Some puzzles with only 17 clues, the fewest a sudoku puzzle can have, can still be solved with simple moves.
What matters is where the clues sit and which moves they leave you.
It’s the moves a puzzle needs
Every move in sudoku is one of a handful of techniques. Some you can see just by looking at the board, like a row with one empty cell. Others only show up once you’ve written notes, and the hardest ones link cells on opposite sides of the board.
A puzzle’s difficulty is the hardest technique it can’t be solved without. If an Easy-looking board suddenly needs an X-Wing halfway through, it’s an Expert puzzle.
The four levels in Bubble Sudoku
Each level only asks for the techniques next to it, and each one builds on the level before.
| Level | What it can ask for |
|---|---|
| Easy | Full House, Hidden Single: Last Remaining and Obvious Single: With Notes |
| Medium | Everything in Easy, plus Obvious Single: Last Possible and Hidden Single: With Notes |
| Hard | Everything in Medium, plus Pointing Pairs, Claiming Pairs, Obvious Pairs, Hidden Pairs and Obvious Triples |
| Expert | Everything in Hard, plus X-Wing, Swordfish and Hidden Triples |
Hard is still fair
A well-made sudoku puzzle has exactly one answer, so every move can be proved. Even an Expert board never needs a blind guess. When you can’t find the next move, it’s there, just in a technique you haven’t tried yet.
How to move up a level
Learn one new technique at a time, then play the level that first needs it. When a hint shows you a move you didn’t see, read the reason and look for the same shape on your next board. Most players jump a level by learning just two or three patterns.