What makes a Sudoku hard? Difficulty levels explained
Two puzzles with the same number of clues can be a five-minute stroll or an hour's work. What decides it is the hardest move the puzzle forces you to make.
Difficulty is about technique, not clues
Many apps label difficulty by how many digits are missing. That's a weak guide: a sparse puzzle can still fall to simple singles, and a crowded one can hide an X-Wing. Among Ninebloom's puzzles, Easy ones start with 24 to 34 clues and Expert ones with 23 to 34: almost the same range. The honest measure is to solve the puzzle the way a person would, always using the easiest move available, and note the hardest move that was ever needed.
That's how Ninebloom rates every puzzle. Its solver works through the techniques in order and gives each puzzle the level of the hardest one it had to use.
The six levels
| Level | The move it adds | What it feels like |
|---|---|---|
| Beginner | Naked singles | Every step is a cell with one digit left. Good for learning the rules. |
| Easy | Hidden singles | Cross-hatching digits through boxes, rows and columns. No pencil marks needed. |
| Medium | Naked pairs and pointing pairs | The first puzzles where pencil marks pay off. |
| Hard | Box/line reduction and naked triples | You'll compare lines against boxes and track three cells at once. |
| Expert | X-Wing | Patterns across two rows or columns, for one digit at a time. |
| Fiendish | At least one of the Swordfish, XY-Wing, XYZ-Wing, simple colouring or unique rectangle | Rare, satisfying moves. Usually one or two per puzzle. |
Each level can also use every move from the levels above it. A Hard puzzle is mostly singles and pairs, with one or two places where box/line reduction or a triple is the only way forward.
How many puzzles at each level
Ninebloom ships 10,200 classic puzzles across the six levels (Beginner 1,500, Easy 2,000, Medium 2,500, Hard 2,000, Expert 1,200, Fiendish 1,000), plus 730 daily puzzles. Every one is checked to have exactly one solution and to need exactly the technique its level says. Each level is played in order, #1, #2 and on, so you can see how far you've come.
The daily puzzle follows the week: Easy on Monday, Medium on Tuesday, Wednesday and Friday, Hard on Thursday and Saturday, and Expert on Sunday.
Moving up a level
- Stay at a level until you rarely need a hint there, then move up.
- When you move up, read the guide for the new technique first. It's the only new thing you need.
- If a puzzle stalls, use the stuck checklist: most stalls are a missed easier move, not a missing harder one.
Questions
Does a puzzle with fewer clues mean it's harder?
Not reliably. In Ninebloom's own puzzles, an Easy puzzle can start with as few as 24 clues, while an Expert puzzle can start with 34 and still need an X-Wing. What matters is the hardest logical move the puzzle requires.
What is the hardest Sudoku technique?
There's no single answer: beyond the twelve techniques on this site come chains, forcing nets and other advanced moves used on the very hardest puzzles. Ninebloom's Fiendish level stops at moves that a person can learn and spot by eye.
Why does an easy puzzle sometimes feel hard?
Usually a missed single. Easy puzzles never need more than hidden singles, so if you're stuck, sweep each digit through every box again before reaching for a harder move.