The usual guess is that fewer clues means a harder puzzle. It is a weak guide. What makes a puzzle hard is the most difficult step you cannot avoid, and two grids with the same number of clues can need very different steps.
Why the clue count is a poor guide
We built 60 puzzles at each level and counted the clues. Hard puzzles came out with 24 to 27, Very hard ones with 22 to 26 — the two ranges overlap. A 25-clue grid can be either, and the count alone cannot tell you which.
There is also a floor. A sudoku with a single solution needs at least 17 clues; that was proved by an exhaustive computer search in 2012. Puzzles made by removing digits one at a time rarely get anywhere near that, so below the mid-20s the count stops moving while the difficulty keeps rising.
What we measure instead
Every new puzzle is solved by the same engine that gives you hints. It works like a careful person: at each step it tries the easiest technique first and only moves to a harder one when nothing easier applies. The level is the hardest technique that solve needed.
| Level | Techniques it adds · clues in our sample |
|---|---|
| Easy | Naked single · 44 |
| Medium | Hidden single in a box, hidden single in a line · about 30 |
| Hard | Naked pair, hidden pair · 24 to 27 |
| Very hard | Pointing pair, box/line reduction, naked triple, hidden triple · 22 to 26 |
| Expert | X-Wing, Y-Wing, simple colouring, Swordfish · 22 to 26 |
A level includes everything below it. An Expert puzzle still needs singles and pairs on the way; it is the single hardest step that puts it at the top.
What actually decided the hard ones
In the same sample we recorded which technique was the hardest step. For Very hard, a pointing pair decided 41 of 60 puzzles and box/line reduction 14; triples and hidden pairs made up the rest. For Expert, simple colouring decided 34 and Y-Wing 26. X-Wing and Swordfish belong to the level, but neither was the deciding step in those 60.
So if Very hard feels like a wall, pointing pairs are the thing to learn first. At Expert, learning to follow chains of two-candidate cells pays off more than hunting for fish patterns.
No guessing, at any level
If the engine cannot finish a puzzle with its thirteen techniques, the puzzle is thrown away and another is built. That is why the hint can always name the technique for the next step, and why no puzzle here needs trial and error.
The label you see is the measured one, not the one you asked for. When a puzzle built for Very hard turns out to need nothing beyond pairs, it says Hard.
Time is not part of the grade. Two people can take ten minutes and forty on the same puzzle; the steps it requires stay the same.