Sudoku difficulty levels explained
Buyers care a lot about difficulty. A book called "expert" that turns out easy gets bad reviews, and so does a "beginner" book that is too hard. Here is what difficulty really means in sudoku.
Clues: the numbers you start with
A sudoku starts with some numbers filled in. These are the clues. Easy puzzles usually give around 36 to 40 clues, medium around 30 to 34, hard around 27 to 29, and expert puzzles often 24 or fewer. Mathematicians have shown that a sudoku with a single solution needs at least 17 clues, so there is a floor to how sparse a puzzle can be.
Solving methods matter too
Clue count is only a rough guide. What really makes a puzzle hard is the kind of thinking needed. Easy puzzles can be finished by finding the one spot where a number can go. Harder ones need methods like looking at pairs of candidates or tracking how numbers in one box limit another. Two puzzles with the same clue count can feel quite different.
One solution, always
A proper sudoku has exactly one solution, so it can be solved by logic without guessing. The puzzle book maker checks every puzzle for this before it goes in your book.
Choosing a level for your book
For a first book, a single clear level is easier to market. Large print books for relaxing usually do well at easy or medium. Books for keen solvers can be hard or expert. Many sellers later publish a series: one volume per level, all with matching covers.
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