Sudoku Difficulty Benchmark: Book Bolt vs. Wisdom Effect

Clue count does not set difficulty. One Book Bolt book had exactly 28 givens in every puzzle, and its puzzles ranged from SE 1.2 to SE 7.2. All of our Master puzzles have 37 givens, and every one needs Fiendish- or Hell-level techniques.

Our levels are more consistent, especially in the middle. Our Easy, Medium, Hard and Master levels barely overlap. Our top three levels overlap at the puzzle level, and we show where below.

Book Bolt does a lot more than sudoku. This study covers only its sudoku generator, not its research, cover or listing tools.

Book Bolt (the sudoku generator in Book Bolt Studio), tested on 26 October 2026: eight settings, 15 consecutive puzzles each, 120 puzzles in total. We tested the four difficulty levels with the default "Filled cells: Auto", and "Hard" with "At least 25 / 35 / 50 / 60 % filled".

Wisdom Effect: one unedited book with seven chapters of 15 consecutive puzzles, 105 puzzles in total.

We entered each puzzle into Sudoku Coach (sudoku.coach) and recorded three ratings. We used the puzzles in print order, without removing or editing any.

There is no standard for "Easy" or "Hard". By Sudoku Coach's names, our own "Medium" would be "Hard" and our "Hard" would be "Vicious". So we don't compare level names. We compare what matters to a reader: how much difficulty varies inside a level, and whether the next level is reliably harder.

"Hard" covers a wide range. On consecutive pages, puzzle #11 scores SE 7.4 (HoDoKu 4,496) and puzzle #12 scores SE 4.0 (HoDoKu 856). With four levels, everything above intermediate shares one label.

"Normal" and "Hard" overlap: 12 of 15 "Normal" puzzles score SE 4.0–4.2, where the easiest "Hard" puzzles sit.

"Easy" leaves almost as few givens as "Hard" (22–25). Most of these puzzles need only simple techniques, but they take a lot of scanning: Sudoku Coach rates 13 as Moderate and 2 as Hard.

No. Book Bolt lets you combine a difficulty level with a minimum share of filled cells. We paired "Hard" with four values, some deliberately extreme, to see which setting takes priority.

The same clue count can mean very different puzzles. At 35 %, all 15 puzzles have 28 givens. Twelve need only singles, and one (#12) needs chains at SE 7.2.

The filled-cells setting wins. At 50 % and 60 %, every puzzle scores SE 1.2, "Hard" or not. The slider does what it promises; a short warning that "Hard" no longer applies would help publishers.

How our puzzles are made: we remove digits from a full grid one at a time. After each step, a solver that uses human techniques rates the puzzle. A puzzle enters our bank only when its rating matches the target level, so the number of givens follows from the difficulty.

What works: the average rises at every level. Medium, Hard and Master do not overlap at all, and all 15 Hard puzzles fall in one category even though their givens range from 25 to 37.

Where we are not perfect: Master overlaps with Insane, and Nightmare overlaps with Inferno, so a single Insane puzzle can be easier than a single Master puzzle. Workload inside Inferno ranges from HoDoKu 1,870 to 14,120. And Master needs harder techniques than Hard but averages slightly less work, because its 37 givens make the rest of the grid quick.

What we will improve next: we want to narrow the Master and Insane bands so they no longer overlap, and keep the workload inside Nightmare and Inferno closer together, so that every puzzle in a top level takes a similar amount of time.

Seven levels let you publish narrow editions, from a gentle beginner book to an Inferno-only book for experts.

One export builds a multi-level book with chapters, a table of contents and matching solutions. In Book Bolt, each level needs its own setup.

Small sample: 15 puzzles per setting, from one run. Another run will differ somewhat.

One rating tool: we used only Sudoku Coach. Sudoku only: we did not test any other part of either product.

Snapshot in time: both products change. These results describe the versions we tested on 26 October 2026.

Different approach: Book Bolt generates puzzles on demand. We draw from a bank that was rated in advance.

For this test, we exported each Book Bolt setting separately. Book Bolt is a trademark of its owner, and Wisdom Effect is not affiliated with Book Bolt. Sudoku Coach is an independent tool and did not take part in this study. Corrections: none yet; each one will be listed here with its date.