Sudoku Difficulty Rating Calculator
Turn a puzzle's given-clue count and the hardest technique it needs into a simple difficulty score and band.
How it works
Score = clue term (fewer givens → higher) + technique tier × 15, capped at 100
Sudoku difficulty is driven far more by which solving techniques a puzzle forces than by the raw number of clues, though very sparse grids do trend harder. This heuristic blends both: a clue term that rises as givens fall, plus a technique term that jumps 15 points per tier from simple singles up to chains and forcing nets. It mirrors how puzzle sites label difficulty — the hardest logical step needed is the dominant signal.
Worked example
A puzzle with 28 givens whose hardest required step is an X-wing (tier 3): the clue term is about (40 − 28) × 2.2 + 8 ≈ 34, capped at 40 it is 34, plus (3 − 1) × 15 = 30, for a score near 64 — the Hard band.
Sources: Mathematics of Sudoku (Wikipedia)
Frequently asked questions
Do fewer clues always mean a harder sudoku?
Not reliably. Clue count trends with difficulty but the solving techniques required matter far more — a 30-clue puzzle needing forcing chains is harder than a 24-clue puzzle solvable with singles. The minimum for a unique 9×9 solution is 17 clues.
What makes a sudoku "diabolical"?
It requires advanced techniques — chains, colouring, forcing nets or extensive candidate elimination — rather than being solvable step-by-step with singles and pairs. Those deep logical steps, not clue scarcity alone, define the top band.
Why is 17 the minimum number of clues?
A 2012 exhaustive computer proof showed no 9×9 sudoku with 16 or fewer givens has a unique solution. 17 is the fewest clues that can still pin down exactly one answer.
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This calculator is for educational purposes. Double-check important results before acting on them.