Sudoku difficulty grader
How hard is your puzzle, really? Enter or paste any sudoku and get an honest grade — plus the exact techniques it takes to solve.
The interactive grader needs JavaScript. The ladder it grades by is documented below — from naked singles to forcing chains.
How grading works
Most sites rate difficulty by counting clues or timing solvers. Both mislead: a 24-clue puzzle can be a warm-up while a 30-clue one demands chains. Stillgrid's grader solves your puzzle the way a person would — it tries the simplest technique first and escalates only when nothing simpler works — then grades the puzzle by the hardest technique it was forced to use. The result also lists every technique on the path, so "Medium" isn't a vibe, it's a claim you can check.
Classic grids are first checked for a unique solution; a grid with zero or several solutions is reported honestly instead of graded.
The technique ladder
- Easy Naked single, hidden single — one candidate left, or one home left for a digit.
- Medium Naked pair, hidden pair, pointing pair — and cage-sum combinations in killer.
- Hard X-Wing — a candidate rectangle across two rows and two columns.
- Diabolical Swordfish and XY-Wing — three-line fish and bivalue pivots.
- Nightmare Coloring, forcing chains, and Almost Locked Sets — following candidate implications until something breaks.
A puzzle that defeats the whole ladder grades as stuck — genuinely beyond Nightmare. The famous "world's hardest sudoku" grids land there, and we'd rather say so than pretend.
Common questions
How is sudoku difficulty graded?
The honest way to grade sudoku difficulty is by which solving techniques a puzzle actually requires, not by counting clues. Stillgrid's grader solves each puzzle the way a human would — simplest technique first, escalating only when stuck — and grades it by the hardest technique it needed, from Easy (singles only) up to Nightmare (chain-based logic).
Why does my puzzle grade as stuck?
Stuck means the grader ran its entire technique ladder — through X-Wing, Swordfish, XY-Wing, coloring, and forcing chains — and still couldn't finish. A few famously extreme puzzles genuinely live beyond the ladder. But the common cause is a typo: a grid with a wrong or missing given often has several solutions, and no logical technique can finish a puzzle that isn't unique.
Can it grade 6×6 and 16×16 sudoku?
Yes. The grader accepts 6×6, 9×9, and 16×16 grids (16×16 uses letters A–G for 10–16), as classic or X-Sudoku — for X-Sudoku the two main diagonals are checked as well. Killer and jigsaw need their cage and region layouts, which a plain digit string can't carry, so they aren't supported here — every killer and jigsaw you play on Stillgrid is already graded.
Is the sudoku grader free?
Yes. The grader is free to use, with no signup — like every puzzle on Stillgrid, which is free to play with no signup needed to start.