How is Sudoku difficulty measured?

A puzzle's difficulty is defined by the single hardest logical technique required to solve it without guessing. Solvers such as Sudoku Explainer work through a puzzle with a fixed ladder of techniques — from naked singles up to forcing chains — and report the hardest rung they had to use as a number, the SE grade. Because the method is fixed and reproducible, the same puzzle always earns the same grade, which is why we rate every board this way instead of using vague words like "easy" or "expert".

Not reliably. The fewest clues a valid Sudoku can have while still holding exactly one solution is 17 — proved by an exhaustive computer search (McGuire, Tugemann and Civario, 2012). But clue count only weakly tracks difficulty: a 17-clue puzzle can be gentle, and a 30-clue puzzle can demand advanced chains. What matters is the technique the layout forces, not the number of blanks.

There are 6,670,903,752,021,072,936,960 — about 6.67 × 10²¹ — fully solved Sudoku grids (Felgenhauer and Jarvis, 2005), which reduce to 5,472,730,538 essentially different ones once symmetry is removed. In practice that means fresh, never-before-seen puzzles at any difficulty will never run out.

Famous "monsters" like AI Escargot (Arto Inkala, 2006) were once marketed as the hardest ever made. The SE grade lets us compare them objectively rather than by reputation: the hardest known puzzles sit around SE grade 11 and above.

See how the SE grade works or the hardest Sudoku puzzles for the monster roster.