Sudoku FAQ — difficulty, SE grade, and the hardest puzzles
Straight answers to the questions people ask most about hard Sudoku.
By The Hardest Sudoku Team · Last updated
TL;DR
A quick reference: how Sudoku difficulty is measured, what the SE grade means, the fewest clues a puzzle can have, what the hardest puzzles demand, and how many Sudoku there are.
How is Sudoku difficulty measured?
Sudoku difficulty is measured by the hardest logical technique a puzzle forces you to use, expressed as an SE grade — not by how many clues it has. The scale runs from about 1.0 for the gentlest puzzles to 11.0 and beyond for the monsters.
What is the hardest Sudoku ever made?
Famous "monster" puzzles such as AI Escargot (Arto Inkala, 2006) were built to be as hard as possible and sit around SE grade 11. Rather than argue by reputation, we grade every puzzle on the same scale so the hardest can be ranked objectively.
What is an SE grade?
The SE grade is the difficulty number produced by Sudoku Explainer, a well-known logical solver. It finds the single hardest technique needed to crack a puzzle by pure logic and reports it as a number, so the same puzzle always earns the same grade.
What is the minimum number of clues a Sudoku can have?
A valid Sudoku with exactly one solution needs at least 17 clues. An exhaustive computer search (McGuire, Tugemann and Civario, 2012) proved that no 16-clue puzzle with a unique solution exists.
Can every Sudoku be solved by logic without guessing?
Any proper Sudoku — one with a single solution — can be solved by logic alone. Easy puzzles fall to simple scanning, while the hardest ones require deep techniques like forcing chains, but guessing is never required.
What is a "clean clear"?
A clean clear means you finished a puzzle with zero mistakes and zero hints. It is decided the moment you make your first mistake or take your first hint, so it is a badge you cannot fake.
What techniques do the hardest Sudoku puzzles require?
Difficulty climbs through a ladder of techniques: naked and hidden singles, locked candidates, pairs, X-Wings and Swordfish, XY-Wings, and finally forcing chains — the deep "if this, then that" reasoning the monster puzzles demand.
How many different Sudoku puzzles are there?
There are about 6.67 × 10²¹ fully solved Sudoku grids (Felgenhauer and Jarvis, 2005), which reduce to 5,472,730,538 essentially different ones. Fresh puzzles at any difficulty will never run out.
Does having fewer clues make a Sudoku harder?
Not reliably. Clue count only weakly tracks difficulty — a 17-clue puzzle can be gentle and a 30-clue puzzle brutal. What matters is the hardest technique the layout forces, not the number of blanks.
What makes Hardest Sudoku different?
We measure difficulty as a precise number instead of a vague label, let you actually play the legendary monster puzzles, and mark a clean clear you cannot fake — with no mid-game ads to break your concentration.