Sudoku Difficulty Levels Explained

A sudoku's difficulty is the hardest technique you need to solve it, not how many digits are printed in the grid. A puzzle that only ever asks for singles is easy however few clues it has. A puzzle that forces one chain of linked candidates is hard however many clues it has. This guide is for beginners and intermediate players who want to know what the labels mean and which level to pick.

The six levels at a glance

sudoku69 has six levels: Easy, Medium, Hard, Expert, Master and Legendary. Each has a rating band on the Sudoku Explainer scale (ER), explained below.

The tier names come from the ladder in our solving techniques guide. Each tier is a family of techniques: basic means counting candidates inside one unit, tough means patterns across units (and cage arithmetic in Killer), diabolical means chains of linked candidates, extreme means grouped links, locked sets and nets, and last-resort means guess and verify.

How is sudoku difficulty rated?

We rate puzzles with Sudoku Explainer, the solver written by Nicolas Juillerat. Its ER scale is the best-known way to put a number on how hard a sudoku is, and we run our own TypeScript port of it.

In plain words:

1. Every solving step has a score. A simple single scores low, a long chain scores high.
2. The solver always plays the easiest step available, then the next easiest, until the grid is solved.
3. The puzzle's rating is the score of the hardest step it had to take.

So one difficult move sets the whole rating. A grid that falls to singles and pairs except for one X-Wing is rated as an X-Wing puzzle, and it lands in Hard. A puzzle the rater cannot solve by logic at all is never published here.

The bands are fixed and do not change with the puzzle's size or type. Easy is 1.0 to 1.9, Medium 2.0 to 2.9, Hard 3.0 to 4.9, Expert 5.0 to 6.9, Master 7.0 to 9.9 and Legendary 10.0 and up. Every puzzle in our catalogue sits in exactly one band.

What makes a sudoku hard?

Three things, in order of importance:

• The hardest technique required. This is the ER rating itself. Moving from singles to a fish pattern is a bigger jump than adding another dozen empty cells.
• How long you have to hunt for it. Some puzzles offer the key pattern in plain sight. Others hide it behind several easier steps you must complete first.
• How many hard steps in a row. One chain is a puzzle. Five chains is an evening.

Clue count is not on the list, and that is the next point.

Why clue count is a poor guide

It is natural to think fewer givens means a harder puzzle. Our own data says otherwise. These are the given-digit counts across our classic 9x9 puzzles:

Legendary puzzles have more clues on average (25.3) than Master puzzles (24.4). Some Hard puzzles have 44 clues, while some Medium puzzles have only 22. The trend is real at the extremes, since Easy puzzles do have the most givens, but the ranges overlap heavily. Where the clues sit matters far more than how many there are. For the counting side of the story, see sudoku maths.

How Killer and 16x16 map onto the same bands

The six levels mean the same thing in every mode, because the ER bands are identical. What changes is the material the techniques work on.

• Classic 9x9. Digits 1 to 9, rows, columns and boxes. Averages by level run from 1.26 (Easy) to 10.26 (Legendary).
• Killer 9x9. Same grid plus cages with sums. Cage arithmetic techniques such as innies and outies take the place of some pattern work. Expert, Master and Legendary Killer puzzles have no given digits at all: the cages alone fix the solution. Averages run from 1.40 to 10.43.
• 16x16. Sixteen symbols (1 to 9, then A to G) in 4x4 boxes. The same techniques apply on a bigger board, so a level takes longer to play even at the same rating. Averages run from 1.20 to 10.14.

A Hard puzzle in any mode needs a tough-tier technique and no more. A Legendary puzzle in any mode needs something near or beyond the top of the ladder. The hardest puzzles in our catalogue are rated 12.8 (Killer), 11.1 (classic) and 10.4 (16x16); see the hardest sudoku puzzles for what sits at the top.

How to pick your level

• New to sudoku or coming back after a break: start at Easy. If you can finish two or three without hints, move up.
• Comfortable with singles and want a challenge: Medium. This is where pairs and pointing pairs first appear.
• You know pairs and triples and want your first named patterns: Hard. Expect X-Wing, Y-Wing and Swordfish, or cage arithmetic in Killer.
• You enjoy long chains and always pencil in candidates: Expert and Master.
• You want the logic pushed to its limit: Legendary.

The rule of thumb is to play at the level where you finish most puzzles but must learn something new about one in three. If you are stuck every time, drop a level and read the technique the level needs. If nothing ever slows you down, go up one. The step-by-step solving guide shows the order to learn techniques so each level feels reachable.