Features / Difficulty you can trust

Sudoku difficulty that means something

Each level is defined by the hardest technique a board really needs, and every board is checked before anyone sees it.

The problem

Most apps grade by counting clues, which barely works: two boards with the same number of clues can sit at opposite ends of the range. Worse, a board labelled expert can turn out to need a guess. When you stall you cannot tell whether you missed something or the board was broken, and that doubt spoils the part of sudoku that is supposed to be certain.

How it works

Every published board is solved again afterwards by a separate checker, which works out the hardest technique the board truly requires rather than trusting what the generator meant to make. Easy is solvable with single candidates alone. Medium needs locked candidates and pairs. Hard needs subsets and X-Wings. When you finish, the app names the hardest technique that board needed, so you can tell whether you improved or just drew an easier one.

10,950: boards audited, every one published to date
0: that needed a guess at any point
0: that required a technique above their stated level
0: that turned out to have more than one solution

Read the grading method in full, including what the checker does and where the argument could be attacked.

Sudoku grid showing an X-Wing, the hardest technique a Hard board can require. — An X-Wing, the hardest technique a Hard board is allowed to need. Easy and Medium boards are held to lower ceilings.

What you would use it for

Know that a stall is yours to fix

If you are stuck, something is always deducible. The board was checked for that before it was published, so the doubt is gone and the search is the puzzle again.

Watch yourself improve on purpose

Finishing a board that needed an X-Wing is a different achievement from finishing one that needed a hidden single. Because the technique is named, you can see which you did.

Read the method and argue with it

The grading is written out in full on the method page, including where the argument could be attacked. It is published so it can be checked.

Learn how to use it

How to play walks through the controls step by step.

The daily puzzleThree boards a day, the same three for every player in the world, new at midnight UTC.
Pencil marks and hintsEverything you need to work a board, and a hint that refuses to solve it for you.
Shaped boardsFour boards that are ordinary sudoku plus exactly one extra rule, playable at every level.

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