Sudoku difficulty levels, and what actually makes a puzzle hard
A Sudoku puzzle's difficulty is not the number of clues it starts with. It is the hardest technique you are forced to use to finish it. This app grades all six of its levels that way: every puzzle is solved by the same engine that gives you hints, and it is filed under the hardest step that solve required.
That is why an Easy grid with 28 given numbers and a Hard grid with 28 given numbers are not the same puzzle. The clue count tells you almost nothing; the technique you get stuck on tells you everything.
| Level | Techniques | Hardest step you will meet |
|---|---|---|
| Easy | 3 | Naked Single |
| Medium | 1 | Hidden Single |
| Hard | 7 | Hidden Triple |
| Expert | 8 | Empty Rectangle |
| Master | 8 | Remote Pairs |
| Extreme | 10 | 3D Medusa |
Easy
Solvable by scanning alone, with no pencil marks.
An Easy puzzle never forces you past a cell with one candidate left. You can finish one without notes, one unit at a time, and that is the point: it is where the grammar of the game becomes automatic. If you are new, the app's interactive tutorial starts you here on a real board.
Techniques this level introduces
Medium
Adds the move where you ask where a number goes, not what a cell holds.
Medium introduces the Hidden Single, and with it the mental flip that separates casual solvers from steady ones: instead of looking at an empty cell and asking what fits, you take a number and ask where it can still go. Most puzzles printed in newspapers live at this level.
Techniques this level introduces
Hard
Notes become necessary; pairs and triples start doing the work.
Hard is where a solve stops being a sequence of placements and becomes a sequence of eliminations. Pointing pairs, box/line reductions and naked and hidden subsets do not fill a cell: they remove candidates elsewhere, and a placement falls out a move or two later. Solvers usually stall here first, and the fix is almost always better notes rather than more staring.
Techniques this level introduces
Expert
Patterns that span the whole grid rather than one unit.
Expert is the level where you stop solving inside one unit at a time. An X-Wing ties two rows to two columns; a Y-Wing links three cells that may sit nowhere near each other; Simple Coloring follows one number across half the grid. These take longer to spot and clear far more when they land.
Techniques this level introduces
Master
Chains and grouped patterns, where the logic runs through several cells.
Master puzzles need chains. Instead of recognising a fixed shape, you follow a run of forced consequences from one cell to the next and use the fact that at least one end must be true. The patterns here, kites, W-Wings, XY-Chains, are all the same idea at different lengths.
Techniques this level introduces
Extreme
Uniqueness arguments, finned fish and multi-digit colouring.
Extreme puzzles reach for techniques that most solvers only ever meet through a hint. Some, like Unique Rectangle and BUG+1, do not reason about the grid at all: they reason about the promise that the puzzle has exactly one solution. Others, like ALS-XZ and 3D Medusa, track several numbers at once. This is the band the app's hint engine was built for.
Techniques this level introduces