Most sudoku apps label their puzzles Easy or Hard based on how many numbers are missing. That’s a rough guide at best. A grid with few clues can be simple, and a grid with plenty of clues can stop you cold.
One Sudoku grades every puzzle differently. Before a puzzle reaches you, a grader solves it the way a person would, trying the simplest techniques first, and records the hardest one it actually needed. That technique decides the level. It also means no puzzle ever requires guessing: if the grader can’t finish a grid with logic, the grid is thrown away.
The upshot is that each level teaches something specific. Work through them in order and you’ll learn sudoku properly, one idea at a time.
Step 1: The rules
Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. (How to Play Sudoku covers the rules in more detail.) That’s it. There’s no arithmetic involved; the digits could just as well be letters or colours.
The only other word you need is candidate: a digit that could still go in an empty cell without breaking a rule. Every technique below is a way of narrowing candidates down until only one is left.
Step 2: Easy — the Naked Single
| Technique | The idea |
|---|---|
| Naked Single | Look at one empty cell. Cross out every digit already in its row, column and box. If only one is left, that’s the answer. |
Easy puzzles in One Sudoku need nothing else. Every move is available by looking at a single cell, which makes Easy the place to get comfortable with the grid and build a rhythm.
You’re ready to move on when you can finish Easy in under 5 minutes without hints.
Step 3: Medium — the Hidden Single
| Technique | The idea |
|---|---|
| Hidden Single | Pick a digit and a row, column or box. If that digit fits in only one cell there, it goes there, even if that cell has other candidates too. |
This is the jump most beginners feel. Every Medium puzzle needs at least one Hidden Single, so cell-by-cell checking eventually runs dry. The fix is to flip your question from “what goes in this cell?” to “where does this 7 go in this box?” and to scan digit by digit, 1 through 9.
You’re ready to move on when Medium takes under 10 minutes.
Step 4: Hard — pairs, triples and pointing
Hard is where notes stop being optional. These techniques don’t place digits directly; they remove candidates, which is what eventually exposes the next single.
| Technique | The idea |
|---|---|
| Naked Pair | Two cells in a house hold the same two candidates and nothing else. Those two digits belong to those two cells, so remove them everywhere else in the house. |
| Hidden Pair | Two digits can only go in the same two cells of a house. Every other candidate in those cells can go. |
| Pointing Pair / Triple | Inside a box, a digit is confined to one row or column. It can’t appear in that row or column outside the box. |
| Box/Line Reduction | The reverse: in a row or column, a digit is confined to one box. It can’t appear elsewhere in that box. |
| Naked Triple | Three cells in a house share three candidates between them. Those digits are taken; clear them from the rest of the house. |
| Hidden Triple | Three digits fit only in the same three cells of a house. Strip everything else from those cells. |
Every Hard puzzle requires at least one of these, and none of them requires anything harder. Learn the pairs and the pointing pair first; they come up far more often than the triples.
You’re ready to move on when you can finish Hard in under 15 minutes.
Step 5: Expert — fish and wings
Expert puzzles need at least one pattern that spans more than one house at a time.
| Technique | The idea |
|---|---|
| X-Wing | A digit fits in exactly two cells in each of two rows, and those cells line up in the same two columns. The digit can be removed from the rest of those columns. (It works with rows and columns swapped, too.) |
| Swordfish | The same logic stretched across three rows and three columns. |
| XY-Wing | Three two-candidate cells link up so that, whichever way the pivot cell goes, one digit is ruled out of any cell that sees both ends. |
That’s the full set. One Sudoku’s Expert level stops at XY-Wing, so if you know these eleven techniques you have everything any puzzle in the app will ask of you.
The target: Expert in under 20 minutes, with no hints. That’s a genuinely good mark.
How the app helps you learn
A few things in One Sudoku are designed specifically for this progression:
- Notes that tidy themselves. Switch on Notes mode to pencil in candidates. When you place a digit, it’s automatically removed from the notes of every cell in the same row, column and box, so your marks stay accurate without the bookkeeping.
- Hints that teach, not just fill. You get three per game. A hint first fixes any wrong entry, since a mistake quietly blocks every technique. After that it points to a Naked Single if one exists, then a Hidden Single, so it shows you the next step rather than a random square.
- No mistake limit. Wrong entries are counted, but they never end the game. Undo is unlimited. Experimenting is how techniques sink in, and there’s no penalty for doing it.
- A Daily Challenge. One puzzle a day, the same for every player, with a streak to keep you coming back. Consistency does more for improvement than long sessions.
Your progress checklist
| Level | Techniques it requires | Target time |
|---|---|---|
| Easy | Naked Single | 5 min |
| Medium | + Hidden Single | 10 min |
| Hard | + Pairs, Triples, Pointing, Box/Line Reduction | 15 min |
| Expert | + X-Wing, Swordfish, XY-Wing | 20 min |
A last piece of advice: play one level above where you’re comfortable. If you never get stuck, you aren’t learning anything new. If you’re stuck on every move, drop back down. Getting stuck a couple of times per puzzle is the sweet spot.
