Sudoku strategy by difficulty
Move up one level at a time and learn only the techniques that level adds. Easy needs two kinds of single. Medium adds pointing pairs, box/line reduction and naked and hidden pairs. Hard adds naked triples and X-Wing, Expert adds XY-Wing, Swordfish and simple coloring, and Evil adds unique rectangles.
This plan follows the site’s own grading and its measured data, and it says which technique to practice first at each step.
Why plan by technique and not by level name?
A level name such as “Hard” means different things on different sites. On this site it has a precise meaning: the hardest technique the puzzle needs comes from the third tier of the solver, as explained in how Sudoku difficulty is graded. That lets you plan practice as a list of skills. When a puzzle at your level stalls, the cause is nearly always a technique you have not learned yet, and the level tells you which tier to look in.
The percentages below come from the site’s stored classic puzzles, 1,000 per level. They show how often each technique was needed, not how long it takes to learn. The frequency guide gives the full table and the method.
Easy: master the two singles
What you need: naked singles and hidden singles, and nothing else. Every Easy puzzle on the site can be finished with those two.
How to practice: scan without pencil marks first. For each digit, check the rows, columns and boxes where it can still go. For each cell, count what its row, column and box already contain. Work through the guided first puzzle if you have not done so, and then play Easy puzzles until finding singles feels quick.
Ready to move up when you can finish an Easy puzzle without a hint and without any wrong placements. If you stall on Easy, re-scan, as the cause is a missed single.
Medium: learn to see candidates
What you need: everything from Easy plus four techniques. Pointing pairs and triples were used in 95% of Medium puzzles. Box/line reduction appeared in 30%, naked pairs in 28% and hidden pairs in 11%.
How to practice: this is the level at which pencil marks become necessary, so start with the pencil marks guide. Learn pointing pairs first, since the data says you will meet one in almost every puzzle. Then add box/line reduction, which is the same idea from the other direction. Learn naked pairs next and hidden pairs last, as they are the least frequent and the hardest to spot.
Ready to move up when you can finish a Medium puzzle without hints and can name the technique each time you remove a candidate.
Hard: add triples and X-Wing
What you need: the Medium toolkit plus naked triples and X-Wing. In the Hard sample, naked triples were used in 58% of puzzles and X-Wing in 46%, and Medium techniques stayed common: pointing pairs 88%, box/line reduction 51% and naked pairs 46%.
How to practice: a Hard puzzle is mostly Medium technique with a few heavier steps, so keep your marks tidy and do the cheaper techniques first. When you are stuck, scan for a naked triple in a row, column or box with several unsolved cells. Then try X-Wing, looking at the digits that are already placed many times. Both are worth learning as patterns, as each is easier to find when you know what shape to look for.
Ready to move up when you can finish Hard puzzles with at most an occasional hint and your pencil marks hold up under checking.
Expert: chains and wings
What you need: XY-Wing, simple coloring and Swordfish. In the Expert sample, XY-Wing was used in 61% of puzzles and simple coloring in 57%. Swordfish appeared in only 4%, so it is the lowest priority.
How to practice: learn XY-Wing first, because it involves three cells and is easier to track on paper. Then learn simple coloring, which needs you to follow a chain of linked cells for one digit. Practice marking the chain on a copy of the grid. Leave Swordfish until the other two are comfortable. Expect to spend longer on each puzzle, and expect to re-check your marks more often, since a single wrong mark can give a false chain.
Ready to move up when you can find and apply an XY-Wing and a coloring chain unaided.
Evil: unique rectangles and the rest
What you need: unique rectangles, which every stored Evil puzzle used, plus everything below it. The Evil sample also leaned more on the earlier techniques than Expert did, with naked pairs in 67% of puzzles against 49% in Expert.
How to practice: learn what makes a unique rectangle sound. The deduction depends on the puzzle having exactly one solution, which every puzzle on this site does. Then practice reading four-cell rectangles in the marks. Evil is defined as needing a unique rectangle or something beyond the twelve techniques, so the grading leaves room for puzzles that need more than this site’s lessons cover.
The plan at a glance
| Level | New techniques | Learn first | Share of sample using it |
|---|---|---|---|
| Easy | Naked singles, hidden singles | Hidden singles | 98% |
| Medium | Pointing pairs, box/line reduction, naked pairs, hidden pairs | Pointing pairs | 95% |
| Hard | Naked triples, X-Wing | Naked triples | 58% |
| Expert | XY-Wing, simple coloring, Swordfish | XY-Wing | 61% |
| Evil | Unique rectangles | Unique rectangles | 100% |
The last column is the share of that level’s 1,000 sampled puzzles that used the technique at least once. The Evil figure of 100% comes from the definition of the level and is not a finding. The Easy figure is high because hidden singles are the only technique above naked singles at that level.
What should you check when you are stuck?
At any level, work through the same list from cheapest to most expensive. This is the order the site’s solver uses, so a hint will follow the same path.
- Look for naked singles: a cell with only one possible digit.
- Look for hidden singles: a digit with only one possible cell in a row, column or box.
- Look for pointing pairs and box/line reduction, which remove candidates using the way boxes and lines overlap.
- Look for naked and hidden pairs.
- Look for naked triples and X-Wing.
- Only then try XY-Wing, simple coloring and Swordfish.
- Finally, check for a unique rectangle.
Whenever a step finds anything, return to the top of the list, since a single removal can create a single. Stalling at a level is rarely about the top of the list. It is much more often something skipped from the cheap end.
Is it the puzzle or your marks?
When a puzzle will not move, check your notes before blaming the level. A missing mark can hide a hidden single, and a wrong mark can produce a false pair. Compare a few cells against the board by listing the digits in the cell’s row, column and box, and see whether the marks match what is left over. If you have been marking for a while, rebuilding the notes in one area is faster than hunting for a single wrong mark. Remember that the Auto-notes button resets every empty cell to the placed-digit result, so it also undoes any eliminations you have reasoned out.
If the marks check out and you have been through the list above, the puzzle probably needs a technique from the next tier. That is the point at which to read the next lesson, not to guess.
Why do players plateau?
Three causes are common, and each has a fix grounded in the data above.
- Learning rare techniques first. Swordfish appears in a small minority of Expert puzzles and hidden pairs in about one Medium puzzle in nine, while pointing pairs appear in most. Learn by frequency within each level, not by how impressive a technique sounds.
- Skipping notes at Medium. Several Medium techniques are invisible without candidates, so a player who never writes marks can stall at that level indefinitely.
- Using the third hint level every time. Being handed the move teaches less than being told the technique and finding the move yourself.
Is clue count a guide?
No. The mean starting clue count in the site’s pools runs from 24.4 on Easy to 24.8 on Evil. Judge a level by which techniques it requires, not by how full the starting grid looks.
What about the variants?
Variants offer fewer levels. The 4×4 board has one, the 6×6 and 16×16 boards have two, and killer Sudoku has three, so on those pages the same plan applies but stops sooner. The techniques also adapt to the extra rules. On the diagonal, hyper and anti-knight boards, the extra constraints remove candidates, which can create patterns sooner, and the techniques work as usual. The unique-rectangle shortcut is only used on boards with no extra constraints beyond rows, columns and regions, so it does not apply on the diagonal, hyper or anti-knight boards. For killer Sudoku, the cage-sum techniques come on top of this list.
Where do hints fit in?
Use hints to learn, not to finish. The first level only points to an area, which is enough when you were close. The second names the technique, so you can look it up in the techniques hub, and the third gives the move. A good habit is to take the second level, apply the technique yourself and only then compare with the third. Hints are turned off in expert mode, so do not rely on them as your only route.
A weekly rhythm
- Play two puzzles at your current level and finish them without hints.
- Read one technique lesson from the next level up.
- Play one puzzle at the next level and try to use the new technique at least once.
- When you stall, note which technique the hint names, and add it to your next lesson.
Start from the home page and pick the level that matches the step you are on. The guides hub has the remaining tutorials, and the pillar guide on how to solve Sudoku puts the whole method in one place.