Sudoku Difficulty Levels Explained
Understanding what makes evil sudoku different from other difficulty levels and what techniques each requires.
Difficulty Comparison
| Level | Clues | Techniques Needed | Avg. Time |
|---|---|---|---|
| Easy | 36-45 | Naked singles only | 5-10 min |
| Medium | 32-35 | Hidden singles, basic elimination | 10-20 min |
| Hard | 28-31 | Pairs, triples, pointing | 15-30 min |
| Evil | 20-27 | X-Wing, Swordfish, XY-Wing, chains | 20-60 min |
| Extreme | 17-22 | All techniques + bifurcation | 30-90 min |
What Makes Evil Sudoku Special?
Evil sudoku represents the highest standard difficulty level on most platforms. It's designed to be solvable without guessing, but requires mastery of advanced techniques.
The key characteristic of evil puzzles is that basic techniques are insufficient. You will hit walls where only X-Wing, Swordfish, or chain-based logic can progress.
Techniques Required for Evil
Always Needed
- Pencil mark management
- Naked and hidden singles
- Locked candidates
- Naked/hidden pairs & triples
Often Required
- X-Wing pattern
- XY-Wing
- Simple coloring
- Box-line reduction
Sometimes Required
- Swordfish
- XY-Chain
- W-Wing
- Unique rectangles
Rarely Required
- Jellyfish
- Advanced chains
- Complex coloring
- ALS techniques
Evil vs Extreme: What's Harder?
Evil
- Clues: 20-27 (typically 22-24)
- Guarantee: Solvable without bifurcation
- Platform: Web Sudoku, most apps
- Target: Advanced human solvers
Extreme/Diabolical
- Clues: 17-22 (often minimal)
- Guarantee: May require trial & error
- Platform: Specialized apps only
- Target: Expert/competition level
The Key Difference
Evil puzzles are designed to be solvable through pure logical deduction. Extreme puzzles may occasionally require "bifurcation" (trying a value and seeing if it leads to a contradiction), which many purists don't consider true logical solving.
The 17-Clue Minimum
Mathematicians have proven that 17 is the minimum number of clues needed for a Sudoku puzzle to have a unique solution. No valid puzzle can exist with fewer clues.
There are approximately 49,000 known 17-clue puzzles, all discovered through extensive computer searches. These represent the absolute hardest puzzles possible within standard Sudoku rules.
Try 17-Clue Challenge →Recommended Progression
Easy (1-2 weeks)
Master naked singles. Get comfortable with the grid.
Medium (2-4 weeks)
Learn hidden singles and basic candidate elimination.
Hard (4-8 weeks)
Master pairs, triples, and locked candidates.
Evil (8+ weeks)
Learn X-Wing, Swordfish, XY-Wing, and chain techniques.
Extreme (Expert)
Master all techniques including advanced chains.
Frequently Asked Questions
What makes evil sudoku harder than hard difficulty?
Evil sudoku has fewer starting clues (20-27 vs 28-31 for hard) and requires advanced techniques like X-Wing, Swordfish, and XY-Wing. Basic techniques like naked singles and hidden singles aren't sufficient to solve evil puzzles - you'll hit walls that only advanced patterns can unlock.
Is extreme sudoku harder than evil?
Yes, extreme/diabolical puzzles are harder than evil. They have fewer clues (17-22) and may require bifurcation (trial and error). Evil puzzles, by contrast, are always solvable through pure logic without guessing - making them the ideal challenge for advanced human solvers.
How many clues does an evil sudoku puzzle have?
Evil sudoku puzzles typically have 20-27 clues, with most having 22-24 clues. Compare this to 36-45 clues for easy puzzles and 17 clues for the mathematically minimum possible.
Can evil sudoku be solved without guessing?
Yes, properly designed evil sudoku puzzles are always solvable through pure logical deduction without guessing. If you need to guess, you've missed a pattern. Only extreme/diabolical puzzles may occasionally require bifurcation.
What's the difference between evil and extreme sudoku?
The key difference is solvability: evil puzzles are designed to be solved through pure logical deduction. Extreme puzzles may occasionally require trial-and-error (bifurcation). Evil is the highest difficulty most players will encounter that remains purely logical.
Ready for Evil?
Test your skills with our free online evil sudoku puzzles.