Sudoku Difficulty Levels Explained: Easy to Evil

Sudoku difficulty is determined not by how many numbers are given at the start, but by which solving techniques are required to complete the puzzle. An easy puzzle can be solved entirely with naked singles and basic scanning. A medium puzzle requires hidden singles and possibly naked pairs. A hard puzzle demands X-Wing patterns and pointing pairs. And the hardest puzzles, often labeled "evil" or "expert," need Swordfish, XY-Wing, and even chains. Understanding this progression helps you know exactly what to learn next and what to expect at each level.

How Puzzle Difficulty Is Determined

Puzzle creators and apps use solving algorithms to rate difficulty. The algorithm attempts to solve the puzzle using techniques in a defined hierarchy, simplest first. The difficulty rating is determined by the most advanced technique required to reach the solution.

This means two puzzles with the same number of given clues can have wildly different difficulties. A puzzle with 25 givens might be easy if the clues are placed to allow simple scanning. Another puzzle with 30 givens might be hard if the arrangement forces the solver into complex pattern recognition.

The number of givens does have a loose correlation with difficulty (easy puzzles tend to have more, 30 to 40, and hard puzzles fewer, 22 to 28), but it is not the defining factor. The required technique set is what determines the experience of solving the puzzle.

Easy: Naked Singles and Basic Scanning

Easy puzzles are designed for beginners and can be solved using only two techniques:

• Naked singles: When all but one candidate has been eliminated from a cell by its row, column, and box constraints, the remaining candidate is the answer. This is the most basic sudoku deduction.
• Cross-hatching (scanning): Checking where a specific number can go within a box by examining the rows and columns that pass through it. When only one cell remains valid, the number goes there.

Easy puzzles typically have 32 to 40 givens. The solver can work through the puzzle by repeatedly scanning and placing naked singles in a cascading chain. No pencil marks are needed.

Typical solve time: 3 to 10 minutes for intermediate solvers, 15 to 30 minutes for complete beginners.

If you are just starting out, see our complete beginner's guide.

Medium: Hidden Singles and Pairs

Medium puzzles introduce techniques that go beyond single-cell analysis:

• Hidden singles: When a number can only go in one cell within a row, column, or box, even though that cell has multiple candidates. This is the most important technique in sudoku and the foundation for everything above easy.
• Naked pairs: Two cells in the same group with exactly the same two candidates. The pair "claims" those two numbers, eliminating them from other cells in the group.
• Hidden pairs: Two numbers that can only go in two cells within a group, even though those cells have other candidates. Allows elimination of the extra candidates from the pair cells.

Medium puzzles typically have 27 to 33 givens. The solver will hit points where naked singles are not available, requiring pencil marks and pair-based deductions to continue.

Typical solve time: 8 to 20 minutes. Medium is where most solvers spend the majority of their development time.

At this level, pencil marks become important. Snyder notation is usually sufficient for medium puzzles.

Hard: Line-Based Eliminations

Hard puzzles require understanding how boxes and lines (rows/columns) interact to eliminate candidates:

• Pointing pairs/triples: When a candidate in a box is confined to a single row or column, it can be eliminated from the rest of that row or column. This box-line interaction is the gateway to advanced solving.
• Box-line reduction: The reverse of pointing pairs. When a candidate in a row or column is confined to a single box, it can be eliminated from other cells in that box.
• X-Wing: A rectangular pattern where a candidate appears in exactly two cells in each of two rows (or columns), aligned in the same columns (or rows). Allows elimination from the intersecting lines.
• Naked triples: An extension of naked pairs to three cells and three candidates. Less common but follows the same logic.

Hard puzzles typically have 24 to 30 givens. The solver needs full or near-full pencil marks and must systematically search for line-based elimination patterns.

Typical solve time: 15 to 40 minutes. Hard puzzles are where the intellectual challenge of sudoku really begins.

Expert: Advanced Pattern Recognition

Expert-level puzzles require techniques that analyze relationships across multiple cells and groups simultaneously:

• Swordfish: An extension of X-Wing to three rows and three columns. When a candidate is confined to three columns across three rows, it can be eliminated from other cells in those columns.
• XY-Wing: A three-cell pattern where a pivot cell connects to two wing cells, creating an elimination based on the wings' shared candidate.
• Simple coloring: Assigning two "colors" to alternating positions of a candidate along a chain of strong links, then using contradictions or consensus to make eliminations.
• Unique rectangles: Exploiting the fact that a valid sudoku has exactly one solution to eliminate candidates that would create a pattern with multiple solutions.

Expert puzzles typically have 22 to 27 givens. Complete pencil marks are mandatory, and the solver must be comfortable with multiple advanced techniques.

Typical solve time: 25 to 60 minutes, or longer for puzzles requiring rare techniques.

Evil/Diabolical: Chains and Beyond

The highest difficulty tier goes by various names: evil, diabolical, extreme, nightmare. These puzzles require the most complex deduction methods:

• Jellyfish: The four-row/four-column extension of X-Wing and Swordfish. Extremely rare but follows the same logic.
• XY-Chains: Extended chains of bivalue cells (cells with exactly two candidates) where each link shares a candidate with the next. Allows elimination based on the chain's endpoints.
• Alternating inference chains (AICs): General chain-based reasoning that links strong and weak inferences across the grid. The most powerful general technique in standard sudoku.
• Almost locked sets (ALS): Groups of cells where the candidates are nearly locked (N cells with N+1 candidates), used for complex eliminations.

Evil puzzles typically have 20 to 25 givens. They may require combining multiple advanced techniques, and some positions may need chain-based reasoning that spans much of the grid.

Typical solve time: 30 minutes to several hours. Some evil puzzles are unsolved by many experienced solvers.

The Technique Hierarchy Summary

Here is the complete hierarchy from simplest to most complex:

1. Easy: Naked singles, cross-hatching
2. Medium: Hidden singles, naked pairs, hidden pairs
3. Hard: Pointing pairs, box-line reduction, X-Wing, naked triples
4. Expert: Swordfish, XY-Wing, simple coloring, unique rectangles
5. Evil: Jellyfish, XY-chains, AICs, almost locked sets

Each level builds on the previous one. You should not attempt to learn XY-Wing before you are comfortable with naked pairs and pointing pairs. The techniques stack, and each new technique makes more sense when you understand what came before it. For every technique ranked with example grids and how often each level needs it, see sudoku techniques in order of difficulty. If you are stuck mid-puzzle, this step-by-step checklist tells you what to try next.

How to Progress Through Difficulty Levels

Easy to Medium

Focus on learning hidden singles. This single technique is the bridge from easy to medium. Once you can reliably spot hidden singles, medium puzzles become accessible. Learn basic pencil marks (Snyder notation) to support pair detection.

Medium to Hard

Master naked pairs and pointing pairs/box-line reduction. These intermediate techniques cover the vast majority of medium-to-hard transitions. Once you can apply them consistently, start learning X-Wing.

Hard to Expert

This is the biggest jump. You need multiple techniques that you may rarely encounter: Swordfish, XY-Wing, coloring. The challenge is that these techniques are needed infrequently, so building recognition speed is slow. Patience and deliberate practice are essential. See our advanced strategies guide.

Expert to Evil

Chain-based techniques require a fundamentally different way of thinking about the puzzle. Instead of looking at isolated groups, you follow logical chains across the grid. This takes significant study and practice. Many excellent solvers never reach this level, and that is perfectly fine.

How Sudoku Royale grades its six levels

Sudoku Royale uses the same principle, with one addition. Each of its six Classic levels has a ceiling on the hardest technique a puzzle may need, and a ceiling on how much searching it takes to find each move. The second part matters because two puzzles that both need only hidden singles can feel very different: in one the next move is always obvious, in the other you scan the whole grid to find it.

That is stricter than the general scheme above: in Sudoku Royale, Medium never asks for pairs and Hard never asks for an X-Wing. It also means each level has one clear technique to learn before you move up. From solves in the app, a typical Easy board has about 44 empty cells, Medium about 51, and Hard about 56, and the median player fills one empty cell every 11 seconds or so. See the guide to unlimited practice puzzles for how the levels play, or Journey for a chapter-by-chapter path through the same techniques.

Difficulty in daily puzzles

Daily puzzles usually sit in the easy to medium range, because they are meant to be finished in one sitting. The Sudoku Royale Daily Challenge climbs through the week, from Monday (naked singles only) to Sunday (hidden singles, with the moves much harder to find), and never needs more than singles.

For advice on improving your speed at your current difficulty level, and practical tips for beginning solvers, check out our dedicated guides.