Classic 9×9 Sudoku has captivated millions of solvers worldwide, but it is only the beginning. Dozens of Sudoku variants add extra rules, different grid shapes, or arithmetic constraints that create entirely new solving experiences. Whether you are looking for a fresh challenge after mastering standard puzzles or you want to explore a specific variant you have heard about, this guide covers every major Sudoku variant in detail.
What Makes a Sudoku Variant?
A Sudoku variant is any puzzle that modifies the classic 9×9 Sudoku format by changing the grid size, altering the box shapes, or adding extra constraints. The core principle — placing digits so each appears exactly once per defined group — remains the same. What changes is which groups exist and what additional rules apply.
Variants range from simple (smaller grids) to extremely complex (multiple overlapping grids with arithmetic constraints). Understanding classic Sudoku techniques like Naked Singles, Hidden Singles, and Naked Pairs provides a strong foundation for tackling any variant.
Mini Sudoku (4×4 and 6×6)
Mini Sudoku reduces the grid size while keeping the same fundamental rules.
4×4 Sudoku
A 4×4 grid uses digits 1–4 and divides into four 2×2 boxes. Each row, column, and box must contain 1 through 4 exactly once. These puzzles are ideal for young children and absolute beginners. They can typically be solved in under a minute using basic scanning.
6×6 Sudoku
A 6×6 grid uses digits 1–6 and divides into six 2×3 rectangular boxes. The additional digits and rectangular box shape introduce more complexity than 4×4 while remaining far simpler than standard 9×9.
Try Mini Sudoku on SudokuPulse to experience these smaller formats firsthand. Mini puzzles are also excellent warm-ups before tackling a full-sized grid.
Killer Sudoku
Killer Sudoku is one of the most popular and challenging variants. It combines classic Sudoku rules with arithmetic:
- The grid is divided into cages (groups of cells outlined by dotted lines).
- Each cage displays a target sum — the digits in that cage must add up to the target.
- No digit can repeat within a cage.
- Standard Sudoku rules still apply: each digit appears once per row, column, and 3×3 box.
Killer Sudoku requires solvers to combine logical elimination with arithmetic reasoning. Knowing the possible digit combinations for small sums (for example, a two-cell cage summing to 3 can only contain 1 and 2) is essential.
For a deep dive, read our full guide on Killer Sudoku rules and strategies.
Diagonal Sudoku (X-Sudoku)
Diagonal Sudoku, also called X-Sudoku, adds one constraint to the classic rules:
- The two main diagonals (top-left to bottom-right and top-right to bottom-left) must also contain the digits 1–9 exactly once.
This extra requirement creates additional eliminations, often making early stages easier (more constraints narrow possibilities faster) but introducing unique patterns in the mid-solve. Diagonal Sudoku is an excellent first variant for solvers already comfortable with standard puzzles.
Hyper Sudoku (NRC Sudoku)
Hyper Sudoku adds four extra 3×3 boxes overlaid on the standard grid. These additional boxes are positioned inside the grid, offset from the standard 3×3 boxes:
- Box positions: rows 2–4/columns 2–4, rows 2–4/columns 6–8, rows 6–8/columns 2–4, rows 6–8/columns 6–8.
- Each of these four extra boxes must contain digits 1–9 exactly once, in addition to the standard row, column, and box constraints.
The extra constraint groups make Hyper Sudoku heavily constrained, meaning puzzles often have fewer givens but are solvable with basic techniques. It is sometimes called Windoku when the four extra boxes are highlighted in a window-like visual style.
Windoku
Windoku is visually identical to Hyper Sudoku — the four extra 3×3 boxes in the same positions. The name refers to the “window pane” appearance of the four shaded regions on the grid. The rules and solving are the same as Hyper Sudoku. Some publishers treat them as the same variant under different names.
Samurai Sudoku
Samurai Sudoku consists of five standard 9×9 grids arranged in an X pattern, with the corner grids overlapping the center grid:
- Each individual 9×9 grid follows standard Sudoku rules.
- The overlapping 3×3 boxes belong to two grids simultaneously, meaning digits must satisfy both grids’ constraints.
Samurai puzzles are large (the combined grid contains 369 cells) and require solvers to move between grids strategically. When stuck in one grid, progress in an overlapping section of another grid can unlock new information. These puzzles require patience and are among the longest to solve.
Irregular Sudoku (Jigsaw Sudoku)
Irregular Sudoku, also called Jigsaw Sudoku, replaces the standard 3×3 rectangular boxes with irregularly shaped regions:
- The grid remains 9×9.
- Nine regions of nine cells each replace the standard boxes. These regions can be any shape.
- Each row, column, and irregular region must contain digits 1–9 exactly once.
The irregular shapes break the visual patterns solvers rely on in classical Sudoku, requiring more careful attention to which cells share a region. This variant is excellent for developing flexible spatial reasoning.
Thermometer Sudoku
Thermometer Sudoku overlays thermometer shapes on the grid:
- Digits along a thermometer must strictly increase from the bulb (round end) to the tip.
- Standard Sudoku rules still apply.
For example, a four-cell thermometer means the bulb cell must contain a digit low enough that three successively larger digits can follow. A bulb cannot be higher than 6 on a four-cell thermo (since 6, 7, 8, 9 is the maximum increasing sequence of length four).
Thermo Sudoku introduces inequality reasoning and combines beautifully with standard elimination techniques. It has become extremely popular through online puzzle communities.
Sandwich Sudoku
Sandwich Sudoku provides an extra clue for each row and column:
- A number outside the grid indicates the sum of the digits between the 1 and the 9 in that row or column.
- Standard Sudoku rules apply.
For example, if a row’s sandwich clue is 0, the 1 and 9 must be adjacent in that row. If the clue is 35 (the maximum for a 9×9 puzzle), all digits 2 through 8 must lie between the 1 and 9.
Sandwich Sudoku requires solvers to think about digit positioning in addition to placement, making it a distinctive challenge.
Arrow Sudoku
Arrow Sudoku places arrow shapes on the grid:
- The digit in the circle (arrow’s origin) must equal the sum of the digits along the arrow’s shaft.
- Digits along the arrow can repeat if they are in different rows, columns, and boxes.
- Standard Sudoku rules apply.
Arrow constraints create powerful deductions. For example, a three-cell arrow emanating from a circle means the circle cell equals the sum of three digits, each between 1 and 9. If the arrow has three cells, the circle cell must be at least 6 (since the minimum sum of three distinct valid digits is 1+2+3=6).
Comparison of Sudoku Variants
| Variant | Grid Size | Extra Constraint | Difficulty (Typical) | Best For |
|---|---|---|---|---|
| Mini (4×4) | 4×4 | Smaller grid, digits 1–4 | Very Easy | Beginners, children |
| Mini (6×6) | 6×6 | Smaller grid, digits 1–6 | Easy | Beginners, warm-ups |
| Classic | 9×9 | None (standard rules) | Easy to Evil | Everyone |
| Diagonal (X-Sudoku) | 9×9 | Two main diagonals | Medium | First variant for experienced players |
| Hyper / Windoku | 9×9 | Four extra 3×3 boxes | Medium | Players wanting more constraints |
| Killer | 9×9 | Cage sums, no repeats in cages | Hard | Arithmetic + logic fans |
| Irregular (Jigsaw) | 9×9 | Non-rectangular regions | Medium–Hard | Spatial reasoning practice |
| Thermometer | 9×9 | Increasing digits along thermos | Medium–Hard | Inequality reasoning |
| Sandwich | 9×9 | Sum between 1 and 9 per line | Hard | Positional logic |
| Arrow | 9×9 | Circle = sum of arrow digits | Medium–Hard | Sum-based deduction |
| Samurai | 5×(9×9) | Five overlapping grids | Hard–Expert | Marathon solving sessions |
How to Get Started With Variants
If you are comfortable with classic 9×9 Sudoku at medium or hard difficulty, you are ready to try variants. Here is a suggested progression:
- Start with Mini Sudoku — Try 4×4 and 6×6 puzzles on SudokuPulse to understand how changing grid size affects the solving experience.
- Try Diagonal Sudoku — The single extra constraint (two diagonals) is easy to understand and integrates naturally with techniques you already know.
- Explore Killer Sudoku — The arithmetic layer adds significant depth. Read our Killer Sudoku guide before diving in.
- Experiment with Thermo or Arrow — These variants are visually intuitive and satisfying when constraints cascade into chain deductions.
- Attempt Samurai — Once you are confident in standard solving and have patience for longer puzzles, Samurai Sudoku offers a rewarding challenge.
Techniques That Transfer Across Variants
Most standard Sudoku techniques work in variants, sometimes with modifications:
- Naked Singles and Hidden Singles — Universal across all variants.
- Naked Pairs and Hidden Pairs — Work in any constraint group (row, column, box, diagonal, irregular region).
- Pointing and Box-Line Reduction — Apply wherever box-line interactions exist.
- X-Wing — Works in any variant with standard rows and columns.
- Pencil marks — Essential for all variants at medium difficulty and above.
Variant-specific techniques (like cage combination logic in Killer Sudoku or thermometer inequality chains) are layered on top of these fundamentals.
Frequently Asked Questions
What are the most popular Sudoku variants?
The most popular variants are Killer Sudoku, Diagonal (X-Sudoku), Mini Sudoku, Samurai Sudoku, and Jigsaw (Irregular) Sudoku. In recent years, Thermometer and Arrow Sudoku have gained significant popularity through online puzzle communities and YouTube channels.
Which Sudoku variant is the hardest?
Difficulty depends on the specific puzzle, but Samurai Sudoku and Killer Sudoku are generally considered the most challenging mainstream variants. Samurai combines five full grids, requiring extended focus, while Killer demands arithmetic reasoning alongside standard logic. Sandwich Sudoku can also be extremely difficult.
Can I play Sudoku variants online?
Yes. SudokuPulse offers Mini Sudoku for smaller grid formats. Dedicated variant platforms, puzzle apps, and YouTube channels like Cracking the Cryptic provide thousands of variant puzzles across all types. Many variants are available as free web-based puzzles.
Do I need to master classic Sudoku before trying variants?
It strongly helps. Most variants build on classic Sudoku rules and techniques. Being comfortable with medium and hard classic puzzles means you already understand elimination, scanning, and candidate tracking — skills that transfer directly to variants.
Are Sudoku variants used in competitions?
Yes. The World Sudoku Championship, organized by the World Puzzle Federation, includes variant rounds alongside classic Sudoku. Competitors must solve Killer, Diagonal, Irregular, and other variants under time pressure. See our history of Sudoku article for more on competitive Sudoku.