Sudoku Variants Compared: All 27 Types Explained

Sudoku has evolved far beyond the classic 9x9 grid. Today there are dozens of recognized variants, each adding unique constraints that change how you think about number placement. This guide compares the 27 types of Sudoku in the app, from the familiar Classic puzzle to the massive Mega 16x16, the fog-covered Fog of War, and the five families added in version 1.2.4 — Skyscraper, Anti-Knight, Windoku, Odd/Even and Column Indexer — so you can find the variant that matches your skill level and interests.

Comparison Table

The table below provides a quick reference for every variant. Use it to compare grid sizes, core rules, difficulty levels, and ideal player profiles at a glance.

Classic 9x9 Sudoku

Classic Sudoku is the foundation of every other variant on this list. The rules are straightforward: fill a 9x9 grid so that every row, every column, and every 3x3 box contains the digits 1 through 9 exactly once. No arithmetic, no extra constraints — just pure logical deduction.

Because of its simplicity, Classic Sudoku scales well across difficulty levels. Easy puzzles provide plenty of given digits for quick solves, while Hard puzzles may require advanced techniques like X-Wings and Swordfish. It is the best place to build the foundational skills you will need for every other variant.

If you are new to Sudoku, start here. Read our complete beginner's guide for rules, strategies, and tips.

Killer Sudoku

Killer Sudoku replaces the pre-filled numbers of Classic Sudoku with dotted-line "cages." Each cage has a target sum, and the digits inside it must add up to that sum without repeating. You still follow the standard row, column, and box constraints, but now you also need arithmetic skills to narrow down candidates.

The key technique in Killer Sudoku is the "Rule of 45," which states that every row, column, and box sums to 45. By subtracting known cage values, you can deduce what remains. Killer puzzles are rated Medium to Expert and are a natural next step for players who have mastered Classic Sudoku.

Learn the full ruleset in our Killer Sudoku rules guide.

Sandwich Sudoku

Sandwich Sudoku uses a standard 9x9 grid with no pre-filled numbers (or very few). Instead, clue numbers appear outside the grid indicating the sum of all digits sandwiched between the 1 and the 9 in that row or column. If a row's clue is 35, you know that the cells between the 1 and 9 in that row must add up to 35.

Solving a Sandwich puzzle requires working out where the 1 and 9 can go first, then using the sandwich sum to constrain the remaining digits. This variant demands a different type of logical reasoning compared to Classic Sudoku, making it feel fresh even for experienced players.

See our Sandwich Sudoku rules guide for detailed strategies.

Mini Sandwich

Mini Sandwich applies the same sandwich-sum concept to a compact 6x6 grid using digits 1 through 6. The "bread" digits are 1 and 6 instead of 1 and 9, and the 2x3 box regions keep the grid manageable.

This variant is an excellent stepping stone. It lets you practice sandwich logic without the complexity of a full 9x9 grid. Puzzles typically take 5 to 10 minutes, making Mini Sandwich ideal for commutes or short breaks.

Diagonal (X) Sudoku

Diagonal Sudoku, sometimes called X-Sudoku, adds one rule to Classic Sudoku: the two main diagonals (from corner to corner) must each contain the digits 1 through 9 without repeats. This single constraint has a surprisingly large impact on solving strategies because the diagonals intersect every box on the grid.

Because each diagonal provides 9 additional unique-digit constraints, Diagonal puzzles often feel slightly easier than an equivalently rated Classic puzzle. However, the diagonal rule also opens up unique techniques that reward players who think about the grid in new ways.

Read our Diagonal Sudoku rules guide for a full breakdown.

Jigsaw Sudoku

Jigsaw Sudoku keeps the 9x9 grid and the digits 1 through 9 but throws out the rectangular 3x3 boxes. In their place are nine irregular, contiguous regions — sometimes called nonominoes or "squiggly" regions. Every row, every column, and every irregular region must still contain each digit exactly once. There is no arithmetic; the challenge comes entirely from the unusual shapes.

Because the regions bend across rows and columns, familiar box-scanning habits have to adapt. Many solvers rely on the "law of leftovers," which compares a band of rows or columns against the regions covering them to force exact digits. The app also offers Mini Jigsaw, a 6x6 version with six regions and digits 1-6 that is a gentle way to learn the technique.

Read our Jigsaw Sudoku rules guide for strategies and worked examples.

Kropki Sudoku

Kropki Sudoku is a standard 9x9 puzzle decorated with small dots on the borders between adjacent cells. A white dot means the two neighbouring digits are consecutive (they differ by 1); a black dot means one digit is exactly double the other (a 2:1 ratio). Just as importantly, the absence of a dot tells you neither relationship holds — a deduction that is easy to overlook but extremely useful.

Black-dot logic is especially powerful because there are only a handful of 2:1 pairs (1-2, 2-4, 3-6, 4-8), so chains of black dots collapse quickly. All the usual row, column, and box rules still apply. A 6x6 Mini Kropki version using digits 1-6 makes the dot logic approachable for newcomers.

Read our Kropki Sudoku rules guide for a full breakdown.

Thermo Sudoku

Thermo (Thermometer) Sudoku overlays thermometer shapes on a standard 9x9 grid. Along every thermometer the digits must strictly increase from the round bulb to the tip — equal digits are never allowed. The usual row, column, and box rules still apply, so Thermo blends familiar Sudoku deduction with a clean visual ordering constraint and no arithmetic at all.

The key idea is position bounds: because the digits increase, a cell's spot on a thermometer already limits its value before any other clue. Long thermometers are the most constrained — a nine-cell thermometer is forced to read 1-9 in order — so they are the best place to start. A 6x6 Mini Thermo version makes those bounds obvious for newcomers.

Read our Thermo Sudoku rules guide for position bounds and worked strategy.

Arrow Sudoku

Arrow Sudoku decorates a standard 9x9 grid with arrows. The digits along each arrow's path must add up to the digit in its circled cell. Unlike Killer cages, digits may repeat on an arrow as long as the normal row, column, and box rules hold — only the sum matters. Because the circle carries the total, it is always the largest digit on its arrow.

Most Arrow deductions come from minimum and maximum path sums: every path cell is at least 1, and the circle can be at most 9, so long paths force small digits and a two-cell path already needs a circle of at least 3 (meaning 1 and 2 are never circles). A 6x6 Mini Arrow version keeps the arithmetic small for beginners.

Read our Arrow Sudoku rules guide for path-sum bounds and strategy.

Fog of War Sudoku

Fog of War Sudoku hides the 9x9 board under a layer of fog. Only a seed cell's box, row, and column are visible at the start; every correct digit clears the fog around it, gradually revealing givens, empty cells — and 2–4 surprise constraints (killer cages, kropki dots, thermometers, or arrows) hidden in the mist. The underlying puzzle is a normal unique-solution Sudoku, but the gradual reveal turns each solve into a small expedition.

Because wrong digits reveal nothing, Fog of War rewards disciplined, deduction-first play more than any other variant. Read our Fog of War Sudoku rules guide for the reveal mechanics and frontier strategy.

Mini Fog of War

Mini Fog of War brings the same hidden-board mechanic to a 6x6 grid with digits 1–6 and 2x3 boxes, hiding 2–3 surprise constraints per game. The small grid keeps reveals fast and the hidden rules easy to reason about, and it is free to play — the ideal first taste of the fog.

See the Mini Fog of War guide for rules and beginner strategy.

Little Killer Sudoku

Little Killer Sudoku places small clue numbers along the edges of the grid with arrows pointing diagonally inward. Each clue gives the sum of all the digits along that diagonal line. Unlike Killer Sudoku cages, digits can repeat on a Little Killer diagonal as long as they do not violate the standard row, column, and box rules.

This variant is one of the hardest 9x9 types because the diagonal sums overlap in complex ways. Solvers need to combine sum logic with careful candidate elimination. Little Killer puzzles are best suited for advanced players who enjoy deep analytical challenges.

Our Little Killer Sudoku guide covers the rules and solving techniques.

Mini 6x6 Sudoku

Mini 6x6 Sudoku shrinks the grid to 6 rows and 6 columns divided into 2x3 boxes, using only digits 1 through 6. The rules are identical to Classic Sudoku, just on a smaller scale. Puzzles can be solved in under 5 minutes, even by beginners.

Mini Sudoku is perfect for children learning logic skills, for adults who want a quick mental exercise, or as a warm-up before tackling harder variants. Despite its simplicity, harder-rated Mini puzzles can still provide a satisfying challenge by offering very few given digits.

Mega 16x16 Sudoku

Mega 16x16 Sudoku is the largest standard variant, featuring a 16x16 grid divided into 4x4 boxes. You place the digits 1 through 16 (or sometimes 0-9 plus A-F in hexadecimal notation) so that each row, column, and box contains every value exactly once.

With 256 cells to fill, Mega puzzles require serious time commitment — often 30 minutes to over an hour. The logic is the same as Classic Sudoku, but keeping track of 16 candidates per cell pushes your working memory and concentration to the limit. This is the marathon of the Sudoku world.

Get started with our Mega 16x16 Sudoku guide.

Skyscraper Sudoku

Skyscraper Sudoku reads each digit as the height of a building and prints numbers outside the grid counting how many buildings are visible looking along that row or column. A taller building hides every shorter one behind it, so the clue describes the order of the digits rather than which digits are present — information no ordinary Sudoku technique produces.

Two clue values do most of the work: a clue of 1 places the 9 in the cell nearest it, and a clue of 9 fills the whole line 1-9 in order. In between, a clue of k caps the nearest cell at 10 − k. The clues are load-bearing — puzzles are generated unique under the visibility rule, so they cannot be solved from the givens alone. A free 6x6 Mini Skyscraper version is included.

Read our Skyscraper Sudoku rules guide for the full clue logic and strategy.

Anti-Knight Sudoku

Anti-Knight Sudoku adds exactly one rule to Classic: no two cells a chess knight's move apart may contain the same digit. A knight moves two cells one way and one at a right angle, so a cell in the interior gains eight new peers scattered into the neighbouring boxes. Nothing is drawn on the board — the constraint is a relationship you have to carry in your head.

Because a knight peer never shares a row or column with its origin, almost every knight elimination is genuinely new information. A cell that "sees" 20 others in Classic Sudoku can see as many as 28 here, which is why Anti-Knight puzzles solve from noticeably fewer givens. A free 6x6 Mini Anti-Knight version is included, and the app highlights a selected cell's knight peers for you.

Read our Anti-Knight Sudoku rules guide for the geometry and strategy.

Windoku

Windoku — also known as Hyper Sudoku or Windmill Sudoku — lays four shaded 3x3 windows inside the grid, each of which must hold the digits 1 through 9 exactly once. The windows sit one cell in from the border, so none of them lines up with a standard box; each straddles the corners of four boxes at once.

The offset placement produces something unexpected: a fifth region that nobody draws. The nine cells where rows 1, 5 and 9 meet columns 1, 5 and 9 also contain each digit exactly once, which follows from the four windows and ties together the four corners and the centre — otherwise the loneliest cells on the board. A free 6x6 Mini Windoku with two windows is included.

Read our Windoku rules guide for the derivation and strategy.

Odd/Even Sudoku

Odd/Even Sudoku marks some cells with a circle (odd: 1, 3, 5, 7, 9) and others with a square (even: 2, 4, 6, 8). Unmarked cells are unrestricted — an absent marker carries no information, which is the variant's one common trap. There is no arithmetic and no geometry to trace, which makes it the most approachable of the 1.2.4 additions.

Its power comes from counting. Every row, column and box holds exactly five odd digits and four even ones, so a region can carry at most five circles and at most four squares. Hit either limit and the parity of every remaining cell in that region is forced. A free 6x6 Mini Odd/Even version splits three and three, which makes the counting arguments even cleaner.

Read our Odd/Even Sudoku rules guide for the parity strategy.

Column Indexer Sudoku

Column Indexer Sudoku, sometimes called 159 Sudoku, adds nothing to the board at all — it changes how you read three columns that are already there. In every row, the digit in column 1 says which column holds that row's 1, the digit in column 5 names the column of the 5, and the digit in column 9 names the column of the 9. Those columns remain ordinary Sudoku columns at the same time.

Deductions arrive in pairs, jumping across a row and back. Within one row, a 5 in column 1 forces a 1 in column 5, a 9 in column 1 forces a 1 in column 9, and a 9 in column 5 forces a 5 in column 9. Column Indexer is the one variant in the app with no Mini 6x6 version: the rule names three digits and three columns at once, and a six-wide grid has neither a centre column nor a matching digit spacing.

Read our Column Indexer Sudoku rules guide for the swap deductions and strategy.

Which Variant Should You Start With?

Choosing the right starting variant depends on your experience level and what kind of challenge you enjoy. Here is a simple decision flow:

• Never played Sudoku before? Start with Mini 6x6. The smaller grid lets you learn the core rules without feeling overwhelmed. Once you can solve Mini puzzles consistently, move up to Classic 9x9.
• Comfortable with Classic Sudoku? Try Diagonal Sudoku or Jigsaw Sudoku next. Diagonal adds only one extra constraint, while Jigsaw keeps pure logic but swaps the boxes for irregular regions — both are a gentle but fresh step up.
• Like relationship clues over arithmetic? Try Kropki Sudoku. Its white and black dots (consecutive and 2:1 pairs) reward pattern spotting rather than addition.
• Prefer visual, no-math variants? Try Thermo Sudoku. Thermometers where digits climb from bulb to tip turn ordering into the whole puzzle — and the position bounds practically guide your pencil.
• Want light sum logic without cages? Try Arrow Sudoku, where a path adds up to its circle and digits may repeat. It is a gentler on-ramp to sum variants than Killer.
• Enjoy math and arithmetic? Go straight to Killer Sudoku or Sandwich Sudoku. Both combine number placement with sum-based reasoning. Killer is more structured; Sandwich is more deductive.
• Want a sense of discovery? Try Fog of War, where the board reveals itself as you solve and surprise rules wait in the mist — or start with the free 6x6 Mini Fog of War.
• Want the hardest challenge? Take on Little Killer for a tough 9x9 variant, or Mega 16x16 if you want sheer scale.
• Short on time? Mini Sandwich gives you variant-style logic in a compact format you can finish in minutes.

There is no wrong order. Every variant builds on the same foundational logic, so skills transfer between types. The best approach is to play what interests you and move on when you are ready for a new challenge.

Difficulty Ranking

The following ranking orders all 27 variants from easiest to hardest based on the typical solving experience. Keep in mind that individual puzzle difficulty varies — a hard Classic puzzle can be tougher than an easy Killer puzzle.

1. Mini 6x6 — Smallest grid, fewest candidates, fastest solves. The easiest entry point into Sudoku.
2. Mini Jigsaw — A 6x6 grid with irregular regions and digits 1-6. The small size keeps the jigsaw twist friendly.
3. Mini Kropki — Consecutive and double dots on a compact 1-6 grid, a gentle introduction to Kropki logic.
4. Mini Thermo — Increasing thermometers on a 1-6 grid, where position bounds are easy to see. A friendly first variant.
5. Mini Arrow — Arrow path sums on a 1-6 grid; the small maximum keeps the arithmetic light.
6. Mini Odd/Even — Circles and squares on a 1-6 grid where the split is three odd and three even. The gentlest variant in the app.
7. Mini Windoku — Two shaded windows on a 1-6 grid. A high density of constraint makes it unusually forgiving.
8. Mini Anti-Knight — The knight constraint on a 1-6 grid, where the ring of peers is small enough to hold in your head.
9. Mini Skyscraper — Visible-building clues on a 1-6 grid; six positions per line is few enough to check at a glance.
10. Mini Fog of War — The fog reveal on a 1-6 grid with 2-3 hidden constraints. Fast reveals keep it friendly despite the surprises.
11. Classic 9x9 (Easy) — The standard starting point with plenty of given digits to work from.
12. Mini Sandwich — Introduces sandwich sums on a manageable 6x6 grid. Slightly harder than plain Mini because of the arithmetic layer.
13. Odd/Even Sudoku — Parity markers halve the candidate list of every marked cell. No arithmetic, no geometry — the easiest step beyond Classic.
14. Diagonal Sudoku — Classic rules plus two diagonal constraints. The extra information can actually make certain puzzles easier to crack.
15. Windoku — Four extra 3x3 regions, plus a hidden fifth. More constraint means fewer givens are needed, so it rarely feels harder than Classic.
16. Kropki Sudoku — Standard 9x9 logic plus dot clues; the "no dot" deduction and 2:1 black-dot pairs add a fresh layer.
17. Thermo Sudoku — Increasing thermometers add position bounds to the 9x9 grid; long thermometers give away a lot, so it stays approachable.
18. Arrow Sudoku — Path-to-circle sums with repeats allowed; light arithmetic layered onto standard 9x9 logic.
19. Jigsaw Sudoku — Pure logic on a 9x9 grid, but irregular regions demand the law-of-leftovers technique and break box-scanning habits.
20. Anti-Knight Sudoku — The rule is simple but invisible; the difficulty is entirely in remembering to fire the eight eliminations every time you place a digit.
21. Fog of War — A hidden 9x9 board with 2-4 surprise constraints. The logic is standard-plus-variants, but limited visibility demands disciplined deduction.
22. Classic 9x9 (Hard) — Fewer givens demand advanced techniques like naked pairs, hidden triples, and X-Wings.
23. Sandwich Sudoku — Requires strong deduction skills to locate the 1 and 9 before filling in the rest.
24. Skyscraper Sudoku — Asks you to reason about the order of digits rather than the set of them, which is a genuinely different habit. The clues are mandatory, not optional.
25. Killer Sudoku — Arithmetic constraints on top of standard Sudoku rules. Demands both logical and numerical fluency.
26. Little Killer — Complex overlapping diagonal sums make this one of the hardest 9x9 variants.
27. Column Indexer — The most abstract variant in the app. Three columns are values and addresses at once, and deductions have to be read in both directions.
28. Mega 16x16 — Same logic as Classic, but 256 cells and 16 candidates per cell push endurance and working memory to the limit.