Every variant builds on the same foundation — each row, column, and 3×3 box must contain the digits 1–9 exactly once — then adds one or more extra rules. Click Watch Demo to see an interactive walkthrough, or Play to jump straight in.
Standard
Classic Sudoku
Easy · Medium · Hard · Expert
Rules: Fill every cell so that each row, each column, and each of the nine 3×3 boxes contains the digits 1–9 exactly once. No digit may repeat within any of those three groups.
Difficulties: Easy puzzles have many given digits and require only basic scanning. Expert puzzles may need advanced techniques like X-Wings or Swordfish.
Daily & Weekly: The same puzzle is generated for all players on the same day/week using a fixed seed. Your time is compared on the leaderboard.
Strategy: Start by scanning for naked singles — cells where only one digit fits after checking the row, column, and box. Then look for hidden singles — a digit that can only go in one cell within a row, column, or box even if that cell has multiple candidates. Use the Notes button (pencil icon) to pencil in candidates before committing.
Extra Regions
✕
Diagonal (X-Sudoku)
Medium difficulty
Rules: All standard sudoku rules apply. In addition, both main diagonals must each contain the digits 1–9 exactly once.
· The main diagonal runs from the top-left cell (R1C1) to the bottom-right cell (R9C9).
· The anti-diagonal runs from the top-right cell (R1C9) to the bottom-left cell (R9C1).
Visual: Both diagonal cells are tinted blue so you always know which cells carry the extra constraint. The center cell (R5C5) sits on both diagonals.
Strategy: Treat each diagonal as a tenth and eleventh "box" — scan them like you would any row or column. The center cell is the most constrained: it must satisfy its row, its column, its box, and both diagonals simultaneously. Start there if it has many given neighbours. Placing a digit on one diagonal often forces its position on the other diagonal via the shared center cell.
Hyper Sudoku
Medium difficulty
Rules: All standard sudoku rules apply. In addition, four inner 3×3 windows (highlighted in green) must each contain the digits 1–9 exactly once.
Window positions (1-indexed rows and columns):
· Top-left window: rows 2–4, columns 2–4
· Top-right window: rows 2–4, columns 6–8
· Bottom-left window: rows 6–8, columns 2–4
· Bottom-right window: rows 6–8, columns 6–8
Each window overlaps with two standard 3×3 boxes. Green-tinted cells belong to one of the four hyper regions.
Strategy: Many cells belong to both a standard box and a hyper window — this double membership drastically narrows candidates. When a digit is placed in a corner box, check whether its row and column also eliminate it from the overlapping hyper window. Treat each green window exactly like a standard box when scanning. Cells at the centre of each window (R3C3, R3C7, R7C3, R7C7) are the most constrained.
Adjacency Constraints
♞
Anti-Knight
Medium difficulty
Rules: All standard sudoku rules apply. In addition, no two cells that are a chess knight's move apart may contain the same digit. A knight moves in an L-shape: two squares in one direction and one square perpendicular (or vice versa).
From any given cell, up to 8 other cells are a knight's move away. For a cell at RrCc, those cells are at offsets (±1, ±2) and (±2, ±1) from (r, c).
Note: No pre-filled digit clues are given — the anti-knight constraint alone, combined with standard rules, makes the puzzle uniquely solvable.
Strategy: The knight's constraint reaches across box boundaries, so a digit placed in one box eliminates candidates in cells two boxes away. Systematically list all knight-move targets when you place a digit. Cells in the very centre of the grid (around R5C5) have the most knight-move neighbours and are thus the most restricted. Corner cells have fewer knight-move targets and can be easier starting points.
♚
Anti-King
Medium difficulty
Rules: All standard sudoku rules apply. In addition, no two diagonally adjacent cells (sharing only a corner) may contain the same digit. This mimics the king's move in chess — but only the diagonal component adds new information, since the orthogonal neighbours are already covered by row and column rules.
For a cell at RrCc, its four diagonal neighbours are at (r−1, c−1), (r−1, c+1), (r+1, c−1), and (r+1, c+1).
Note: Like anti-knight, no digit clues are given — the constraint alone makes the puzzle solvable.
Strategy: The diagonal adjacency rule adds constraints at every box boundary corner. When two 3×3 boxes meet at a corner, the digits at those corners can no longer match. Pay close attention to where boxes touch diagonally — this is where the anti-king rule adds the most new eliminations beyond standard sudoku logic.
≠±1
Non-Consecutive
Medium difficulty
Rules: All standard sudoku rules apply. In addition, no two orthogonally adjacent cells (sharing an edge — not a corner) may contain consecutive digits. Two digits are consecutive if they differ by exactly 1 (e.g. 3 and 4, or 8 and 9).
· 4 and 5 cannot be horizontal or vertical neighbours.
· 4 and 6 can be neighbours (difference is 2).
· Diagonal adjacency is not affected — only the four orthogonal neighbours matter.
Strategy: Every digit has at most two "forbidden neighbours" (the digit above and below it). For the digit 1 that means only 2 is forbidden; for 9 only 8 is forbidden. For 5, both 4 and 6 are forbidden from all four orthogonal neighbours simultaneously — making 5 one of the hardest digits to place and a great starting point for eliminations. This constraint eliminates roughly half of all candidate placements, so fewer given digits are needed for a unique solution.
Between-Cell Constraints
XV
XV Sudoku
Medium difficulty
Rules: All standard sudoku rules apply. In addition, marks are placed between adjacent cells:
· X between two cells means those two cells sum to exactly 10.
· V between two cells means those two cells sum to exactly 5.
· No mark between two cells means they do not sum to 10 or 5. This negative constraint is just as important as the marks themselves.
Marks appear between horizontally and vertically adjacent cells only. The marks are rendered directly on the grid between the relevant cells.
Strategy: Use the negative constraint aggressively — whenever two unmarked adjacent cells share a candidate pair that would sum to 10 or 5, eliminate that pair immediately. Valid X pairs: {1,9} {2,8} {3,7} {4,6}. Valid V pairs: {1,4} {2,3}. Note 5+5=10 is impossible (no repeats) and single-digit 5 cannot form a V. When you know one cell of an X or V pair, the other is determined by subtraction.
⚫⚪
Kropki Sudoku
Medium–Hard
Rules: All standard sudoku rules apply. Dots appear between orthogonally adjacent cells:
· White dot — the two cells are consecutive (they differ by exactly 1). E.g. 3 & 4, or 7 & 8.
· Black dot — one cell is exactly double the other. Valid pairs: {1,2} {2,4} {3,6} {4,8}.
· No dot between two cells — they are neither consecutive nor in a 1:2 ratio. This negative constraint must be enforced.
Dots appear between all adjacent pairs where the relationship holds; a missing dot is a definite "neither" relationship.
Strategy: Black dots are powerful — only four valid pairs exist ({1,2} {2,4} {3,6} {4,8}), and knowing one cell pins the other to at most one value. White dots give two options but the negative constraint (no dot = not consecutive) is often more useful: most adjacent pairs in a finished puzzle are not consecutive, so cells with no dots between them eliminate a surprising number of candidates. Start with black dots and corner/edge cells where fewer neighbours exist.
Line Constraints
German Whispers
Medium–Hard
Rules: All standard sudoku rules apply. One or more green lines are drawn through a sequence of cells. Any two cells that are adjacent on the line must have values that differ by at least 5.
· Adjacent on the line means directly next to each other in the sequence — not just grid neighbours.
· The constraint applies to every consecutive pair along the line, not just the endpoints.
· Cells not on any green line are unaffected.
Strategy: The digit 5 can never appear on a whisper line — no adjacent digit can differ from 5 by 5 or more while also being in range 1–9 (4 differs by 1, 6 differs by 1; you'd need 0 or 10). Every cell on the line must be either low (1–4) or high (6–9), and they must strictly alternate: low → high → low → … along the line. If you know any one cell's value, you immediately know whether the next must be low or high, cutting candidates in half. Start with the endpoints — they only have one neighbour on the line.
Palindrome Sudoku
Medium difficulty
Rules: All standard sudoku rules apply. One or more purple lines mark palindrome sequences. The digits along each line, read from either end, form the same sequence — like a word palindrome but with numbers.
· The first cell and the last cell of a palindrome line must contain the same digit.
· The second cell and the second-to-last cell must match.
· This continues inward — for a line of length n, cells at positions i and n−1−i must be equal.
· For odd-length lines, the middle cell has no mirror and is unconstrained by the palindrome rule (but still constrained by standard rules).
Strategy: Mirrored cells are linked — placing a digit in one immediately fixes its mirror. Treat mirrored pairs as a single entity when scanning rows, columns, and boxes. If two cells that mirror each other lie in the same row, column, or box, they cannot share the same digit — which means that line position is impossible and eliminates candidates immediately. Short lines (length 2 or 3) are the most restrictive: a 2-cell palindrome forces both cells to be equal, which combined with row/col/box rules means only one digit can work.
Group Constraints
Renban Sudoku
Medium difficulty
Rules: All standard sudoku rules apply. Purple outlined groups of connected cells must collectively contain a set of consecutive digits — no gaps, no repeats, in any order within the group.
· A renban group of size n must contain exactly n consecutive digits. For example, a group of 4 cells could hold {3,4,5,6} in any arrangement.
· The digits within a renban group are all distinct (no repeats), and they form an unbroken run.
· Cells not in any renban group are unaffected by this rule.
Strategy: Renban groups act like mini-regions with a consecutive-run constraint. For a group of size n, the minimum possible digit is 1 and the maximum is 10−n (since you need n consecutive values up to 9). A group of 7 cells can only use {1–7}, {2–8}, or {3–9}. Combine this range restriction with the values already placed in the group's row, column, or box to determine which run is possible. If you know even one digit in a group, you can bound the entire group's range.
Killer Sudoku
Hard — no digit clues given
Rules: All standard sudoku rules apply. No pre-filled digit clues are given. Instead, the grid is partitioned into cages marked with dashed outlines. Each cage shows a small number in its corner — that is the sum of all digits within the cage. Two additional rules apply:
· Digits within a cage must sum to the cage's target number.
· No digit may repeat within a cage (even if the cage spans multiple rows or columns).
A cage can contain 1 to 9 cells. A 1-cell cage immediately reveals that cell's digit.
Strategy: Learn the "killer combos" for small cages — a 2-cell cage summing to 3 can only be {1,2}; summing to 17 can only be {8,9}. Unique combos also exist for 2-cell sums of 4 ({1,3}), 16 ({7,9}), and many others. For large cages, the sum constrains the average value per cell. Use the "45 rule": every row, column, and box sums to 45 — subtract the known cage sums within a region to find the remainder, which must be accounted for by the remaining cells. This often reveals a hidden single-cell cage equivalent.
Parity Constraints
E/O
Even/Odd Sudoku
Medium difficulty
Rules: All standard sudoku rules apply. Every cell is pre-marked with its required parity:
· Blue cells must contain an even digit: 2, 4, 6, or 8.
· Amber/yellow cells must contain an odd digit: 1, 3, 5, 7, or 9.
Every cell carries a parity mark — there are no neutral cells. The parity of every cell is fixed and shown visually by the background tint. You only need to determine the exact digit.
Strategy: Parity marks immediately halve the candidates for every cell — blue cells have 4 options, amber cells have 5. Scan for rows, columns, or boxes where parity marks leave only one slot for a particular even or odd digit. For example, if a row has only one blue cell, all four even digits must compete for it — combine that with the box and column to find which one fits. Even digits (4 of them) are slightly more constrained than odd ones (5 of them), so start with blue cells when stuck.