Hexadoku Online: The 16-Symbol Sudoku Challenge

Hexadoku is a 16×16 logic puzzle played with sixteen unique symbols — digits 1 through 9 and letters A through G — across 256 cells divided into sixteen 4×4 boxes. Each symbol must appear exactly once in every row, every column, and every box, with exactly one logically reachable solution guaranteed. Play Hexadoku free at SudokuPro across six difficulty levels — no sign-up or registration required.

What Is Hexadoku?

Hexadoku is the 16×16 version of Sudoku. Instead of the familiar 9×9 grid with nine digits, Hexadoku expands the puzzle to sixteen rows, sixteen columns, sixteen 4×4 boxes, and sixteen symbols: 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, and G.

The name combines the idea of hexadeca — sixteen — with doku, the same root associated with Sudoku. It describes the puzzle directly: a Sudoku-style logic grid built around sixteen distinct symbols.

The letters A–G do not carry mathematical value. They work exactly like numbers: each one is a logical placeholder that must appear once per row, once per column, and once per 4×4 box. After one or two puzzles, most solvers stop reading the letters as letters and start treating them as natural parts of the symbol set.

Hexadoku is also known as 16×16 Sudoku, Super Sudoku, or Alphabet Sudoku. If you enjoy classic Sudoku but want a larger grid, denser candidate logic, and more advanced pattern recognition, Hexadoku is one of the strongest variants to play.

Characteristics of Hexadoku

• Grid: 16×16 = 256 cells total
• Boxes: Sixteen 4×4 square sub-grids
• Symbol pool: Digits 1–9 plus letters A, B, C, D, E, F, G
• Starting clues: 36–120 depending on difficulty level
• Unique solution: Every puzzle is verified to have exactly one logically reachable answer
• Core rule: Every row, column, and 4×4 box must contain every symbol exactly once
• Best feature: The larger grid creates more frequent advanced patterns than standard 9×9 Sudoku

Hexadoku keeps the elegance of classic Sudoku while multiplying the solving space. The rules stay simple, but the grid becomes much more demanding because each empty cell may begin with up to sixteen candidates instead of nine.

Hexadoku Difficulty Levels

Six levels, all free, no registration required:

• Hexadoku Easy — 110–120 starting clues; the gentlest entry into 16-symbol solving
• Hexadoku Medium — 92–105 clues; hidden singles and pointing pairs across 16 rows
• Hexadoku Hard — 78–90 clues; naked pairs, hidden pairs, and locked candidates
• Hexadoku Expert — 62–74 clues; X-Wing, Swordfish, Jellyfish, and first Squirmbag patterns
• Hexadoku Extreme — 50–60 clues; full fish hierarchy, XYZ-Wing, and 12–18-link AIC chains
• Hexadoku Evil — 36–48 clues; Kraken fish, ALS chains, and 18–30-link AIC networks

Why Hexadoku Is the Ultimate Sudoku Variant

Hexadoku sits near the top of mainstream Sudoku variants because three factors compound with every difficulty level.

Scale. At 256 cells, Hexadoku is more than three times larger than classic 9×9 Sudoku. Every familiar technique still applies, but each one now operates across a wider deduction web. A single placement can affect a long chain of rows, columns, boxes, and candidate sets.

The extended symbol set. Adding A–G to the usual digits increases the working memory required for every solve. This is part of the challenge. Once the symbols become familiar, many players find that returning to 9×9 Sudoku feels unusually spacious and clear.

Pattern density. With 16 rows and 16 columns, advanced patterns appear more often. X-Wing, Swordfish, Jellyfish, Squirmbag, ALS chains, and AIC networks all have more room to form. This makes Hexadoku a strong training ground for solvers who want consistent practice with advanced logic.

Ready to begin? Start with Hexadoku Easy, or review foundational strategy at the full How to Play Sudoku guide. For a different scale, explore 12×12 Sudoku or the classic 9×9 Sudoku.