How to Play 16x16 Sudoku

16x16 sudoku (also called giant sudoku or hexadoku) is played on a grid of 16 rows and 16 columns. You fill every cell with one of 16 symbols so that each row, each column and each 4x4 box contains every symbol exactly once. It is the classic 9x9 game scaled up, with the same logic and no arithmetic. This guide is for players who know 9x9 and want to try the bigger grid.

What are the rules of 16x16 sudoku?

• The grid has 256 cells: 16 rows, 16 columns and sixteen 4x4 boxes.
• There are 16 different symbols, and each appears exactly once in every row, column and box.
• Some cells are filled in at the start (the givens). You place the rest.
• A well-made puzzle has one solution, which you can reach by logic alone. You never need to guess.

That is the whole rule set. Only the size changes: a 4x4 box holds 16 cells instead of 9, and each unit needs 16 symbols instead of 9.

Which symbols does 16x16 sudoku use?

Sixteen symbols do not fit into the digits 1 to 9, so puzzles borrow letters. On sudoku69 the symbols are 1 to 9 followed by A, B, C, D, E, F and G, one character per cell. Other sites and books use other sets: 0 to 9 then A to F (the hexadecimal digits, hence "hexadoku"), or plain numbers 1 to 16. The symbols are only labels. Nothing in the puzzle depends on their value, so A is not "10" in any arithmetic sense. If you switch between sites, check the symbol set before you start.

How is 16x16 different from 9x9?

The rules and techniques are the same. What changes is scale:

• More candidates per cell. An empty cell can start with up to 16 candidates, so notes fill up fast.
• More cells to scan. Each row, column and box has 16 cells, so finding the one place a symbol can go takes longer.
• Longer sessions. There are 256 cells to resolve instead of 81.
• Bigger boxes. A 4x4 box is a stronger constraint per symbol than a 3x3 one, and box scans are usually the quickest way in.

9x9 and 16x16 compared

The given counts come from the puzzles we generated ourselves. On 16x16 the Easy range is 130 to 154 givens and the Legendary range is 88 to 101. Even our hardest 16x16 puzzles keep over a third of the grid filled (94 of 256 on average). Fewer givens do not automatically mean a harder puzzle, because difficulty follows the hardest technique needed, not the clue count.

How do you solve a 16x16 sudoku?

1. Start with the fullest box. Pick the 4x4 box, row or column with the most symbols already placed. Fewer symbols are missing, so fewer cells compete for them.
2. Scan by symbol. Choose a symbol, for example C. Look at each box in turn and ask which cells in it are not blocked by a C in the same row or column. When only one cell is left, place it. This is a hidden single.
3. Then scan by cell. For an empty cell, list the symbols already in its row, column and box. If only one remains, place it. This is a naked single. Both are covered in naked and hidden singles.
4. Pencil in candidates once singles run out. Write small notes in each open cell. Update them every time you place a symbol, or the notes turn into a source of mistakes.
5. Use the same advanced techniques as 9x9. Pairs, pointing pairs, X-Wing and the rest all apply. The patterns are the same, only bigger.

Tips for a big grid

• Work box by box in a fixed order so you do not lose your place.
• Keep notes tidy. Most 16x16 errors come from stale candidates, not from weak logic.
• Take breaks. A fresh look often shows a single you scanned past.
• Fix a wrong placement as soon as you spot it. One error spreads across 256 cells.
• Start at the Easy level to get used to the letters, then move up.

How hard is a 16x16 sudoku?

Difficulty levels are the same as for 9x9 on sudoku69: Easy, Medium, Hard, Expert, Master and Legendary, set by the Sudoku Explainer rating of the hardest step needed. Easy is rated 1.0 to 1.9 and Legendary starts at 10.0. Our 16x16 Easy puzzles peak at 1.5 and our hardest 16x16 puzzle is rated 10.4. A puzzle our rater cannot solve by logic is never published. Easy 16x16 puzzles are gentle in logic but take longer than a 9x9 Easy puzzle because there is more grid to scan.

Where can you play 16x16 sudoku?

You can play 16x16 sudoku on sudoku69 at 16x16 Easy and the five levels above it, using the 1 to 9, A to G symbols described above. For other grid sizes, see giant and mini sudoku for background.