What Is Mini Sudoku?

Mini Sudoku is a smaller version of Sudoku. The standard board is a 6×6 grid split into six boxes, each two rows tall and three columns wide. A few cells start filled in. You fill the rest so that every row, every column and every box contains the digits 1 to 6 exactly once. A well-made puzzle has one solution, and you reach it by logic alone: no guessing, and no arithmetic.

“Mini” simply sets it apart from the familiar 9×9. The 6×6 is the version LinkedIn features among its daily games, which is how a lot of people meet it today. The same rules stretch to other sizes too: this site also has 8×8 boards with 2×4 boxes and 10×10 boards with 2×5 boxes, and 4×4 versions with 2×2 boxes are common in puzzle books for younger solvers.

Every cell belongs to exactly one row, one column and one box, and it has to differ from every other cell in all three. On a 6×6 that is 12 other cells: five in its row, five in its column and two more in its box. Because there are only six digits, a cell is settled as soon as five different digits appear among those twelve neighbours. That is the first thing every solver looks for, and it is called a naked single.

The second idea turns the question around. Instead of asking which digit fits a cell, ask where a digit can go. If a row still needs a 4 and only one of its empty cells can take one, that cell is the 4, whatever else it might have held. Those two ideas, naked singles and hidden singles, solve most mini puzzles on their own. The harder boards add pencil marks and a few elimination patterns, all explained with real examples on the techniques page.

The numbers behind the grid are small enough to count. There are exactly 28,200,960 ways to fill a blank 6×6 grid legally, a figure you can check by brute force on a laptop in seconds. For a classic 9×9 the count is about 6.67 sextillion (6,670,903,752,021,072,936,960, computed by Bertram Felgenhauer and Frazer Jarvis in 2005). Puzzles on this site start with roughly 10 to 20 clues on a 6×6, and the fewer clues and the less obvious their placement, the harder the board.

The idea underneath Sudoku is much older than the puzzle. A Latin square is a grid in which each symbol appears once in every row and every column, and the Swiss mathematician Leonhard Euler studied them in the 18th century. A Sudoku grid is a Latin square with one extra rule: the boxes.

The puzzle as we know it first appeared in 1979, in a Dell puzzle magazine in the United States, under the name Number Place. It is generally credited to Howard Garns, an American puzzle constructor. In 1984 the Japanese publisher Nikoli began running it under a longer Japanese name, soon shortened to Sudoku, which means roughly “the digits must be single”. From 1986 Nikoli capped its puzzles at 32 clues and placed them symmetrically, and it has always favoured hand-made puzzles over computer-generated ones, a style that still shapes how good Sudoku is written.

The worldwide craze came two decades later. In November 2004 The Times in London began printing Sudoku, using puzzles produced by a computer program written by Wayne Gould, a New Zealander living in Hong Kong. Within about a year Sudoku was running in newspapers around the world. Variants followed quickly: killer Sudoku with its sum cages, giant 16×16 grids, and smaller grids like the 6×6 as a quicker way in. Today the 6×6 has a new audience as a short daily puzzle, including on LinkedIn.

• Size. 36 cells instead of 81, and six digits instead of nine. There is less to scan and less to remember.
• Box shape. Nine is a perfect square, so classic boxes are 3×3 squares. Six is not, so mini boxes are 2×3 rectangles. A box lines up with its side neighbour along two rows and with the boxes above and below along three columns, which makes row scans and column scans feel slightly different.
• Neighbours. Each cell has 12 others it must differ from, against 20 on a 9×9. Fewer neighbours means shorter candidate lists, so on easy boards you can often solve without writing pencil marks at all.
• Clues. A classic 9×9 needs at least 17 clues to have a unique solution, a result proved by computer search in 2012. Minis get by with far fewer because the grid is smaller.
• Time. A 6×6 is built for a short break. A 9×9, especially a hard one, is a longer sit-down puzzle.

What does not change is the logic. Every technique you learn on a 6×6 works on a 9×9, which is why the mini makes such a good practice ground.

Mini Sudoku sits in a family of grid puzzles that share the same “no repeats” idea. The main differences are the grid, the extra rule, and whether you need to do sums.

If you like Mini Sudoku, killer Sudoku is the natural next step when you want some arithmetic, and Queens scratches the same itch with shapes instead of digits.

You can play right here, free, with no account. There is a daily 6×6 that everyone gets on the same day, an unlimited mode for as many boards as you like, a library of 900 puzzles across 6×6, 8×8 and 10×10, and printable PDF packs with answer keys. New to the rules? The how-to-play guide walks through a real board step by step, and the LinkedIn Mini Sudoku guide has tips for players of LinkedIn's version.

More questions about playing on this site are answered in the Mini Sudoku FAQ.