How to Play Mini Killer Sudoku

Step 1: Understand the Grid and Basic Rules

Mini Killer Sudoku is defined by its smaller grid size. The most common version is 6x6, using the digits 1 through 6. You will also find 4x4 puzzles using digits 1-4. The standard Sudoku rules apply: every row, column, and outlined 2x3 or 2x2 box must contain all digits exactly once.

The key addition is the cage system. The grid is divided by dashed or colored lines into shapes called cages. Each cage has a small number in its corner, which is the sum of all digits inside that cage. No digit may repeat within a cage, even if the cage spans multiple rows, columns, or boxes. This rule is crucial for solving and is explained in detail in our guide to Killer Sudoku Cage Rules.

Step 2: Start With the Single Remaining Digit in a Cage

The most direct opening move is to find a cage where all but one digit are already placed. Subtract the sum of the known digits from the cage's total. The result is the digit for the empty cell. This is a powerful form of a Naked Single, where the cell's value is forced by arithmetic.

For example, in a 6x6 puzzle, a 3-cell cage has a printed sum of 12. If two cells in that cage are already solved as 3 and 4, the remaining cell must be 5 (because 12 - 3 - 4 = 5). Always scan for these quick placements at the start and after each new digit you enter.

Step 3: Use Minimum and Maximum Sums to Restrict Candidates

When you can't place a digit directly, use cage combinatorics. Calculate the minimum and maximum possible sums for a cage based on its size. For a cage with N cells in a 6x6 puzzle, the minimum sum is 1+2+...+N, and the maximum is (6+5+...+(7-N)).

This creates a 'sandwich' for valid digit combinations. If a cage's printed sum is very low or very high, it immediately eliminates many possibilities. For instance, a 3-cell cage in a 6x6 grid with a sum of 6 can only be {1,2,3}. This restriction helps you perform candidate elimination within rows, columns, and boxes.

• Write small pencil marks for possible digit pairs/triplets inside cages based on their sum.
• Cross-check these cage combos with the standard Sudoku constraints in the row and column.

Step 4: Apply Standard Sudoku Techniques on a Smaller Scale

Don't forget the classic techniques. After using cage logic to restrict candidates in a row, column, or box, look for Hidden Singles—the only cell left that can hold a specific digit. On small grids, these opportunities arise frequently.

Also, watch for interactions between cages and boxes. If a cage is entirely contained within one box, the sum of its digits is a fixed number you can use for elimination. Conversely, if a cage spills over box lines, you must consider the sum constraint across the boundary.

Step 5: Avoid Common Mini Killer Traps

A major pitfall is misapplying 'innie/outie' logic from larger Killer Sudokus. In a standard 9x9 puzzle, rows and columns sum to 45. In a 6x6, they sum to 21 (1+2+3+4+5+6). You can only subtract the printed sums of cages that are fully contained within a row or column from this total. If a cage crosses the boundary of your row, its partial sum is unknown, so the calculation fails.

Another trap involves misidentifying cage spans. If you think a cage covers a full row, its sum must be 21. A different printed total is a clear sign your assumed cage boundary is wrong; re-check the dashed lines. Finally, don't confuse this with Little Killer Sudoku Rules, which uses diagonal sums outside the grid. Mini Killer Sudoku cages are always inside the grid.

• Double-check cage borders. A single dashed line defines the separation.
• Remember the no-repeat-in-cage rule applies even if the cage spans a box. This is a common oversight.