What is the best strategy to start a Killer Sudoku?
A consistent opening strategy removes guesswork from the start of every Killer Sudoku puzzle. Follow these four steps in order and you will always have a productive direction.
Step 1 - Identify All Forced Cages
Before anything else, scan every cage for forced combinations - Those with only one possible digit set. Mark the candidates for each cell in such cages immediately. Key ones to memorise: 2-cell cages of 3 ({1,2}) and 17 ({8,9}); 3-cell cages of 6 ({1,2,3}) and 24 ({7,8,9}).
Step 2 - Apply the 45 Rule to All 27 Units
Work systematically through all 9 boxes, then all 9 rows, then all 9 columns. For each unit, add up cage sums entirely within it and subtract from 45. If the result reveals a single-cell innie or outie, place it immediately. Even two-cell results with a known sum restrict candidates significantly.
This step often produces the largest burst of free information at the start of medium and harder puzzles.
Step 3 - Use Cage Overlap With Units
Look at where cages overlap with rows, columns, and boxes. If all cells of a cage lie within a single row (or column), the cage's digits must appear in that row and can be removed as candidates from the rest of that row outside the cage. This is equivalent to the pointing pair technique in classic Sudoku, applied to cage digit sets.
Step 4 - Switch to Standard Sudoku Techniques
Once the Killer-specific techniques are exhausted, continue as a standard Sudoku with partial pencil marks. Apply naked and hidden singles, naked pairs, and (on harder puzzles) X-Wing and beyond. The boundary between Killer-specific and standard techniques blurs naturally as you gain experience.
For Easy Killer Puzzles
On Easy difficulty, steps 1 and 2 alone typically solve the puzzle or reduce it to a series of naked singles. You can usually skip pencil marks entirely for Easy if you follow the first two steps carefully.