Guide
Classic vs Killer Sudoku: rules, difficulty and strategy
The rule differences
Classic Sudoku hands you a partially filled grid. Every row, column and 3×3 box must end up containing each digit from 1 to 9 exactly once, and a well-formed puzzle has exactly one solution.
Killer Sudoku keeps that rule and adds cages: dashed outlines covering one or more cells with a small total in the corner. The digits in a cage must add up to that total, and a digit cannot repeat inside the same cage. Because Killer grids usually start empty, the cage sums are your only entry point.
Difficulty compared
Difficulty is not a straight line between the variants. An easy Killer puzzle with plenty of small cages can be friendlier than an evil Classic grid that needs chains and forcing nets. What changes is the type of difficulty.
Classic gets harder by removing givens and requiring deeper techniques: hidden pairs, X-Wing, swordfish, colouring. Killer gets harder by using larger cages, where a 3-cell cage summing to 15 has many combinations and gives you almost nothing until other constraints narrow it down.
Strategy shifts when you move to Killer
Learn the locked sums. Some totals have only one combination: 3 in two cells is 1+2, 17 in two cells is 8+9, 6 in three cells is 1+2+3, 24 in three cells is 7+8+9. These are free placements on almost every board.
Use the rule of 45. Every row, column and box totals 45. If the cages inside a box add up to 40 and one cell sticks out, that cell is 5. This "innie and outie" technique has no equivalent in Classic Sudoku.
Pencil-mark combinations, not just candidates. In Classic you note which digits can go in a cell. In Killer you first note which digit sets can fill a cage, then intersect them with the row, column and box constraints. Our Sudoku cage-sum calculator lists every valid combination instantly.
Which should you play?
Play Classic if you want a quick, meditative session or you are still building speed with scanning and pencil marks. Play Killer if Classic has stopped surprising you and you enjoy arithmetic deduction. Many players alternate: Classic on a commute, Killer when they have a longer, quieter stretch.
If you want more variety after that, SodokuDX also has 16×16 Hexadoku and mini 6×6 and 4×4 grids.
Frequently asked questions
What is the main difference between Classic and Killer Sudoku?
Classic Sudoku gives you starting digits and no arithmetic. Killer Sudoku usually starts with an empty grid and adds dashed cages with a target sum, so you combine normal Sudoku logic with addition.
What are the Killer Sudoku rules?
Each row, column and 3×3 box holds the digits 1–9 once. In addition, the digits inside each dashed cage must add up to the small number printed in its corner, and no digit may repeat inside a cage.
Is Killer Sudoku harder than Classic Sudoku?
For most players yes, because there are few or no given digits and you must find cage combinations before normal techniques apply. Once you learn cage sums, an easy Killer can be gentler than an evil Classic grid.
Which Sudoku variant should a beginner start with?
Start with Classic Sudoku on easy or medium until scanning and pencil marks feel automatic, then move to Killer Sudoku and use a cage-sum calculator while you learn the combinations.