What Is Killer Sudoku? How to Solve It (Beginner's Guide)

If you've mastered regular sudoku and want a fresh twist, killer sudoku is the natural next step. It keeps everything you know about sudoku and adds a layer of arithmetic that makes the logic richer — without ever getting harder than simple addition. Here's exactly what killer sudoku is and how to start solving it.

The rules

A killer sudoku is a normal 9×9 sudoku grid overlaid with dashed outlines called cages. Two rule sets apply at once:

• Sudoku rules: fill the grid so 1–9 appears exactly once in every row, every column, and every 3×3 box.
• Cage rules: the digits in each dashed cage must add up to the small number printed in its corner — and no digit repeats within a cage.
• There are usually very few (or no) given digits; the cage sums carry the information instead.

The '45' rule

Every row, column and 3×3 box contains the digits 1–9, which always add up to 45. That single fact is your most powerful tool. If the cages inside a row account for 40, the leftover cell must be 5. Whole boxes work the same way: add the cages that sit fully inside a box, and the remainder must come from the cells that stick out. Hunting for these 'almost complete' regions is how most killer sudokus crack open.

Learn the cage combinations

Because digits can't repeat in a cage, small cages have very few possibilities — memorizing a handful gives you instant footholds:

• Two cells summing to 3 → must be 1+2. Summing to 4 → 1+3. Summing to 16 → 7+9. Summing to 17 → 8+9.
• Three cells summing to 6 → 1+2+3. Summing to 7 → 1+2+4. Summing to 23 → 6+8+9. Summing to 24 → 7+8+9.
• These 'locked' cages fix a small set of digits in a region even before you know their order — often enough to unlock a crossing line.

A simple solving approach

• Mark the locked cages (like 3-in-two = 1,2) — they pin digits to small areas.
• Use the 45 rule on rows, columns and boxes that are nearly covered by cages to deduce single cells.
• Apply normal sudoku scanning as digits appear — each one eliminates candidates in its row, column and box.
• Remember the no-repeat-in-a-cage rule to rule out candidates a cage can't use.
• Keep alternating between cage logic and sudoku logic; never guess — there's always a next deduction.

Try one now

The best way to learn is to solve a gentle one. Our free killer sudoku generator makes easy, medium and hard puzzles, each guaranteed to have a single unique solution. Play in your browser — it flags conflicts and can check or reveal your work — or print a clean PDF with the cage sums and an answer key. Start on easy, lean on the 45 rule, and killer sudoku will quickly become your new favorite.