WHAT STAYS THE SAME AS STANDARD SUDOKU
Killer Sudoku uses the same 9×9 grid with the same three constraints as standard Sudoku:
- Every row must contain the digits 1–9 exactly once.
- Every column must contain the digits 1–9 exactly once.
- Every 3×3 box must contain the digits 1–9 exactly once.
These three rules are non-negotiable. Every deduction in Killer Sudoku still relies on them, the same way they do in standard Sudoku. If you can solve a basic Sudoku, you already know all the grid logic you need.
THE ONE NEW RULE: CAGES
The difference is that Killer Sudoku removes the given digits entirely. Instead, the grid is divided into groups of cells called cages, outlined in dotted lines. Each cage has a number in the corner — the cage sum.
The cage rule has two parts:
- The digits inside the cage must add up to the cage sum.
- No digit may repeat within a single cage.
That second part — no repeats within a cage — is crucial. A cage sum of 4 in a 2-cell cage cannot be solved with two 2s. It must be 1+3. A cage sum of 3 in a 2-cell cage must be 1+2 (the only combination that totals 3 without repeating).
Cages can span across row, column, or box boundaries — they do not have to stay within one region. Most cages contain 2–5 cells. Large cages (6+ cells) appear less often and are usually solved last.
STARTING A PUZZLE: UNIQUE-SUM CAGES FIRST
Some cages have only one possible combination of digits. These are your entry points — start with them before anything else.
The most useful ones to memorise:
- 2-cell cage summing to 3 — must be 1+2 (only option).
- 2-cell cage summing to 4 — must be 1+3.
- 2-cell cage summing to 17 — must be 8+9.
- 2-cell cage summing to 16 — must be 7+9.
- 3-cell cage summing to 6 — must be 1+2+3.
- 3-cell cage summing to 23 — must be 6+8+9.
- 3-cell cage summing to 24 — must be 7+8+9.
When a cage has a unique combination, you know exactly which digits belong in it — you just do not know their order yet. Still, knowing which digits are committed to those cells eliminates them as candidates from all other cells in the same rows, columns, and boxes. This is often enough to unlock the rest of the region.
THE REGION SUM TECHNIQUE
This technique is more powerful than it sounds. A complete row, column, or box always contains 1 through 9 — which sums to 45. If you know the cage sums of every cage that fits entirely within a region, and some cages extend beyond the region, you can deduce exactly which digits cross the boundary.
Example: in a box where the four cages wholly inside sum to 37, the remaining cells must contain 45 − 37 = 8. That means whichever cage extends out of the box contributes exactly 8 from its cells inside the box — immediately constraining which digits can be in those cells.
You do not need to use this on every puzzle at Tier 1, but recognise it when a region's cages are nearly all inside the boundary. It is the key technique that makes Killer Sudoku feel logical rather than guesswork.
COMMON BEGINNER MISTAKES
A few errors come up repeatedly in new solvers:
- Forgetting the no-repeat rule inside cages. Standard Sudoku only forbids repeats within rows, columns, and boxes. In Killer Sudoku, cages add a fourth constraint. Forgetting this leads to cages like 1+1+2 for a sum of 4, which is illegal.
- Treating cage combinations as fixed before placement. Knowing a 2-cell cage must be 8+9 tells you the digits but not which cell gets which. You still need to use row/column/box logic to determine placement.
- Ignoring cages that span two regions. A cage crossing a box boundary constrains two regions at once. New solvers often focus on the cage sum and miss that the cell positions narrow digits in both boxes.
- Skipping pencil marks for multi-option cages. When a cage has more than one possible combination (a 2-cell cage summing to 9 could be 1+8, 2+7, 3+6, or 4+5), write the candidates in each cell the same way you would in standard Sudoku. Eliminating one candidate from any cell automatically eliminates the corresponding partner digit from the other.
HOW HARD IS IT COMPARED TO STANDARD SUDOKU?
At beginner level, Killer Sudoku is not harder than standard Sudoku. Unique-sum cages give you immediate digit information in the first move — something that does not exist in standard Sudoku, where you must wait for enough given digits before any cell becomes forced.
The difficulty curve is steeper, however. Tier 3 Killer Sudoku requires the region-sum technique and the ability to hold multiple cage combinations in mind simultaneously. Tier 4–5 introduces overlapping constraints between cages in adjacent boxes that can feel opaque until the technique clicks.
The recommendation: start at Tier 1, which provides puzzles solvable with just the unique-sum approach. When those feel comfortable and you want more, Tier 2 introduces multi-option cages that require standard Sudoku scanning to narrow down. Move to Tier 3 when Tier 2 is no longer surprising you — that is the natural threshold for the region-sum technique.