Killer Sudoku: rules, cages and solving techniques

What is Killer Sudoku?

Killer Sudoku follows all the usual Sudoku rules and adds dotted cages. Each cage shows a target sum for the digits inside it. DIGIGMA uses 9×9 grids, usually with very few — or no — given digits.

A cell is one square. A box is a 3×3 group of cells. A cage can cross rows, columns or boxes: its outline and target sum are what define it.

Goal of the game

Fill the grid with the digits 1–9. As in standard Sudoku, each digit appears once in every row, column and 3×3 box. On top of that, the digits in each cage must add up exactly to the target shown. Every DIGIGMA puzzle has a logical solution, so you never need to guess.

Rules

• Standard Sudoku rules still apply: 1–9 appear once in every row, column and 3×3 box.
• The digits in each cage must add up exactly to the number shown.
• A digit cannot repeat within the same cage.

For example, a two-cell cage totalling 8 cannot be 4 + 4: both digits in a cage must be different.

How to start a grid

Start with an Easy puzzle from the Killer Sudoku library. Select a cell and enter a digit. If several possibilities remain, use Draft to note candidates.

Look first for the most constrained cages. A two-cell cage with a very small or very large total often has only one or two possible combinations. Then check those possibilities against the row, column and 3×3 box.

A simple step-by-step method

1. Start with cages that have very few possible combinations, such as 3, 4, 16 or 17 across two cells.
2. Use Draft when more than one combination is still possible.
3. Rule out combinations that clash with a digit already used in the row, column or 3×3 box.
4. Use standard Sudoku constraints to decide which digit goes in which cage cell.
5. When a cage is nearly complete, subtract the known digits from its target to find what remains.
6. After every placement, scan the rows, columns and boxes again for newly forced digits.
7. Keep switching between cage logic and Sudoku logic — that interplay is what solves the puzzle.

Useful cage combinations

You do not need to memorise every possible sum. A handful of combinations come up again and again and are worth recognising.

Combine cage logic with Sudoku

Suppose a cage must contain 1 and 2. If one of its cells sits in a row that already contains a 1, that cell must be 2. The same idea works with a column or 3×3 box.

When you’re stuck

• Recalculate the remaining total in cages that are partly filled.
• Check every remaining combination against the relevant rows, columns and 3×3 boxes.
• Look for cages that now have only one possible combination left.
• Return to standard Sudoku logic — solving a cage often creates a forced digit elsewhere.
• Avoid guessing. DIGIGMA puzzles are designed to move forward by deduction.

Common mistakes

• Looking only at cage sums and forgetting the Sudoku rules.
• Repeating a digit inside the same cage.
• Keeping a combination that has already become impossible because of a row, column or box.
• Spending too long on a large cage when a small cage is much more constrained.

Practical tips

• After placing a digit, update the remaining total for its cage.
• Cages contained entirely within one 3×3 box are often easier to exploit.
• In Draft, track both Sudoku candidates and cage-sum possibilities.
• Once the method feels natural, try a puzzle from the Killer Sudoku library.

Interactive tutorial

The tutorial below uses real cage logic with the same keypad as a normal puzzle. You will start with a one-cell cage, then use a column constraint to solve a two-cell cage. After that, open the Killer Sudoku library and try a full puzzle.

FAQ

Is Killer Sudoku harder than Sudoku?

It adds one extra layer — cage sums — but Easy puzzles are still very approachable. The key is learning to combine cage logic with standard Sudoku.

Do cages allow repeated digits?

Not on DIGIGMA. A digit never repeats inside a cage.

Should I solve cages or Sudoku first?

Use both at the same time. Start with the tightest cages, then immediately use rows, columns and boxes to narrow the possibilities.

Do I need arithmetic skills?

No. Basic addition is enough, and the common combinations quickly become familiar.

What should I do when I get stuck?

Recalculate the remaining cage totals, remove impossible combinations, then return to standard Sudoku singles.