How to play Killer Sudoku

Sudoku rules plus cages: the digits in each dotted cage add up to its total and never repeat. Also known as Sum Sudoku, Addoku.

▶ Play Killer Sudoku ⊞ Open the solver ∑ Combination calculator

The rules

The grid is 9×9 with nine 3×3 boxes, and the usual Sudoku rules apply: each row, column and box holds 1–9 once.
Dotted outlines called cages cover every cell. The small number in a cage's corner is its total.
The digits in a cage must add up to the total.
A digit cannot repeat inside a cage — even if the cage spans two boxes.
Usually no digits are given at the start: everything comes from the cage totals.

Playing Killer Sudoku on Puzzle Solvers

Choose a grid size and a difficulty, then press Generate Puzzle.
Click a cell and type a digit, or use the on-screen keypad. Notes switches to pencil marks.
Undo steps back; Force places one digit that has only one possible value; Check marks mistakes in red without giving away the answer.
Stuck? Send to Solver opens the same puzzle in the step-by-step solver. Send to Slate opens it as a board on the Slate whiteboard, to solve in front of a class.

Open the Killer Sudoku game →

Using the Killer Sudoku solver

① Build puzzle: click cells to group them into a cage, then type the cage total. Load the sample to see how it works.
② Check: the solver confirms the cages and totals make a valid puzzle.
③ Solve: step through cage arithmetic, box logic, hidden singles and naked pairs, each one explained.
Need combinations while you solve? Open the cage calculator.

Open the solver →

A worked example

A two-cell cage shows 4. The pairs that add to 4 with different digits are only 1+3 (2+2 would repeat a digit). So the cage holds a 1 and a 3 in some order.

Next to it, a three-cell cage in the same box totals 23. Three different digits adding to 23 must be 6+8+9. With 1, 3, 6, 8 and 9 accounted for, the box's remaining four cells must hold 2, 4, 5 and 7.

Strategies

Start with one-combination cages. Some totals can be made only one way: 3 in two cells is always 1+2, 17 in two cells is 8+9, 6 in three cells is 1+2+3 and 24 in three cells is 7+8+9.
Use the 45 rule. Every row, column and box adds up to 45. Add the cages that lie wholly inside a box; the difference from 45 is the sum of the cells left over — often a single cell.
Must-have digits. If a digit appears in every possible combination of a cage, it is definitely in the cage. A 4-cell cage totalling 12 must contain 1 and 2.
Combine with Sudoku logic. Cage combinations narrow the candidates; ordinary Sudoku singles and pairs then finish the job.
Innies and outies. When cages poke slightly into or out of a box, the 45 rule gives the sum of just those poking cells.

Common mistakes

Repeating a digit inside a cage because the cage crosses a box border — it is still not allowed.
Forgetting that cage totals are fixed sums, not products or differences.
Skipping the 45 rule. Nearly every hard Killer Sudoku needs it.

Where it comes from

Killer Sudoku grew out of Japanese sum-placement puzzles and was made famous by British newspapers in 2005. It is now one of the most popular Sudoku variants.

Frequently asked questions

Can a Killer Sudoku cage repeat a digit?

No. Unlike some arithmetic cage puzzles, digits never repeat inside a Killer Sudoku cage.

What is the 45 rule?

1+2+…+9 = 45, so every row, column and 3×3 box adds up to 45. Subtracting the cage totals you know from 45 reveals the sum of the cells that are left.

Where can I look up cage combinations?

Use the cage calculator, or the printable cage combinations table.

Is Killer Sudoku harder than Sudoku?

Usually, because there are no givens. But the cage totals add information, and easy Killer puzzles can be solved with combinations alone.

Can the solver explain each step?

Yes. Build the cages in the Killer Sudoku Solver and it explains every deduction — cage arithmetic, box logic, singles and pairs.

Keep going

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