Killer Sudoku
Sudoku with a twist: fill the grid using the sums of the dotted cages. Every generated puzzle has exactly one solution, and the cage helper lists which digit combinations can still make each sum.
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Select a cell to see the possible digit combinations for its cage.
How to use the Killer Sudoku
- Pick a level. Easy and Medium start with a few digits filled in; Hard and Expert give you only the cage sums.
- Select a cell and tap a number on the pad or type 1–9. Use the arrow keys to move around.
- Use the cage helper to see which combinations of different digits add up to the selected cage's total.
- Take notes: switch Notes on (or hold Shift while typing) to pencil in candidates.
- Complete the grid so every row, column and 3×3 box holds 1–9 and every cage adds up to its total without repeating a digit.
Killer sudoku combinations cheat sheet
These cage totals can be made in only one way, so they are the best places to start. Remember that digits never repeat inside a cage.
| Cells | Total | Only combination |
|---|---|---|
| 2 | 3 | 1 + 2 |
| 2 | 4 | 1 + 3 |
| 2 | 16 | 7 + 9 |
| 2 | 17 | 8 + 9 |
| 3 | 6 | 1 + 2 + 3 |
| 3 | 7 | 1 + 2 + 4 |
| 3 | 23 | 6 + 8 + 9 |
| 3 | 24 | 7 + 8 + 9 |
| 4 | 10 | 1 + 2 + 3 + 4 |
| 4 | 11 | 1 + 2 + 3 + 5 |
| 4 | 29 | 5 + 7 + 8 + 9 |
| 4 | 30 | 6 + 7 + 8 + 9 |
| 5 | 15 | 1 + 2 + 3 + 4 + 5 |
| 5 | 35 | 5 + 6 + 7 + 8 + 9 |
A two-cell cage totalling 10 can never contain a 5 (5 + 5 repeats a digit), and a two-cell cage can never total less than 3 or more than 17.
Solving techniques that work
- The 45 rule: every row, column and box adds up to 45. Subtract the cages that sit fully inside a box from 45 to find the total of the remaining "innie" cells. If one cage pokes out of the box by a single cell, that "outie" equals the cage total minus what is left inside.
- Combination elimination: a 3-cell cage of 20 can be 3+8+9, 4+7+9, 5+6+9 or 5+7+8. If none of its cells can be 9, it must be 5+7+8.
- Odd and even: a two-cell cage with an odd total always holds one odd and one even digit.
- Classic sudoku logic still applies: naked and hidden singles, pairs and pointing pairs all work alongside the cage arithmetic.
Frequently asked questions
What are the rules of killer sudoku?
Fill the 9×9 grid so that every row, column and 3×3 box contains the digits 1 to 9 once. The grid is also divided into dotted cages: the digits in each cage must add up to the small number in its corner, and a digit can't repeat inside a cage.
What is the 45 rule in killer sudoku?
The digits 1 to 9 add up to 45, so every row, column and box totals 45. If the cages lying completely inside a box add up to 38, the cells of that box that are left over must total 45 − 38 = 7. Using this rule on rows, columns and boxes is the key technique in harder puzzles.
Which cage sums are the easiest to solve?
Sums with only one possible combination: 3 in two cells is always 1+2, 4 is 1+3, 16 is 7+9 and 17 is 8+9. In three cells, 6 is always 1+2+3, 7 is 1+2+4, 23 is 6+8+9 and 24 is 7+8+9. The cage helper shows these instantly.
How are the difficulty levels different?
Easy puzzles start with about 14 digits filled in and use mostly small cages. Medium starts with about 5 digits. Hard and Expert give no digits at all, only cage sums, and Expert uses larger cages that allow many more combinations.
Does every puzzle have a unique solution?
Yes. Each puzzle is generated in your browser and checked by a solver; if a second solution is possible the cages are adjusted until only one remains.
Can I play with the keyboard?
Yes. Arrow keys move, 1–9 enter a digit, Shift+1–9 adds a note, Backspace erases, N toggles notes mode, H gives a hint, P pauses and Ctrl+Z undoes.
Is killer sudoku harder than normal sudoku?
Usually yes at the same level, because you start with few or no digits and must use arithmetic as well as logic. On the other hand, small cages with unique combinations give you footholds that a normal sudoku doesn't have.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.