Play killer sudoku free online, with a daily puzzle and as many new ones as you like in Standard or Tough. Tap any square and the cage helper lists every set of digits that can make its cage total, so you spend your time on logic instead of sums.
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How to play killer sudoku
Killer sudoku keeps every rule of a normal sudoku and adds one more layer:
- Every row, column and 3 by 3 box holds the digits 1 to 9 exactly once.
- The grid is split into cages, shown with dashed outlines. The small number in the top-left corner of a cage is the total of the digits inside it.
- A digit can never repeat inside a cage, even when the cage crosses into another box.
There are few or no starting numbers. Everything comes from the cage totals and the usual sudoku rules.
Here is the simplest case. A cage of two squares with a total of 3 can only hold 1 and 2, because 1 + 2 is the only way to make 3 with two different digits. You do not know which square is which yet, but you already know no other square in that row (if the cage lies in one row) can be a 1 or a 2. Moves like this one get a killer started.
Using the board
Tap a square and type or tap a digit. Notes switches to pencil marks, Check marks squares that are wrong, Undo steps back and Hint fills one square for you. The cage helper under the board shows the possible digit sets for the selected cage and crosses out any set that clashes with digits you have already placed in it. Your game and timer are saved on your device.
Standard puzzles use cages of two to four squares (now and then a one-square cage, which is simply a given). Tough puzzles use bigger cages of up to five squares, which leaves you with fewer, looser totals to work from.
Killer sudoku techniques
Learn the cages with only one answer
Some totals can only be made one way. Spot them and you get free digits for your notes straight away.
| Cage size | Total | Only possible digits |
|---|---|---|
| 2 squares | 3 | 1 + 2 |
| 2 squares | 4 | 1 + 3 |
| 2 squares | 16 | 7 + 9 |
| 2 squares | 17 | 8 + 9 |
| 3 squares | 6 | 1 + 2 + 3 |
| 3 squares | 7 | 1 + 2 + 4 |
| 3 squares | 23 | 6 + 8 + 9 |
| 3 squares | 24 | 7 + 8 + 9 |
| 4 squares | 10 | 1 + 2 + 3 + 4 |
| 4 squares | 11 | 1 + 2 + 3 + 5 |
| 4 squares | 29 | 5 + 7 + 8 + 9 |
| 4 squares | 30 | 6 + 7 + 8 + 9 |
| 5 squares | 15 | 1 + 2 + 3 + 4 + 5 |
| 5 squares | 35 | 5 + 6 + 7 + 8 + 9 |
The 45 rule
The digits 1 to 9 add up to 45, so every row, column and box totals 45. This is the most useful tool in killer sudoku. Add up the cages that sit completely inside a box. If they cover all but one square of the box, that square is 45 minus their total. If the cages inside a box add to 38 and one square is left over, that square is 7.
It also works the other way. If a group of cages covers a whole box plus one square that pokes out of it, the square sticking out equals the cages' total minus 45. You can use the same trick across two or three rows or columns at once, where the total is 90 or 135.
Rule out the impossible middle
Even cages with many combinations give something away. A two-square cage of 10 can be 1 + 9, 2 + 8, 3 + 7 or 4 + 6, but never 5, since 5 + 5 would repeat a digit. A three-square cage of 8 has to contain a 1, because its only sets are 1 + 2 + 5 and 1 + 3 + 4. The helper tells you when every remaining set shares a digit like this.
Mix cage logic with normal sudoku
Once a few digits are in, ordinary sudoku moves take over: a 1 locked in a cage within one row rules 1 out of the rest of that row, and singles start to appear. Switch between the two kinds of thinking and the grid opens up.
More sudoku to try
If you want classic rules with a sharper edge, play evil sudoku, where every hint names the technique. Samurai sudoku joins five grids into one, printable sudoku sheets are ready for a pencil, and Canoku swaps the numbers for Canadian emoji.