Play killer sudoku free online
Killer sudoku is sudoku with sums. There are no numbers printed in the grid to start you off. Instead, the grid is divided into cages, groups of squares drawn with a dotted line, and each cage has a small number in its corner: what its digits add up to. Choose Easy, Medium or Hard. Every puzzle has exactly one answer and can be solved by reasoning alone: the computer works each one through before you see it. Nothing to sign up for, and a puzzle you start is saved in this browser.
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How to play killer sudoku
The rules are the rules of sudoku, plus one about the cages:
- Rows, columns and boxes. Fill every square with a digit from 1 to 9 so that each row, each column and each of the nine 3 by 3 boxes (the heavy lines) holds every digit once.
- Cages add up. The digits in a cage add up to the number in its corner.
- No repeats in a cage. A digit appears at most once in a cage, even when the cage crosses from one box into another.
A cage of two squares marked 3 can only be a 1 and a 2, so you know those two squares straight away, though not yet which way round. A cage of two marked 10 could be 4 + 6, 3 + 7, 2 + 8, 1 + 9, but never 5 + 5, because a digit cannot repeat in a cage.
Controls
Tap or click a square, then tap a number below the grid. Tap the same number again to clear it. Turn on Notes to jot small possible numbers in a square instead; writing a number takes it out of the notes in its row, column, box and cage for you. Erase clears a square, and Undo takes back any move, as often as you like.
With a keyboard, the arrow keys move around the grid and the number keys write. Hold Shift with a number to write a note, or press N to switch notes on and off. Backspace or Delete erases, U undoes and H asks for a hint.
Two of the tools help when you are stuck, and the result says when you used them. Hint fills in one square and says why in one plain sentence; if one of your numbers is wrong, it finds that first. Check marks in red any number that is not right, without filling anything in. A number that breaks a rule you can see, such as a digit twice in a row or a cage over its sum, shows in red as soon as you write it.
The 45 rule
The digits 1 to 9 add up to 45, so every row, every column and every box adds up to 45. This is the most useful idea in killer sudoku. Look for a box, row or column that the cages almost fill. Say the top left box holds three cages that sit wholly inside it, adding up to 15, 11 and 12, and one square that belongs to a cage reaching into the next box. The three cages make 15 + 11 + 12 = 38, so that last square must be 45 − 38 = 7.
It works the other way too. If the cages that cover a box add up to 52, and only one square of them sticks out of the box, that square is 52 − 45 = 7. Two rows side by side add up to 90, three add up to 135, and the same counting works for them; the hard puzzles need it.
Killer sudoku combinations
Most cages can be filled several ways: a cage of three adding up to 15 has 8. A few sums can be made only one way, and they are the best places to start. These are all of them for cages of two to five squares:
| Squares | Sum | Digits |
|---|---|---|
| 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 | 16 | 1 2 3 4 6 |
| 5 | 34 | 4 6 7 8 9 |
| 5 | 35 | 5 6 7 8 9 |
The pattern is easy to remember: the smallest and largest sums for each size are made only one way, and so is the next one in from each end. A cage of nine squares is a whole row's worth of digits and always adds up to 45. For every size and sum, use the killer sudoku calculator.
Killer sudoku tips for beginners
- Start with the sure cages. A single square on its own is its sum. Then find the cages from the chart above: a 2-square 17 is 8 and 9, a 3-square 6 is 1, 2 and 3.
- Use the 45 rule early. Scan each box, then each row and column, for one or two squares the whole cages leave out.
- Write the combinations as notes. For a cage of two marked 4, note 1 and 3 in both squares. When one of them is settled, the other follows.
- Follow a cage inside a box. If a cage that must hold a 9 lies within one box, no other square in that box can be 9.
- Mind the repeats. A 2-square cage of 4 is 1 and 3, never 2 and 2, which rules out a 2 in either square.
- Check before you go further. If the puzzle stops making sense, press Check: one early slip can make everything after it look impossible.
Easy, medium and hard killer sudoku
- Easy puzzles have small cages of two or three squares and some squares on their own. The cage sums and the ordinary sudoku rules are all you need, so they are the place to learn.
- Medium puzzles have larger cages and few single squares. Expect to use the 45 rule on a row, column or box, and to notice a digit that must sit in one line of a box.
- Hard puzzles have the largest cages, up to six squares, and need the 45 rule across two or more rows or columns side by side.
At every level there is one answer and a way to reach it by reasoning, so a guess is never needed. The daily killer sudoku changes level through the week.