Killer Sudoku Online Free
Killer Sudoku is Sudoku with sums. The grid starts empty. Instead of printed digits you get cages: groups of cells outlined with a dashed line, each with a small number in its corner. The digits inside a cage must add up to that number, and the usual Sudoku rules still hold for every row, column and 3×3 box.
This page gives you an unlimited supply of new Killer Sudoku puzzles in four levels, Easy, Medium, Hard and Expert. Every board is generated in your browser and checked to have exactly one solution, so you never have to guess. Notes, hints, undo and a timer work just as in our classic Sudoku, and the line under the board lists every combination that fits the cage you have selected. There is nothing to download and no account; your game is saved in the browser.
Killer Sudoku rules
- Each row, column and 3×3 box contains the digits 1 to 9 exactly once, as in normal Sudoku.
- The digits in a cage add up to the number printed in its top-left corner.
- A digit cannot repeat inside a cage, even when the cage runs across two boxes.
Because every row, column and box holds 1 to 9, each of them adds up to 45, and the whole grid adds up to 405. That one fact is behind most of the clever moves in Killer Sudoku. On Easy, a few cages are a single cell; a one-cell cage simply tells you that cell's digit.
How to play Killer Sudoku on this site
- Select a cell and type a digit, or tap it on the number pad. The cell's row, column, box and cage are shaded, and the cage outline turns blue.
- Read the combinations line under the board. For a cage of 3 cells adding to 7 it shows 124, the only possibility. Once you have placed a digit in a cage, combinations without that digit drop out. You can switch the line off in Settings if you prefer to work them out yourself.
- Use notes (the pencil, or the N key) to write candidates. When you place a digit, it is removed from the notes in the same row, column, box and cage.
- Mistakes and hints work as in the classic game: three mistakes end the game unless you take a second chance, and a hint fills in one cell.
- Change level with the tabs at the top or the New Game button. Each level keeps its own statistics, which you can see under the chart icon.
How to start a Killer Sudoku
1. Find the cages with only one combination
Some sums can only be made one way. A 2-cell cage of 3 must be 1 and 2, and a 2-cell cage of 17 must be 8 and 9. You do not know which cell gets which digit yet, but you know both digits, and that alone removes them from the rest of the row, column or box the cage sits in. The ones worth learning first:
| Cells | Sums with one combination |
|---|---|
| 2 | 3 = 1+2 · 4 = 1+3 · 16 = 7+9 · 17 = 8+9 |
| 3 | 6 = 1+2+3 · 7 = 1+2+4 · 23 = 6+8+9 · 24 = 7+8+9 |
| 4 | 10 = 1+2+3+4 · 11 = 1+2+3+5 · 29 = 5+7+8+9 · 30 = 6+7+8+9 |
The full list for every cage size is on the Killer Sudoku combinations chart, with a calculator for the cages in between.
2. Use the rule of 45
Pick a row, column or box. Add up the cages that lie completely inside it. If one cell of that house is left over, its digit is 45 minus that total. That cell is called an innie. If instead the cages covering a house stick out of it by one cell, the cell outside, an outie, equals the total of those cages minus 45.
Example: a box holds cages of 15, 8, 11 and 6 that stay inside it, plus one cell that belongs to a cage reaching into the next box. The four cages add up to 40, so the leftover cell is 45 − 40 = 5. The same works with two leftover cells: they must add up to the difference, which often narrows them to one or two pairs.
3. Then solve it like Sudoku
Once a few digits are placed, ordinary Sudoku logic takes over: a digit that fits only one cell of a row, column or box goes there, and a cell with one candidate left is solved. The difference is that each cage keeps narrowing things down. When a 3-cell cage of 15 already holds a 9, the other two cells add up to 6 and can only be 1+5 or 2+4.
Killer Sudoku tips
- Write combinations, not just candidates. For a cage that can be 1+8, 2+7, 3+6 or 4+5, the useful note is often which digits are missing from all of them.
- Look for digits a cage must contain. Every combination of 4 cells adding to 12 includes 1 and 2. If those cells all sit in one row, no other cell in that row can be 1 or 2.
- Small and large sums are your friends. A low sum needs low digits and a high sum needs high digits, so they cut candidates fastest. Middle sums such as 10 in two cells allow four pairs and say little on their own.
- Extend the rule of 45 to two houses. Two neighbouring rows add up to 90 and three add up to 135. When one house has no innie, a pair of houses often does.
- Check the cage sum before you commit. If two cells of a 3-cell cage of 20 hold 9 and 8, the last cell must be 3, not "something small".
If you are new to Sudoku itself, the beginner's guide and the solving techniques page explain singles, pairs and locked candidates, which you will also use here.
Killer Sudoku difficulty levels
Every puzzle starts from a solved grid split into one-cell cages. Neighbouring cages are merged one at a time, and a merge is kept only if the puzzle still has exactly one solution. When the board reaches the level's number of cages, a step-by-step solver checks which techniques it needs, and boards that do not fit the level are thrown away. The figures below come from 40 boards per level.
| Level | Cages | Largest cage | What it takes |
|---|---|---|---|
| Easy | 32–35, about 6 of them single cells | 4 cells | Cage combinations and singles; no rule of 45 needed |
| Medium | 28–31 | 5 cells | The rule of 45 at least once |
| Hard | 25–27 | 6 cells | The rule of 45, and usually locked candidates, naked pairs or a digit a cage must place in one row, column or box |
| Expert | 22–24 | 6 cells | Usually more than that: sums across two or three houses, longer chains of reasoning |
Is Killer Sudoku harder than normal Sudoku?
Not necessarily. An empty grid looks intimidating, but the cage sums carry a lot of information, and an Easy Killer can feel lighter than a Hard classic puzzle. What changes is the kind of thinking: you add small numbers in your head and reason about combinations, not only about where each digit can go. Expert Killer puzzles, on the other hand, can take a long time even for experienced solvers.
Where Killer Sudoku comes from
Killer Sudoku mixes Sudoku with the sums of Kakuro. Its invention is usually credited to the Tokyo puzzle setter Tetsuya Nishio, building on an idea of one of his students. In Japan it is called samunanpure, "sum number place". It first appeared outside Japan in The Times in 2005 and has been a newspaper favourite ever since, also under names such as Sumdoku and Addoku.
Frequently asked questions
What is a cage in Killer Sudoku?
A cage is a group of cells outlined with a dashed line. The small number in its corner is the total of the digits inside it, and no digit may repeat within the cage.
Can a number repeat in a Killer Sudoku cage?
No. The digits in a cage are always different, even when the cage crosses from one 3×3 box into another. So a 2-cell cage of 4 must be 1 and 3, never 2 and 2.
What is the rule of 45 in Killer Sudoku?
Every row, column and box adds up to 45. If the cages inside one of them leave a single cell uncovered, that cell is 45 minus the cages' total. If the cages covering it stick out by one cell, that cell is the cages' total minus 45.
Do I need to guess to solve these puzzles?
No. Every board on this page is checked to have exactly one solution before it is shown, so there is always a logical next step, although on Expert it can take a while to find.
Why do some Easy puzzles have one-cell cages?
A one-cell cage gives you that cell's digit for free. Easy boards keep a few of them as footholds; from Medium up they are rare.
Is Killer Sudoku easier than normal Sudoku?
It depends on the puzzle. Easy Killer boards are gentle because the cage sums give a lot away. Harder ones ask for more arithmetic and combination work than classic Sudoku of a similar level.