How to play Killer Sudoku
Killer Sudoku combines a normal sudoku grid with a little addition. The grid is divided into dashed cages, each with a target sum, and on harder levels there are no starting digits at all: the cage totals are your only clues. Every puzzle here is generated fresh, has one unique solution, and adapts to how well you played the last one.
The rules
- Fill every cell with a digit from 1 to 9.
- Each row, each column and each 3×3 box must contain every digit exactly once, just as in classic sudoku.
- The digits inside each dashed cage must add up to the small number in the cage's top-left corner.
- A digit may not repeat inside a cage, even if the cage spans more than one box.
Click a cell and type or tap a digit; arrow keys move the selection, Backspace clears. A wrong digit turns red and counts as an error. The cage total turns green once the cage is complete and correct, and red as soon as it holds a repeat or its digits already exceed the target.
Getting started
Start with the cages that allow only one set of digits. Suppose the cage covering row 1, columns 1 and 2 shows a 3. Two different digits summing to 3 can only be 1 and 2, so those two cells hold 1 and 2 in some order. Now look along row 1 and down columns 1 and 2: if a 1 is already placed in column 2, then row 1, column 2 is the 2 and row 1, column 1 is the 1.
The page helps with this. When you select a cell, the panel below the grid lists every digit combination that fits its cage, striking out sets that conflict with digits you have already placed there. Number buttons for digits that cannot appear in that cage are greyed out. Use the Notes button, or press N, to switch to pencil marks and record candidates; placing a correct digit automatically removes it from the notes in its row, column, box and cage.
Strategies and tips
- Learn the unique combinations. Some cages have only one possible set: 3 in two cells is 1+2, 4 is 1+3, 16 is 7+9 and 17 is 8+9. In three cells, 6 is 1+2+3, 7 is 1+2+4, 23 is 6+8+9 and 24 is 7+8+9.
- The rule of 45. Every row, column and box adds up to 45. If the cages lying entirely inside a box add up to 38 and one cell of the box is left over, that cell must be 7. The same trick works across two or three rows or columns at once.
- Innies and outies. When a cage pokes one cell out of a box, add up the cages that cover the box, subtract 45, and the difference is the value of the protruding cell.
- Shared digits. If a two-cell cage must be 1+2 and both cells are in the same row, then 1 and 2 are used up in that row, which removes candidates from the other cages crossing it.
- Watch cage sizes. A large cage with a small total, or a small cage with a large total, is very restrictive. A five-cell cage of 15 must be exactly 1+2+3+4+5.
How the difficulty adapts
The page uses five levels, 400 rating points apart: Easy (1000), Medium (1400), Hard (1800), Expert (2200) and Master (2600). Each new puzzle uses the level nearest to your rating. Easy uses cages of at most three cells plus 24 starting digits; Medium allows four-cell cages with 12 digits; Hard has four-cell cages and no digits at all; Expert and Master allow cages of up to five and six cells.
Finishing within the level's target time (10 minutes on Easy up to 30 on Master) with three errors or fewer earns +80. More errors, or running over time, costs 160, and each hint costs another 80. The rating never drops below 800. A few clean solves in a row move you up a level, and a rough patch moves you back.
This matters because a puzzle that stays a little beyond your comfort zone keeps you reasoning instead of following habits. A challenge that rises with your skill is better training than one you can do on autopilot. You can read more about what research says about puzzles and brain health.
A short history of Killer Sudoku
Killer Sudoku grew out of the sudoku boom of the mid-2000s, when British newspapers were looking for fresh variations to keep readers interested. The Times was among the first to print it under the name Killer Sudoku. The puzzle borrows its addition from Kakuro, the cross-sum puzzle, and its row, column and box rules from sudoku, which makes it a natural next step for players who find classic grids too easy.
Frequently asked questions
Do I need to be good at maths?
Only simple addition is involved. The combination panel lists every possible digit set for the selected cage, so you can focus on the logic rather than the sums.
Why are there no starting digits?
From the Hard level upwards, the cage totals alone are enough to fix the solution. The generator checks that exactly one solution exists before giving you the puzzle.
What does a hint do?
Select a cell that is empty or wrong and press Hint to reveal its correct digit. Each hint costs 80 rating points.
Is my progress saved?
If you are signed in, your rating is saved to your account. Otherwise it resets on reload, and you can pick a level with the difficulty buttons.