9 Puzzles

How to play Killer

Play Killer

Killer is Sudoku with the given numbers taken away and sums put in their place. It's also called Killer Sudoku. It looks fierce, but the arithmetic never goes beyond adding small numbers, and the logic is friendly once you know the one trick it's built on.

The rules

  • Every row, every column and every 3×3 box holds the numbers 1 to 9 exactly once, as in Sudoku.
  • Dashed lines divide the grid into cages. The small number in a cage's corner is its sum: the digits inside add up to it.
  • A digit never repeats inside a cage, even when the cage spans two boxes.
  • There are no numbers to start from. Everything comes from the sums.

Every puzzle on this site has exactly one solution, and you can always reach it by reasoning, without guessing.

179171111107741316178141112128121015917910816141714546141110
An easy puzzle with 36 cages and no givens. From Easy puzzle #1 — play it
179171111107741316178141112128121015917910816141714546141110754896231821345976396127584479653812215478693638912457142539768987261345563784129
Solved. Each row, column and box holds 1 to 9 once, and the digits in every cage add up to its sum.

Your first moves

Start with the cages that can only be made one way. Two cells that add up to 3 must be 1 and 2; two that add up to 17 must be 8 and 9. You won't know which way round yet, but nothing else can go there.

179171111107741316178141112128121015917910816141714546141110123456789123456789
The 17 cage has two cells, and the only two different digits that add up to 17 are 8 and 9. Every other candidate in it is crossed out. From Easy puzzle #1 — play it

The other thing every Killer player uses is the rule of 45. The digits 1 to 9 add up to 45, so every row, column and box does too. When the cages lying wholly inside a box (or a row, or a column) cover all of it but one cell, that cell is whatever is missing from 45.

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The cages wholly inside box 9 add up to 39. The whole of box 9 adds up to 45, so the one cell left over, at row 7, column 7, is 45 − 39 = 6. From Easy puzzle #3 — play it

From there, Killer plays a lot like Sudoku: look for the digit that has only one place left in a row, column or box, and for the cell that has only one digit left. The strategy guide covers the rest, from cage combinations to the rule of 45 across several rows.

Playing on this site

  • Tap a cell, then a number on the pad. Tap the same number again to clear it.
  • Notes switches the pad to pencil marks. When you place a number, it's removed from the notes in that row, column, box and cage for you.
  • The cage of the selected cell is shaded lightly, so you can see what you're adding up.
  • A number repeated in a row, column, box or cage is marked straight away, and so is a cage whose digits overshoot its sum, or fill it and miss — its sum is struck through. Check shows any cell that doesn't match the solution, but only when you ask.
  • Hint names the next deduction and where to look, without giving away the number. One hint per puzzle is free; after that there's a one-minute wait between hints.

Keyboard

  • Arrow keys move; number keys place a number; Shift + number writes a note.
  • N toggles notes, Z undoes, Y redoes, H asks for a hint.

Difficulty and the daily puzzle

Every tier is a full 9×9 grid. What changes is what you need to know:

  • Easy: cage sums, singles and the rule of 45 for a single cell.
  • Medium: the rule of 45 for two or three cells at once, and cages that claim a digit for a row, column or box.
  • Hard: the rule of 45 across two or three rows or columns, or two boxes, and Sudoku's triples and X-Wing.

Each day's first puzzle is the same for everyone, and they follow the week from Monday's gentlest to Saturday's toughest, with Sunday the next toughest. After it, Next puzzle gives you as many more as you like.