Killer Sudoku

How to play Killer Sudoku

Killer Sudoku combines the pure logic of classic Sudoku with a dash of mental arithmetic. As in ordinary Sudoku, you fill a 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. The twist is that the grid usually begins almost empty — instead of given numbers, it is carved into dotted-outline groups called cages, and a small number in the corner of each cage tells you the total its digits must add up to. On top of that, no digit may repeat inside a single cage. Those two extra rules turn arithmetic into a powerful deduction tool and give Killer Sudoku its devoted following. Play it here in your browser with pencil marks, a running cage total, a mistake counter and a timer, at three difficulty levels, and earn a ranked score every time you solve.

The goal

Complete the whole 9×9 grid. Every row, column and 3×3 box must hold each of the digits 1 to 9 exactly once; every cage must add up to exactly the target printed in its top-left corner; and no cage may contain the same digit twice. When the final cell is filled correctly the puzzle is solved. Every puzzle here has exactly one solution, guaranteed by the generator, so you never need to guess — careful logic will always get you there.

The rules

• Standard Sudoku still applies: each of the nine rows, nine columns and nine 3×3 boxes must contain the digits 1 through 9 with no repeats.
• Every cage — the dashed-outline group of cells — must contain digits that add up exactly to the small sum shown in its top-left corner.
• A digit may never be repeated within a single cage, even if the cage total could otherwise be made another way.
• There are few or no starting numbers. The cages and their sums are the clues; on Hard the grid can begin completely empty.

Cages and sums

A cage is a group of two, three or four connected cells drawn with a dashed border. The number in its top-left corner is the sum of all the digits inside it. Because digits cannot repeat in a cage, the sum together with the cage size tells you a great deal: a two-cell cage summing to 3 can only be 1+2; a three-cell cage summing to 6 can only be 1+2+3; a two-cell cage summing to 17 can only be 8+9. Learning these "cage combinations" is the heart of Killer Sudoku. Every cage here has a valid, non-repeating set of digits, and all 81 cells belong to exactly one cage — so the cage sums across the whole grid always total 405, which is 45 for each of the nine rows.

Controls

• Tap a cell to select it, then tap a number on the pad (or press 1-9) to enter it. Tap Erase, or press 0 or Backspace, to clear a cell.
• Turn on Notes (or press N) to add small pencil-mark candidates to a cell instead of a final answer — perfect for tracking which digits a cage still allows.
• Arrow keys move the selection around the grid. The cage you are working in is highlighted, and a live readout shows its current total against its target sum.
• Use Hint to reveal one correct cell when you are stuck. Hints and mistakes both lower your final score, so use them sparingly.

Difficulty levels

• Easy — smaller cages (mostly pairs and triples) plus a handful of revealed digits to get you started.
• Medium — a balanced mix of cage sizes with only a few given digits.
• Hard — larger four-cell cages and, in most puzzles, no starting digits at all. Pure cage logic.

Solving techniques

Killer Sudoku rewards a mix of ordinary Sudoku scanning and a little arithmetic bookkeeping. A few reliable techniques:

• The rule of 45: every row, column and 3×3 box sums to 45. If a box is almost entirely covered by cages, add those cage sums; the difference from 45 gives the total of the remaining cells that spill outside — often pinning down a single value.
• Innies and outies: extend the rule of 45 across several rows, columns or boxes at once. Compare the total of the cages inside a region to 45 times the number of lines to find the value of a lone cell poking in (an innie) or out (an outie).
• Cage combinations: memorise the forced sets, especially the extremes. Low sums (3, 4, 6, 10) and high sums (30, 29, 17, 16) leave the fewest options and are the best place to start filling.
• Cross off duplicates: because a digit cannot repeat in a cage, a value already used elsewhere in that cage’s row, column or box is doubly constrained. Pencil marks make these eliminations easy to see.
• Work the overlaps: where a cage shares a row or box with another, the digits it uses up limit what its neighbours can be. Chaining these limits together is almost always faster than guessing.

Winning and scoring

The puzzle is won the moment every cell holds its correct digit. Your result is submitted to the leaderboard as points, where higher is better: score = max(1, 10000 − seconds − mistakes × 200). In other words you begin from a notional 10,000, lose one point per second and 200 points for every wrong entry or hint used, and never drop below 1 (scores are capped at 99,999). Solve quickly and cleanly for the best result. Scores are tracked per difficulty, so a fast Hard solve and a fast Easy solve each keep their own place on the board.

Frequently asked questions