How to play

Killer sudoku is a 9×9 sudoku with an extra layer of clues. The grid is divided into cages, groups of cells outlined with a dashed line. Each cage has a small number in its corner.

1. Every row, column and 3×3 box must contain the digits 1 to 9, each exactly once, just like classic sudoku.
2. The digits inside a cage must add up to the number in its corner.
3. A digit cannot repeat inside a cage, even when the cage crosses a box border.

Harder killer puzzles start with no digits at all. The cage sums are the only clues, and they are enough to pin down exactly one solution.

Strategy

Killer sudoku uses all the classic sudoku techniques, plus a set of its own based on arithmetic.

• Unique combinations. Some sums can only be made one way. A 2-cell cage of 3 must be 1 and 2. A 2-cell cage of 17 must be 8 and 9. A 3-cell cage of 6 is always 1, 2 and 3, and a 3-cell cage of 24 is always 7, 8 and 9. Learn these first: they are the starting points of most puzzles.
• Excluded digits. Even when a cage has several combinations, some digits never appear. A 2-cell cage of 10 can be 1+9, 2+8, 3+7 or 4+6, but never 5, because 5+5 would repeat.
• The rule of 45. The digits 1 to 9 add up to 45, so every row, column and box sums to 45. If cages cover a box except for one cell, subtract the cage sums from 45 to get that cell. If a cage sticks out of a box by one cell, the same trick gives the cell outside.
• Multiple units. Two adjacent rows total 90 and three total 135. Adding up whole cages across several rows or boxes and comparing with these totals reveals a lot in hard puzzles.
• Cage and line overlap. If a cage must contain a 9 and the whole cage sits in one row, no other cell in that row can be 9.
• Classic sudoku moves. Once digits appear, singles, pairs and pointing work exactly as they do in normal sudoku.

A useful order of attack: mark the unique-combination cages, then apply the rule of 45 to every box, then fall back on ordinary sudoku logic.

History

Killer sudoku combines the rules of sudoku with the sum clues of kakuro. It became widely known in 2005, during the sudoku boom, when British newspapers including The Times began printing it under the name Killer Sudoku next to their regular puzzles. Some publishers sell it as sum sudoku.

Playing on Summoned Games

• Four levels. Easy uses small cages of one to three cells and starts with 12 digits filled in. Medium uses cages of two to four cells with 4 digits filled in. Hard and Expert use bigger cages, up to five and six cells, and usually start from an empty grid.
• Unlimited puzzles. Every game is generated fresh, and a solver checks that it has exactly one solution before you see it.
• Daily puzzle. One medium puzzle per day, the same for every player, with a streak counter.
• Cage sum helper. Tap a cell and the strip under the grid shows its cage total, how many cells it has, and how much is left to place in the empty ones.
• Notes. Pencil marks are cleared automatically when you place that digit in the same row, column, box or cage.
• Error marking. A wrong digit turns red and adds to the mistakes counter. A cage that repeats a digit or adds up to the wrong total is flagged too.
• Undo, Erase and Hint. A hint fills one correct digit into the selected cell, or a random open one.
• Leaderboards for puzzles solved and for the fastest Hard or Expert solve.
• Auto-save keeps your grid, notes and timer if you leave mid-puzzle.
• Rules card. A How to play screen in the menu walks through the rules with small examples.

On a phone, tap a cell and then a digit on the pad. On a keyboard, move with the arrow keys, type 1 to 9, erase with Backspace or Delete, and press N or F for notes, H for a hint, U or Ctrl+Z to undo and Esc for the menu.

FAQ

What is the difference between killer sudoku and normal sudoku? The sudoku rules stay the same, but instead of starting digits you get cages with sums. Harder killer puzzles give you no digits at all, only the cage totals.

Can a digit repeat inside a cage? No. Each cage holds different digits, even if it stretches across two boxes.

What is the rule of 45? Every row, column and box holds 1 to 9, which adds up to 45. Comparing that total with the cage sums inside a box often gives you a cell directly.

Does every puzzle have one solution? Yes. The generator runs a solver on every puzzle and only keeps it once the solution is unique.

Is it free to play? Yes. It runs free in your browser on phone or desktop, with no ads.