Killer Sudoku — free online puzzle, no download

Killer Sudoku takes ordinary Sudoku and removes the one thing you normally start with: the given numbers. There are none. Instead the grid is divided into dashed cages, each printed with a small target sum, and two extra rules apply — the digits in a cage must add up to that sum, and no digit may repeat inside a cage. Everything else is unchanged: every row, every column and every 3×3 box still holds 1 to 9 exactly once.

That combination is what makes it satisfying. With no givens the puzzle looks impossible for about thirty seconds, and then the arithmetic starts handing you certainties that plain Sudoku never offers. Every puzzle here is generated fresh and verified to have exactly one solution before it is dealt, so it is always solvable by logic and never by guessing. Free, no download, and a part-finished grid is saved if you close the tab.

How to play

• Tap a cell, then tap a digit on the pad. Tap ⌫ Erase to clear it.
• Every row, column and 3×3 box holds 1–9 once, exactly as in normal Sudoku.
• Every dashed cage must add up to the small number in its corner, and no digit repeats inside a cage.
• Turn on Notes to pencil in candidates — with no givens, you will want them.
• Digits that clash with a row, column, box or cage turn red as you place them.

The sums that give themselves away

The fastest way into a Killer Sudoku is to find the cages that have only one possible combination of digits. Because digits cannot repeat inside a cage, small cages with extreme sums are effectively pre-solved. A two-cell cage summing to 3 must be 1+2. Summing to 4 it must be 1+3. Summing to 16 it must be 7+9, and 17 must be 8+9. Three-cell cages behave the same way at the extremes: 6 can only be 1+2+3 and 24 can only be 7+8+9. You do not know the order yet, but you know the contents, and that is usually enough to eliminate those digits from every other cell in the row, column or box the cage sits in.

The second tool is the rule of 45. Every row, column and 3×3 box contains 1 through 9, so it sums to 45. If a box is covered entirely by cages except for one stray cell, that cell equals 45 minus the sum of those cages — a single digit, deduced without touching a candidate list. The same trick works across two or three boxes at once: total their cages, subtract from 90 or 135, and whatever is left belongs to the cells that spill outside.

The third habit is simply to be suspicious of large cages. A five-cell cage summing to 25 tells you almost nothing on its own, and beginners waste a lot of time staring at them. Work the small cages and the 45s first; the large cages resolve themselves once their neighbours are pinned. If you want the version without the arithmetic, plain Sudoku is here too, and Nonogram scratches a similar deduce-from-numbers itch with pictures instead.

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More logic puzzles: Sudoku, Nonogram, Minesweeper and Lights Out. Or browse all games.