How to play Killer Sudoku

Killer Sudoku takes the logic of ordinary Sudoku and folds in a gentle dose of arithmetic. Instead of starting with a scatter of given digits, the board is carved into dotted-outline groups called cages, each printed with a small total in its corner. The cages are the clues — read them carefully and the whole grid unlocks one cell at a time.

Goal

Fill every empty cell so that each row, each column, and each of the nine 3×3 boxes contains the digits 1 through 9 exactly once — the familiar Sudoku rule. On top of that, the digits inside each cage must add up to the number shown in its top-left corner, and no digit may repeat within a single cage. Every puzzle we generate is checked for one — and only one — solution before it reaches your screen, so you never have to guess.

Cages and sums

A cage can be any contiguous shape of two, three, four or five cells. Because the digits in a cage are all different, the total tells you a lot: a two-cell cage summing to 3 must be 1 and 2; a two-cell cage summing to 17 must be 8 and 9. These forced pairs — combined with the row, column, and box constraints — are the backbone of the solve. When a cage is complete and its total is wrong (or a digit repeats), its number turns red to warn you.

Controls

• Click or tap a cell to select it, then tap a number on the pad or press a key from 1 to 9 to place that digit.
• Turn on Notes to pencil-in the candidates you are still weighing. A pencil mark clears automatically once that digit becomes impossible in its row, column, box, or cage.
• Erase empties the selected cell; Hint fills it with the correct digit when you are truly stuck; Undo steps back through your moves.
• Any digit that breaks a row, column, or box turns red, so mistakes are easy to spot. The timer runs until the grid is complete.

Difficulty

Easy uses small cages and reveals a few starter digits for a relaxed warm-up. Medium gives only a couple of helpers, hard removes them entirely, and expert leans on larger five-cell cages with no given digits at all — pure cage logic. Pick your level before you start a new puzzle.

Tips

• Hunt for cages with only one possible combination — the smallest and largest totals are the most revealing.
• Use the "rule of 45": every row, column, and box sums to 45, so a cage that almost fills one often pins the leftover cell.
• Lean on Notes. Killer puzzles reward laying out candidates and chipping away with small, certain eliminations.