How to play Killer Sudoku

Sudoku is a logic puzzle in which you fill a grid with digits so that every row, column and box holds each digit exactly once.

Fill the grid so every cage adds up to its sum.

About Killer Sudoku

Killer Sudoku starts with an empty grid divided into cages of one to five connected cells, each printed with its sum. The digits in a cage must add up to that sum and may not repeat within it, on top of the usual row, column and box rules.

Each cage's sum is printed in its first cell. Boards built mostly from small cages tend to rate easier, because a small cage gives its digits away, while large ones hide them and rate harder. Whatever the rating, the cages alone fix the answer, since a Killer board has no givens to lean on.

Start with cages that allow only one set of digits: a one-cell cage simply is its sum, and two cells adding up to 4 must be 1 and 3. Then use the 45 rule on a box: take the cages lying wholly inside it away from 45, and what is left is the total of its remaining cells, the ones in cages crossing its edge.

Cages here hold one to five cells and may cross from one box into the next. Even the largest have telling sums: five cells adding up to 15 can only be 1 to 5, five adding up to 35 only 5 to 9, and three adding up to 24 only 7, 8 and 9. Working out which digit sets fit a cage counts as an easy step when a board is rated, since no Killer board can be started without it, so a Medium board is one that also needs pointing pairs, box-line reduction or naked pairs, and a Hard one needs more than those. Over 300 generated boards, Easy, Medium and Hard each came up roughly a third of the time.

The goal of Sudoku

Fill every empty cell so that each row, each column and each box holds every digit exactly once. The classic grid is nine by nine, with nine boxes of three by three and the digits 1 to 9; the 6x6 grid has six boxes of two rows by three columns and the digits 1 to 6. Every puzzle has exactly one solution.

How to set up Sudoku

Choose Easy, Medium, Hard or Any as the difficulty of your next new puzzle. Easy puzzles need only the simplest deductions, Medium ones also need eliminations such as pointing pairs and naked pairs, and Hard ones need techniques beyond those.

Sudoku rules

Select a cell and enter a digit. Turn on Notes to pencil in candidate digits instead; notes are reminders, not moves, and writing a digit in a cell clears its notes. Erase empties a cell, Undo takes back your last move, and Restart lays the same puzzle out again.

A digit repeated in a row, column, box or cage is marked as a conflict, and so is a full cage that does not add up to its sum. A digit that breaks no rule but is still wrong is left unmarked, unless you turn on Check entries in the settings.

Hint fills in the next cell that logic forces from your current position, or points out your first wrong entry. Hints are unlimited, but a puzzle finished after a hint, or with Check entries on, is not recorded as a win.

The full Sudoku guide, with strategy tips and common questions