How to play Calcudoku

Calcudoku is Sudoku's arithmetic cousin. There are no boxes and no numbers to start from — just cages, each with a small sum to work out. You'll use addition and times tables, but mostly you'll use logic.

The rules

• A grid of 4×4 holds the numbers 1 to 4, a 6×6 grid 1 to 6, an 8×8 grid 1 to 8.
• Every row and every column holds each number exactly once.
• The heavy lines divide the grid into cages. The clue in a cage's corner is a target and an operation, and the numbers in the cage must make the target:
  • + they add up to it;
  • × they multiply to it;
  • − (two cells) the bigger minus the smaller is it;
  • ÷ (two cells) the bigger divided by the smaller is it.
• A cage with no sign has one cell, and that's its number.
• A number can appear twice in a cage, as long as the two aren't in the same row or column.

Every puzzle on this site has exactly one solution, and you can always reach it by reasoning. You'll also see this puzzle called Mathdoku or simply a cage puzzle.

Your first moves

Start with the free ones: a cage with a single cell is simply its number.

Next, the small cages. A two-cell cage can usually be made in only a few ways, and the row and column rule cuts that down further.

From there, Calcudoku plays a lot like Sudoku: look for the number that has only one place left in a row or column, and for the cell that has only one number left. The strategy guide covers the techniques Medium and Hard need.

Playing on this site

• Tap a cell, then a number on the pad. Tap the same number again to clear it.
• Notes switches the pad to pencil marks. When you place a number, it's removed from the notes in that row and column for you.
• The cage of the selected cell is shaded lightly, so you can see what you're adding up.
• A number repeated in a row or column is marked straight away, and so is a full cage that misses its target — its clue is struck through. Check shows any cell that doesn't match the solution, but only when you ask.
• Hint names the next deduction and where to look, without giving away the number. One hint per puzzle is free; after that there's a one-minute wait between hints.

Keyboard

• Arrow keys move; number keys place a number; Shift + number writes a note.
• N toggles notes, Z undoes, Y redoes, H asks for a hint.

Sizes and the daily puzzle

• Easy: 4×4.
• Medium: 6×6.
• Hard: 8×8.

Each day's first puzzle is the same for everyone, and they follow the week from Monday's gentlest to Saturday's toughest, with Sunday the next toughest. After it, Next puzzle gives you as many more as you like.