What Is Shikaku? Rules and Beginner Strategy

What is Shikaku?

What is Shikaku, sometimes written Rectangles or Divide by Box? It is a grid dividing logic puzzle. Each cell number tells you the area of the rectangle it belongs to, and your job is to split the entire grid into rectangles so that every rectangle contains exactly one number and that number equals the rectangle area in cells. When you finish, no cell is left uncovered and no rectangles overlap. Shikaku is prized for how a purely spatial idea produces clean, guess free deductions.

The rules

Shikaku keeps its rule set tidy, which makes what is Shikaku quick to teach.

• Divide the whole grid into rectangles, including single row or single column shapes.
• Every rectangle must contain exactly one numbered cell.
• The number equals the area of its rectangle, meaning width times height in cells.
• Rectangles never overlap and together cover every cell.

Beginner strategy

The smoothest way to learn what is Shikaku rewards is to begin with the clues that have the fewest possible shapes.

1. Start with ones, since each must be its own single cell rectangle.
2. For a prime number like five or seven, the rectangle can only be one cell wide, so it is a straight strip in one of two directions.
3. Look at clues near a corner or edge, where the wall limits how a rectangle can extend.
4. When two numbers compete for the same cells, the arrangement that lets both be satisfied is forced.
5. Fill certain rectangles first, then use the shrinking free space to pin down the rest.

Factor pairs matter

Every number in Shikaku hides its possible shapes in its factors. A six can be one by six, six by one, two by three, or three by two, so listing those options tells you exactly which rectangles are even possible. A clue that is prime has far fewer choices, which is why primes are such reliable starting points. Thinking in factor pairs turns a vague hunt into a short list of candidates you can test against the walls and against neighboring clues.

Avoiding overlaps

The most common Shikaku mistake is extending a rectangle so far that it swallows a second number, which is never allowed. Each rectangle owns exactly one clue. When you sketch a possible rectangle, check that it captures only its own number and does not stray into a neighbor's territory. Because every valid Shikaku has one unique tiling, careful checking always beats forcing a shape that feels right but crowds another clue.

If you like carving the grid

Shikaku shares Sudoku love of clean regions and provable moves, with luck playing no part. If you enjoy fitting shapes into a grid, the block placement puzzle Astro Blocks at play.google.com/store/apps/details?id=com.snailpixel.astroblocks offers a livelier take on the same spatial thinking. And when you want the calm, no guessing logic of Sudoku without the arithmetic, Shapedoku keeps the classic row, column, and box rules while replacing digits with nine glowing shapes. Play the free daily challenge at app.shapedoku.com or explore the solver on shapedoku.com.