What is Shikaku?
Shikaku is a Japanese rectangle logic puzzle, also known as Divide by Squares or Rectangles. The grid contains number clues, and each number tells you the exact area of one rectangle.
This free Shikaku online game lets you play six grid sizes from 5x5 to 10x10, with easy, medium and hard puzzles. Every generated puzzle is checked by an exact-cover solver so the clues have one solution.
- Divide the whole grid into rectangles.
- Each rectangle must contain exactly one number clue.
- The number must equal the rectangle's area.
- Rectangles cannot overlap or leave gaps.
- The completed board covers every cell exactly once.
How to play Shikaku online
Drag across the grid to draw a rectangle around exactly one numbered clue. If the rectangle area matches that clue and contains no other clue, it is placed on the board.
Use Check for feedback, Hint to place one correct rectangle, Undo to step back, Erase to remove the selected rectangle, Solution to reveal the grid, and New puzzle to generate another unique Shikaku challenge.
- Begin with small numbers because they have fewer possible shapes.
- Remember that a 6 clue could be 1x6, 2x3, 3x2 or 6x1.
- Look for clues near edges and corners because the board limits their rectangles.
- Watch how one rectangle blocks space for neighbouring clues.
Shikaku rules
The rules of Shikaku are simple, but the deductions can be satisfying. A clue belongs to one rectangle only, and the area of that rectangle must match the clue number.
A rectangle is allowed to be long, thin, wide or square. It is not allowed to contain two clues, overlap another rectangle or leave an unreachable gap in the grid.
- A 1 clue is a one-cell rectangle.
- A prime number clue can only make a 1-by-n or n-by-1 rectangle.
- A clue in a corner has fewer possible rectangles.
- Two rectangles may share an edge, but they stay separate regions.
- A solved Shikaku has no empty cells.
Shikaku strategy tips
Good Shikaku strategy starts by listing the possible rectangles for each clue. If a clue has only one shape that fits, place it first and use its border to narrow the rest of the grid.
Harder Shikaku puzzles often depend on space pressure. Even when a clue has several possible rectangles, only one choice may leave enough room for the neighbouring clues to take their required areas.
- Solve forced 1 clues and corner clues early.
- Use factor pairs to imagine every possible rectangle size.
- Eliminate shapes that would trap an empty cell.
- Check whether a rectangle would steal space from another clue.
- When stuck, switch from one clue to the empty spaces around it.
Shikaku grid sizes and difficulty
Small 5x5 and 6x6 Shikaku puzzles are good for learning the rectangle rules. Larger 8x8, 9x9 and 10x10 boards create more factor choices and more interaction between distant clues.
Easy puzzles use more local deductions and smaller rectangles. Medium puzzles mix compact and stretched regions. Hard puzzles use fewer clues, larger areas and longer chains of reasoning, but the solver still verifies a single solution.
- 5x5 Shikaku is a quick warm-up.
- 6x6 and 7x7 Shikaku are friendly practice sizes.
- 8x8 and 9x9 Shikaku add more strategic space management.
- 10x10 Shikaku gives the largest online challenge on this page.
- Easy, medium and hard change rectangle sizes, clue density and deduction pressure.