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.
A worked Shikaku example
The quickest way to feel how Shikaku works is to eliminate impossible rectangles for a single clue. Suppose a 6 sits near the left edge of a 5x5 board. Its factor pairs are 1x6, 6x1, 2x3 and 3x2 — but on a five-row, five-column grid a 1x6 or 6x1 strip simply cannot fit, because no row or column is six cells long. Those two shapes are gone before you draw anything.
That leaves a 2x3 or 3x2 block, and both must still include the clue and cover empty cells only. If a neighbouring clue already needs the cell directly below the 6, the 3-tall option is blocked, leaving just the 2x3 wide rectangle. One clue has been solved purely by ruling out shapes that do not fit, and its fixed border now shrinks the options for every clue around it.
- List the factor pairs of the clue (6 = 1x6, 6x1, 2x3, 3x2).
- Throw out any shape longer than the grid allows.
- Throw out shapes that would cover a second clue or run off the board.
- If only one shape survives, place it and use its edges as new walls.
- Re-check the neighbouring clues, which now have less room.
The area-sum check
Every Shikaku has a hidden balance: because the rectangles cover the whole grid exactly once, the clue numbers always add up to the total number of cells. A 7x7 board has 49 cells, so its clues must total 49. That makes a quick sanity check, and on small or nearly finished boards it becomes a real solving tool.
Use it both ways. If you have placed several rectangles and the clues you have left sum to the number of empty cells, you are on track; if they do not, a rectangle is wrong. Late in a puzzle the leftover area often pins the last clue exactly: when one unsolved clue's number equals the cells still free, its rectangle simply fills the remaining space.
- All the clue numbers added together equal the grid's cell count.
- A 7x7 grid totals 49; a 10x10 totals 100.
- Placed rectangles plus remaining clues should always balance.
- If the totals do not match, an existing rectangle is wrong.
- When one clue equals the free cells left, it takes all of them.
Shikaku, a Nikoli classic
Shikaku was popularised by Nikoli, the Japanese publisher behind many of the world's best-loved logic puzzles, including Sudoku and Nurikabe. Its Japanese name, roughly 'divide into squares,' describes the whole game: cut the grid into rectangular blocks so each one carries a single area clue.
Like other Nikoli puzzles, a good Shikaku is built to have exactly one solution reachable by pure logic, with no guessing required. You will also see it in English under the names Divide by Squares and Rectangles. The version here keeps that one-solution standard and adds grid sizes from 5x5 to 10x10, so you can grow from quick warm-ups to longer challenges.