Shikaku Puzzle
Shikaku
8×8 — A bigger board, and some clues only give way once you notice a cell that only one clue can reach.
Standard play. Timer runs. Hints available.
Shikaku — also called Cellblocks, Rectangles, or Divide by Box — is a rectangle-partition puzzle from Nikoli. You get a grid with scattered numbers and need to divide it into non-overlapping rectangles so that each rectangle contains exactly one number equal to its area. Two rules, no exceptions. A cell marked “6” belongs to a rectangle covering 6 cells. Could be 2×3, could be 1×6 — you figure out which one fits.
What is Shikaku?
Shikaku was invented in 1989 by Yoshinao Anpuku, a university student in Kyoto, and published by Nikoli — the same company that brought Sudoku, Kakuro, and Nurikabe to the world. The Japanese name is 四角に切れ (shikaku ni kire), which translates roughly to “cut into rectangles.” Nikoli has published more than 14 books of Shikaku puzzles since then.
Outside Japan, the puzzle goes by a few different names. The Guardian newspaper runs it daily as “Cellblocks.” Nikoli's own English name is “Divide by Box.” Some publishers just call it “Rectangles.” The rules are identical regardless of the label.
What makes Shikaku unusual among grid puzzles is the interaction model. Instead of toggling cell states or placing numbers, you draw rectangles. Click one corner, drag to the opposite corner, and the cells in between get claimed. It feels different from anything else in the category — more construction than deduction, even though the underlying logic is pure constraint satisfaction.
The two rules
Most grid puzzles have four to six rules. Shikaku has two:
- Full coverage. Divide the entire grid into non-overlapping rectangles. Every cell belongs to exactly one rectangle. Squares count as rectangles.
- Number = area. Each rectangle contains exactly one number, and that number equals the rectangle's area. A cell showing “8” belongs to a rectangle of 8 cells — maybe 2×4, maybe 1×8.
That's it. No connectivity constraints, no adjacency restrictions, no row or column uniqueness. All the difficulty comes from working out which rectangle a clue has to take, and the levels differ by how deep that reasoning has to go.
For solving techniques and worked examples, read the full strategy guide.
How to play
Drawing rectangles on the grid:
- Draw — drag from one corner to the opposite corner, with a mouse or a finger. A single click or tap drops a single square, so dragging is the only way to take more than one cell.
- Remove — click or tap any rectangle you have already drawn to take it off the board. Or use undo.
- Show possibilities — click a numbered cell without dragging to see all valid rectangle placements for that number.
Each placed rectangle gets a distinct pastel color so you can tell them apart at a glance. Invalid placements — wrong area, multiple numbers inside, or overlaps — get flagged immediately. Keyboard shortcuts: arrow keys navigate, Enter starts/ends selection, Delete removes a rectangle.
A math puzzle in disguise
Nikoli calls Shikaku “a great math trainer for kids,” and the reason is built into the rules. Every number on the grid is a factoring problem: 12 can be 2×6, 3×4, 4×3 or 6×2, and you work out which of those shapes fits the space you have. A 1×12 strip is 12 as well, but no board here is twelve cells wide, so it never comes up. The board size sets how much arithmetic there is, not how hard the thinking gets — on a 10×10 a clue of 15 can only be 3×5 or 5×3, and the factoring is the easy part. The work is in the reasoning each level asks for.
That makes Shikaku a natural fit for classrooms. The multiplication practice is real but doesn't feel like homework, because the spatial reasoning keeps it from being rote. Parents and teachers looking for something beyond flashcards might find the easy grids useful — 6×6, around ten clues, and every rectangle forced by its own number.
Also known as
Shikaku has more aliases than most Nikoli puzzles. The Guardian newspaper publishes it daily as “Cellblocks.” Nikoli's official English name has changed over the years — first “Divide by Squares,” then “Divide by Box.” Some publishers use “Rectangles” as a generic descriptor. The puzzle community has mostly settled on “Shikaku” as the universal name, which is what we use here.
Difficulty levels
Five levels, and each step changes exactly one thing. The board grows at medium and expert; the thinking deepens at hard and einstein. So hard is medium’s board asking a harder question, and einstein is expert’s board asking a harder one.
| Level | Grid | What to expect |
|---|---|---|
| Easy | 6×6 | About ten clues, and every rectangle is forced outright by its own number. Nothing to work out — read it off and draw it. |
| Medium | 8×8 | A bigger board, and some clues only give way once you notice a cell that only one clue can reach. |
| Hard | 8×8 | Medium’s board, with far more of that same reasoning — about a third of the solve rather than a sixth. |
| Expert | 10×10 | A bigger board, hard’s reasoning, and clues whose every remaining placement covers one shared cell. |
| Einstein | 10×10 | Expert’s board and the deepest step: ruling a placement out because it would strand a cell no clue could reach. |
Key strategies at a glance
Start with a clue that has only one legal rectangle. A 1 is a single cell. A 2 in a corner can only go one way. A prime clue can only ever be a single line of cells, one row or one column long. Every easy board is built out of nothing else, and every harder board still starts here.
When those run out, turn the question around and look at a cell instead of a clue. If only one clue can reach a cell, that clue has to cover it, whichever of its shapes turns out to be right. That is the move medium introduces, and hard asks for it over and over.
Two deeper steps carry expert and einstein. If every placement a clue has left covers one cell, that cell belongs to the clue already, before you know where the rest of its rectangle goes. And a placement can be ruled out by what it leaves behind: if drawing it would strand a cell that no remaining clue could reach, it is wrong, even though nothing about the rectangle itself breaks a rule.
Frequently asked questions
What is shikaku?
A grid logic puzzle from Nikoli. Divide the grid into rectangles so each rectangle contains exactly one number equal to its area.
Is shikaku the same as cellblocks?
Yes. The Guardian newspaper publishes this puzzle as “Cellblocks.” Nikoli calls it “Divide by Box.” Same rules, different names.
How is shikaku different from sudoku?
Sudoku fills cells with digits using row/column/box uniqueness. Shikaku draws rectangles where each number matches the rectangle's area. Sudoku is about elimination; Shikaku is about spatial partitioning and factoring.
Is shikaku good for kids?
Nikoli markets it as a math trainer. Every placement is a factoring exercise — figuring out that 12 = 2×6 or 3×4. The easy 6×6 grids work well for kids learning multiplication: around ten clues, and every rectangle forced by its own number.
What does “shikaku” mean?
The full Japanese name is 四角に切れ (shikaku ni kire), meaning “cut into rectangles.” 四角 (shikaku) means quadrilateral.