Shikaku: one rule, endless deduction

Updated July 2026

Shikaku (四角に切れ, "divide by squares") is a Japanese grid puzzle popularised by Nikoli, the publisher behind Sudoku's rise. It has exactly one rule, takes a minute to learn, and rewards real logical deduction rather than guessing.

The rule

Divide the grid into rectangles so that every rectangle contains exactly one number, and that number equals the rectangle's area in cells. Every cell must belong to a rectangle. That's the whole game.

A "4" can be a 1×4 strip, a 4×1 strip, or a 2×2 square. A "5" — because 5 is prime — can only be a 1×5 or 5×1 strip. This little asymmetry is where all the logic lives.

Try it — a 5×5 board

Drag (or click two corners) to draw a rectangle. It locks in when it covers exactly one number matching its area. Tap a locked rectangle to remove it. Fill the whole board to win.

Five solving techniques

1. Primes first. Prime numbers (2, 3, 5, 7) can only form strips, which slashes their possible placements. Start there.

2. Corners and edges. A number near a corner has far fewer legal rectangles than one in the open middle. The board's border does half your work.

3. Orphan cells. Find a cell that only one clue can possibly reach — that cell forces part of that clue's rectangle immediately.

4. Let big numbers collide. Large rectangles need room. Sketch every legal placement of a big clue; the cells shared by all of them are certain.

5. Cross-hatch the leftovers. Every cell must be covered, so an unreachable-looking region tells you some nearby rectangle must stretch into it.

Where the idea went next

Shikaku's core move — carving a grid into pieces that satisfy local constraints — turned out to be fertile ground. Pentomino puzzles ask you to tile with fixed five-cell shapes. Fillomino frees the pieces from being rectangles. And our own Artisan of Glass takes the carving idea somewhere warmer: panes instead of rectangles, a small set of allowed shapes instead of areas, plus a coloring rule — touching panes must differ, and same-colored neighbours fuse into one pane. Solve the board and it becomes a glowing stained-glass window.

If Shikaku is the kind of puzzle you like, it's the same muscle — with a prettier reward at the end.

Like this? The full game is free on iPhone and iPad.