Shikaku is a logic puzzle where you draw rectangles on a numbered grid. Each rectangle must contain exactly one number, and that number tells you the rectangle's area (how many cells it covers). Fill the entire grid to win.
1Each number = one rectangle
Every numbered cell belongs to exactly one rectangle. The number tells you how many cells that rectangle covers.
2Area must match
A "6" means the rectangle covers exactly 6 cells. It could be 1×6, 2×3, 3×2, or 6×1 — but always 6 cells total.
3No overlaps
Each cell belongs to exactly one rectangle. You can't draw a rectangle over cells that are already filled.
4Fill every cell
The puzzle is solved when every cell is covered. No gaps allowed — the rectangles must tile the entire grid.
Let's solve a simple 4×4 grid together. Here's the starting position:
┌───┬───┬───┬───┐ │ 2 │ │ 3 │ │ ├───┼───┼───┼───┤ │ │ │ │ │ ├───┼───┼───┼───┤ │ 4 │ │ │ 1 │ ├───┼───┼───┼───┤ │ │ │ │ │ └───┴───┴───┴───┘
Step 1: Start with the "1". The cell with "1" in the bottom-right is already a complete 1×1 rectangle. Mark it done.
Step 2: Handle the "2" in the top-left corner. A "2" in a corner can only go one direction — horizontally (1×2) or vertically (2×1). Since it's in the top-left corner, try a 1×2 horizontal rectangle covering cells (0,0) and (0,1).
Step 3: Place the "3". The "3" is in the top row. It could be 1×3 horizontal or 3×1 vertical. Since it's in row 0, a 1×3 horizontal would extend to (0,2), (0,3), (0,4) — but the grid is only 4 wide. So try 3×1 vertical: (0,2), (1,2), (2,2).
Step 4: Place the "4". The "4" is in row 2, col 0. It could be 1×4, 2×2, or 4×1. A 2×2 square covering (2,0), (2,1), (3,0), (3,1) fits perfectly.
Step 5: Fill the gaps. The remaining cells form a 2×1 rectangle in (1,0)-(1,1) and a 1×2 in (3,2)-(3,3). Assign them to the nearest numbers or check if they complete existing rectangles.
❌ Forcing a shape that doesn't fit
If a "6" is in a 2-wide column, you can't use a 3×2 rectangle — it's too wide. Always check the available space before committing.
❌ Forgetting the "exactly one number" rule
A rectangle can't contain two numbers. If your rectangle covers both a "3" and a "2", it's invalid — even if the area is 5.
❌ Leaving gaps
Every cell must be covered. If you finish placing all numbers but there are empty cells, something is wrong — go back and check.
| Puzzle | Core Skill | Key Difference |
|---|---|---|
| Shikaku | Spatial reasoning | Draw rectangles matching area numbers |
| Sudoku | Number logic | Fill digits 1-9 in rows/columns/boxes |
| Akari | Line-of-sight | Place bulbs, illuminate cells, block by walls |
The best way to learn Shikaku is to play. Start with a 5×5 grid — it's small enough to solve in a few minutes, but challenging enough to teach you the core techniques.
Shikaku (四角に切れ) is a logic puzzle by Nikoli where you divide a numbered grid into rectangles. Each rectangle must contain exactly one number, and the number equals the rectangle's area.
Sudoku uses digits 1-9 in rows/columns/boxes. Shikaku uses area numbers to define rectangles. Both are logic puzzles, but Shikaku is about spatial reasoning, not number placement.
Well-designed Shikaku puzzles have exactly one valid solution. The numbers are chosen to guarantee uniqueness.
Start with cells numbered 1 (already complete). Then work from corners and edges where fewer rectangle shapes are possible. Count the factors of each number to list all possible shapes.