How to Play Shikaku

A complete guide to rules, controls, and solving techniques.

The Goal

Divide the entire grid into non-overlapping rectangles. Each rectangle must contain exactly one clue number, and the rectangle's area must equal that number.

That's the whole game. Simple to state, but the puzzles can require deep logical thinking to solve — especially at higher difficulties where multiple constraints interact.

The Rules in Detail

Rule 1: Cover every cell

Every cell in the grid must belong to exactly one rectangle when the puzzle is solved. No cells left over, no cells in two rectangles at once.

Rule 2: One clue per rectangle

Each rectangle contains exactly one number (the clue). If a rectangle would contain zero or two clues, it's invalid.

Rule 3: Area must match the clue

If a clue says 6, the rectangle it belongs to must have an area of exactly 6 cells. That could be a 1×6, 6×1, 2×3, or 3×2 rectangle — all are valid as long as the area is 6.

Rule 4: Only rectangles (no L-shapes)

Shikaku only allows axis-aligned rectangles. You cannot place an L-shaped region, a diagonal line, or any other irregular shape.

A Simple Example

Consider a 4×4 grid with clues at these positions:

┌─────┬─────┬─────┬─────┐
│  4  │     │     │     │
├─────┼─────┼─────┼─────┤
│     │     │  2  │     │
├─────┼─────┼─────┼─────┤
│     │     │     │     │
├─────┼─────┼─────┼─────┤
│     │  6  │     │  4  │
└─────┴─────┴─────┴─────┘

Working through it:

The clue 2 at row 2, col 3 can be 1×2 (horizontal) or 2×1 (vertical). Look at what fits given the grid edges and nearby clues.
The clue 4 at the top-left can be 1×4, 4×1, or 2×2. A 4×1 going down, or 1×4 going right.
The clue 6 near the bottom can be 2×3, 3×2, 1×6, or 6×1 — but the grid is only 4 wide, so 6×1 is impossible. Try 2×3 or 3×2.
The clue 4 at the bottom-right similarly has limited options due to grid edges.
Each constraint reduces options for neighboring clues. Eventually, only one valid arrangement remains.

Every Shikaku puzzle generated on this site has a unique solution — there is always exactly one correct arrangement.

Controls

Mouse / Touch

Drag across the grid to draw a rectangle. While dragging, a dashed outline shows your current selection.
A green outline means the placement is valid (one clue inside, area matches, no overlap).
A red outline means the placement is invalid.
Release to place the rectangle (green only; red placements are rejected).
Click an existing rectangle to select it. A selected rectangle shows a highlight border.
With a rectangle selected, click the Delete button in the toolbar (or press Delete) to remove it.

Keyboard

Key	Action
`Ctrl`+`Z`	Undo the last placement or deletion
`Ctrl`+`Y`	Redo
`Delete` / `Backspace`	Remove the selected rectangle
`Escape`	Cancel current drag / deselect

The Toolbar

Below the grid, the action bar provides:

Undo / Redo — Step backward and forward through your move history. There's no penalty for undoing.
Delete — Remove the currently selected rectangle.
Reset — Clear all your placed rectangles and start the current puzzle over.
Hint — Get a hint that suggests or reveals part of the solution. Each hint is counted and shown in your stats.
Achievements — View your earned achievements and progress.
Help — Quick reference for rules and controls (you're reading the full version now).

The Hint System

If you're stuck, the hint button reveals a rectangle from the puzzle's solution. Hints are tracked and displayed in your completion stats, so you can aim for hint-free solves as a personal challenge.

Hints do not unlock automatically — you choose when to use them. There's no limit, so use as many as you need while learning.

Difficulty Levels

Difficulty	Grid Size	What Makes It Hard
Easy	5×7	Most placements are forced — only one valid rectangle for the clue
Medium	7×9	Some ambiguity requires you to consider multiple clues together
Hard	9×12	Complex constraint chains; requires methodical elimination
Extreme	12×14	High ambiguity with long deduction chains
Expert Unlock	15×17	Large grids with deep interdependencies
Insane Unlock	18×20	Very large; requires tracking many possibilities simultaneously
Nightmare Unlock	21×24	Maximum grid size; extensive backtracking typically required

Solving Strategies

See the full Shikaku Strategy Guide for in-depth techniques. The short version:

Start with small clues. A clue of "1" is always a 1×1 square. A clue of "2" can only be 1×2 or 2×1. Resolve these first.
Work the corners and edges. Clues near corners have far fewer valid rectangle orientations, making them the easiest to pin down.
Eliminate by area. If a clue is 12 and the only rectangles that fit are 3×4 and 4×3, you know which cells must be inside that rectangle regardless of orientation — the intersection of both options is committed.
Look for cells with only one possible owner. If only one clue's candidates cover a particular cell, that clue must own it.

Completion and Stats

When you correctly place rectangles covering every cell of the grid, the puzzle is solved. A completion screen shows:

Your solve time
Move count (each rectangle placed or removed counts as one move)
Hints used
Whether you set a personal best time for that puzzle

Best times are saved locally in your browser and persist across sessions.

Progress and Session Restore

Your current puzzle progress is saved automatically as you play. If you close the tab or browser and return later, your progress is restored — no account or login needed. All data stays in your browser's local storage.