How to play

Shikaku is played on a square grid with numbers scattered across it. You divide the whole grid into rectangles.

1. Every rectangle contains exactly one number.
2. The number gives the rectangle's area, counted in cells. A 6 can be 1×6, 6×1, 2×3 or 3×2.
3. Squares count as rectangles, so a 4 can also be 2×2.
4. Rectangles cannot overlap, and together they must cover every cell of the grid.
5. The puzzle is solved when the last cell is covered.

Each puzzle has exactly one correct way to divide the grid.

Strategy

Place the 1s first. A 1 is always its own single cell. Then look at 2s: a 2 boxed in on three sides has only one shape left.

Know the shapes of each number. Prime numbers like 5, 7, 11 and 13 can only be a single row or column of cells. A 4 can be a strip or a 2×2 square, and a 12 has up to six. Fewer shapes means fewer places to check.

Look for cells only one number can reach. Pick an empty cell and ask which numbers could stretch a rectangle over it. If only one can, that number's rectangle must include the cell, which often fixes its shape.

Watch for other numbers in the way. A rectangle can never cover a second number. Large numbers squeezed between other clues often have just one shape that avoids them all.

Work in from the corners and edges. A corner cell can only be covered from two directions. Numbers near the border lose half their options because rectangles cannot leave the grid.

Count what a region needs. When placed rectangles wall off a small region, the numbers inside it must add up to its size. Use that to rule out shapes that would leave an awkward leftover cell, like a single empty square that no number can reach.

Erase freely. If a rectangle blocks progress elsewhere, remove it and try another shape. Undo steps back through your moves.

History

Shikaku is one of the pencil puzzles published by Nikoli, the Japanese puzzle publisher that also popularised sudoku. In English it is also called Rectangles or Divide by Box, and sometimes spelled Sikaku. The Japanese word shikaku means a square or a four-sided shape.

Playing on Summoned Games

Pick a grid by difficulty: 5×5 easy, 7×7 medium, 10×10 hard or 15×15 expert. New puzzles are generated when you ask for one, so they never run out, and each is checked to have exactly one solution. The daily puzzle is a 10×10 grid, the same for everyone that day, and a daily streak tracks the days in a row you solve it.

The game stops rectangles that break the rules: covering two numbers, covering none, the wrong area, or overlapping another rectangle. Each refused rectangle adds one to your mistake count and shows the reason. The Candidates button outlines every rectangle the selected number could still be. The win screen shows your time, mistakes and how many times you used candidates.

Solved puzzles count toward the puzzles solved leaderboard, and 10×10 grids, the daily included, have a leaderboard for the fastest time. Your best time per size shows in the menu. Your game saves as you play, so you can close the tab and resume later.

On a phone or with a mouse, tap one corner of a rectangle, then the opposite corner, or drag from one corner to the other. Tap a placed rectangle twice, or select it and press Erase, to remove it. Keyboard players use the arrow keys, Enter or Space to pick corners, Delete to erase, H for candidates and U to undo.

FAQ

Does every puzzle have only one solution? Yes. Each puzzle is checked by a solver before you see it, and any puzzle with a second solution is changed until it has exactly one.

Can a rectangle be a square? Yes. A square is a rectangle, so a 4 can be 2×2 and a 9 can be 3×3.

What are candidates? A help option. With it on, selecting a number outlines every rectangle that number could still form, given the rectangles you have already placed. Using it is counted on the win screen.

Is Shikaku the same as Rectangles or Divide by Box? Yes. These are other names for the same puzzle.

Is it free? Yes. It runs in the browser with no ads and no download.