How to Make a Shikaku Puzzle (With a Unique Solution)

Five steps to make a Shikaku by hand

1. Draw a grid — 5×5 or 6×6 is ideal for a first attempt.
2. Split it into rectangles. Mix sizes; avoid too many 1s and very long strips.
3. Write each rectangle’s area in one of its cells.
4. Erase the rectangles and give the numbers to a friend — or solve it yourself a day later.
5. Test uniqueness. If you find a second valid answer, move a number and test again.

Two ambiguity traps (verified by our solver)

Most broken homemade puzzles fail in one of two ways. Each example below was checked by computer: the left version has two solutions, the right has exactly one.

Trap 1: diagonal dominoes

Trap 2: the swappable block

The pattern is general: whenever two neighbouring rectangles together form a bigger rectangle that could be cut a different way, check that the numbers still decide the cut.

What makes a good Shikaku

• A logical path. Solvers should always have at least one certain move — no guessing.
• A good opening. One or two easy entry points, usually near corners.
• Variety. A mix of primes, squares and composite numbers keeps every region interesting.
• Few 1s. A 1 is already solved, so it adds little.

How computer generators do it

Our generator works like a careful human designer: it randomly divides the grid, places a number in each rectangle, then runs an exhaustive solver. If a second solution exists, it moves the numbers in the rectangles that differ and checks again. Finally it grades the puzzle by the techniques required. Read the details on our methodology page, or test your own creation in the Shikaku solver — it reports whether a puzzle is unique.

Questions