Shikaku vs Sudoku

If you like Sudoku, you already have every skill Shikaku needs. Both are solved by pure deduction, both have exactly one solution, and neither ever requires a guess. What differs is the object you are reasoning about.

The rules, side by side

Sudoku. Fill every cell with a digit from 1 to 9 so that no digit repeats in any row, any column, or any 3×3 box. You are told some digits; you deduce the rest. The board is always 9×9, and the answer is a grid of symbols.

Shikaku. Divide the whole grid into rectangles so that each rectangle contains exactly one number, and that number equals the number of cells it covers. You are told the numbers; you deduce the rectangles. The board is any size, and the answer is a partition of the grid.

Four real differences

What you deduce

A Sudoku cell has a hidden value, and every deduction narrows a set of nine candidates down to one. A Shikaku cell has a hidden owner: which rectangle it belongs to. That makes Shikaku feel geometric rather than symbolic. Where a Sudoku player thinks "this cell is a 4", a Shikaku player thinks "this cell has to belong to that 6".

The constraint is local, not global

In Sudoku, placing a 7 constrains eight other cells in its row, eight in its column, and its whole box, some of them far away. In Shikaku, placing a rectangle constrains only the cells it touches and its immediate neighbours. Shikaku deductions stay local, which is why the puzzle scales so naturally: a 12×12 is not just a bigger 7×7, it holds more independent little arguments at once.

Arithmetic

Sudoku has none, despite the digits, which are really just nine distinguishable symbols; you could play with colours. Shikaku has a little: you need to know that 12 is 2×6 or 3×4. Neither asks you to add anything, which is what separates both from Kakuro, where clues are sums.

The finish

A finished Sudoku is a wall of digits, and reading whether it is right takes a scan. A finished Shikaku is a mosaic, and being right is something you can see from across the room. That is a small thing, but it is why Shikaku boards look the way they do.

Which is harder?

Neither, as families. Both stretch from a two-minute warm-up to something that will hold you for an hour. What is true is that the difficulty knobs are different. A hard Sudoku is one whose deductions are rare and deep. A hard Shikaku is one where the direct deductions run out and you must assume a placement and follow the consequences until something breaks.

Kaku measures exactly that, counting how many times a human solver would have to look ahead, and grades the board on the count rather than on its size. There is more on this in the difficulty ladder, and the deductions themselves are in Shikaku techniques.

Common questions

Is Shikaku harder than Sudoku?

Neither is harder as a family; both range from gentle to brutal. They are hard in different ways. Sudoku asks you to track which of nine symbols can live in a cell. Shikaku asks you to see shapes, because a number is not a symbol, it is the area of a rectangle you have not drawn yet.

Do you need arithmetic to play Shikaku?

Only counting and, informally, factor pairs. A 12 can be 1x12, 2x6, 3x4 or their rotations. There is no addition, no sums to balance, and unlike Kakuro no arithmetic constraints between clues.

Does Shikaku ever require guessing?

No, exactly as in Sudoku. A well-made puzzle has one solution reachable by deduction alone. Hard puzzles may require you to follow a chain of consequences several steps deep, but a chain of consequences is proof, not a guess.

Is Shikaku the same as Rectangles or Divide by Squares?

Yes. Shikaku (四角, Japanese for square or rectangle) is also published as Rectangles, Divide by Squares and Sikaku. The rules are identical.

Where to start

Read how to play Shikaku, which works a real 5×5 board from empty to solved in seven placements. It takes about five minutes, and Sudoku players tend to finish their first puzzle immediately afterwards.