THE SHORT VERSION
Sudoku: a 9×9 grid, partially filled with digits 1–9. Fill the empty cells so every row, column, and 3×3 box contains each digit exactly once. You place values into a fixed structure.
Shikaku: a grid with a few numbered cells. Draw lines to divide the grid into rectangles (or squares). Each rectangle must contain exactly one numbered cell, and that number must equal the rectangle's area. You define the structure — the grid is blank except for clue numbers.
Sudoku is a placement puzzle. Shikaku is a partitioning puzzle. They don't share a single solving technique.
SIDE BY SIDE
What you place
Sudoku: digits 1–9 into empty cells
Shikaku: rectangle boundaries across the grid
Starting information
Sudoku: 17–35 pre-filled digit cells
Shikaku: a few numbered clue cells (the rest are blank)
Constraint type
Sudoku: uniqueness — no digit repeats in any unit
Shikaku: area match — each rectangle's size equals its clue
Arithmetic required
Sudoku: none (1–9 are labels, not values)
Shikaku: factoring — you need the factors of each clue number
Grid partitioned at the end
Sudoku: every cell has a digit; the grid is intact
Shikaku: the grid is sliced into non-overlapping rectangles
Japanese origin
Sudoku: popularised by Nikoli in the 1980s
Shikaku: also a Nikoli puzzle (仕掛) from the same era
HOW SHIKAKU ACTUALLY WORKS
Every Shikaku clue is a number N. That number is the area of the rectangle you'll draw around it. If a clue reads 6, you need a rectangle with area 6 — meaning 1×6, 6×1, 2×3, or 3×2 shapes. The clue cell must be somewhere inside that rectangle.
The constraint is coverage: every cell in the grid must belong to exactly one rectangle, and no rectangle may contain two clue numbers. This means rectangles adjacent to walls, corners, or neighbouring clues quickly become forced — only one factoring fits in the available space.
There is no digit placement in Shikaku. The "answer" is a set of drawn rectangles that tiles the grid perfectly.
WHAT DOESN'T TRANSFER FROM SUDOKU
Sudoku's core toolkit — naked singles, hidden singles, pointing pairs, X-wings — depends on uniqueness within rows, columns, and boxes. None of those apply to Shikaku. Shikaku has no units to scan, no candidate lists to manage, and no elimination chains of the Sudoku type.
Sudoku solvers sometimes approach Shikaku by trying to "place cells" the way they'd place digits. This doesn't work. Shikaku is geometric: you're reasoning about spatial shapes and which factored rectangles fit in a region, not about which value belongs in a cell.
WHAT DOES TRANSFER
The elimination mindset carries over: both puzzles are solved by ruling out invalid configurations and confirming forced ones. In Shikaku, the equivalent of a "forced digit" is a clue whose area, combined with its position near a wall or another clue, leaves only one valid rectangle shape and orientation.
Corner and edge discipline transfers too. Sudoku players learn to scan corners and edges first because they have fewer candidate cells. Shikaku's corner and edge clues are similarly constrained — a prime-number clue in a corner can often only be placed one way.
Start Shikaku at corner and edge clues. The walls force the rectangle shape; only the orientation is free.
WHICH IS HARDER?
At entry level, Shikaku is more approachable. Corner clues are immediately forced; clue 1 is a trivial 1×1 square. A beginner can make visible progress on an easy Shikaku within the first minute.
At high difficulty, both puzzles require holding the whole board in your head. Hard Shikaku demands reasoning about how adjacent rectangles share boundaries — a change on one side of the grid can invalidate a rectangle on the other. Hard Sudoku demands long multi-unit elimination chains that Shikaku never produces.
They're genuinely different cognitive experiences. The players who say Shikaku is "easier than Sudoku" are usually comparing entry-level grids. At master difficulty, they're equally demanding — just demanding in different ways.
WHEN TO MAKE THE SWITCH
If you play Sudoku: try Shikaku when you want to apply spatial and geometric reasoning rather than digit elimination. Shikaku uses your grid-reading instincts in a completely different direction. It's a mental reset — same puzzle table, different tools.
If you play Shikaku: try Sudoku when you want a puzzle where the constraints interact across the whole grid simultaneously, and where the candidate-elimination loop creates longer logical chains. Sudoku's triple-constraint structure (row + column + box) is the Shikaku area-match rule's opposite in flavour.