Shikaku Tips & Strategy
You know the rules — now solve faster and bigger. Strong Shikaku play is about reading every number as a menu of shapes (its factor pairs) and committing only what is proved. These are the techniques that carry you from relaxed 5×5 boards to clean, guess-free 7×7 solves. New to the game? Start with how to play Shikaku.
The strategy, in order
- Translate numbers into factor pairs — a 12 is 1×12, 2×6 or 3×4 (plus rotations); a prime is always a straight strip. The factors ARE the possible shapes.
- Sweep the forced moves first — 1s, corner numbers and primes near edges usually have exactly one legal placement.
- Claim forced cells — if only one number’s possible rectangles can reach a cell, that cell belongs to that number, even before you know the exact shape.
- Use the no-second-number rule aggressively — most candidate shapes die because they would swallow another clue; check this before anything else.
- Leave flexible numbers for last — a big composite number placed early is a guess; placed late, it is forced by its trimmed surroundings.
- When stuck, find the loneliest empty cell — the cell fewest rectangles can reach is where the next proof lives.
Think in factor pairs, not in shapes
The single biggest upgrade: stop staring at the grid and start listing factors. Every clue is an area, and an area can only be made by side lengths that multiply to it. So a 7 is one of two strips, a 9 is a square or a strip, and a 12 is a four-entry menu. Primes and 1s have the shortest menus — that is why they anchor the solve. Composites have long menus — that is why placing them early is where wrong solves come from.
The elimination order strong solvers use
Forced first, flexible last. Work 1s → corners → edge primes → forced cells → composites. Each committed rectangle subtracts cells from every neighbouring menu, so the order matters: the cheap proofs early make the expensive numbers trivial late. On a 7×7 this discipline is the whole difference between a smooth solve and a tangle of erased rectangles.
Want the fundamentals spelled out — what the numbers mean, the three rules, a first walkthrough? That is the beginner guide. Ready boards at both sizes, plus a shared daily, are on the main Shikaku page.
Shikaku strategy: FAQ
What is the best starting move in Shikaku?
Mark every 1 (always a single cell), then place corner numbers and primes — their shape menus are shortest, so their rectangles are usually forced immediately.
How do I handle big numbers like 12 or 16?
Leave them for last. Big composites have many possible shapes early on; after the forced rectangles around them are committed, usually only one legal shape remains.
Why does my board end with an impossible empty cell?
A rectangle you committed earlier was plausible but not proved, and it stranded that cell. Prefer forced moves — if a shape is not provable yet, work elsewhere and come back.
Does the same strategy work on 7×7 boards?
Yes — nothing new is needed, the menus are just longer. The factor-pair habit and the forced-first order matter more as the grid grows, not less.