Nurikabe Puzzle
Nurikabe gives you a grid with a handful of numbered cells scattered across it. Everything else is blank. Your job is to decide which empty cells are “sea” (black) and which belong to numbered “islands” (white). Each number tells you exactly how many cells its island contains. The sea has to form one connected mass with no 2×2 pools, and islands can never touch each other side-to-side. That dual constraint — building islands while maintaining a connected sea — is what makes it click.
Nurikabe
5×5 — Small grids where island isolation and basic unreachable-cell logic get you through.
Standard play. Timer runs. Hints available.
What is Nurikabe?
Nurikabe is a logic puzzle from Nikoli, the Japanese publisher behind Sudoku, Kakuro, and Hitori. It first appeared in Puzzle Communication Nikoli #33 in March 1991, created by a puzzle author going by the name renin (れーにん). The puzzle takes its name from the yokai nurikabe — a creature from Japanese folklore that manifests as an invisible wall, blocking travelers on dark roads. In the puzzle, the sea of black cells acts as that wall, separating and surrounding the numbered islands.
What sets Nurikabe apart from most grid puzzles is the interplay between two different connectivity requirements. The islands must stay separate (they can't touch each other orthogonally), but the sea must stay unified (all black cells form one connected region). Every cell you place affects both sides of that equation. Mark a cell as sea, and you might connect two sea regions but accidentally cut off an island's only growth path. Mark it as island, and you might complete one island but create a 2×2 pool or disconnect the sea. The tension between these constraints is what makes even small grids satisfying to work through.
The four rules
Every Nurikabe puzzle follows four constraints:
- Island size matches the clue. Each numbered cell belongs to a white island containing exactly that many cells, including the numbered cell itself.
- One clue per island. Each island contains exactly one numbered cell. You won't find two numbers in the same island.
- Connected sea. All black cells must form a single orthogonally connected group. No isolated patches of sea allowed.
- No 2×2 pools. No 2×2 block of cells may be entirely black. The sea flows in channels, never in wide pools.
One consequence of rules 1 and 2 together: islands belonging to different clues can never touch orthogonally. If they did, they would merge into a single island with two clues, violating rule 2.
For solving techniques and worked examples, read the full strategy guide.
How to play
Each cell cycles through three states when you click or tap:
- Unmarked — not yet decided
- Black (sea) — you believe this cell is part of the sea
- White (island) — you've confirmed this cell belongs to an island
Numbered cells are always part of their island and can't be toggled to black. The white state for non-numbered cells is a solving aid — it marks cells you've confirmed as island territory. Keyboard shortcuts: arrow keys to navigate, Space/Enter to cycle, B for black, W for white, Delete to clear.
Also known as
Nurikabe goes by a few other names depending on the publisher. Conceptis Puzzles markets it as “Islands in the Stream,” which describes the visual result — white islands scattered in a river of black cells. Wikipedia also lists “Cell Structure” as an alternative. Neither alias has much search volume on its own, and “nurikabe” is the universally recognized name in the puzzle community.
Difficulty levels
| Level | Grid | What to expect |
|---|---|---|
| Easy | 5×5 | Small grids where island isolation and basic unreachable-cell logic get you through. |
| Medium | 7×7 | The 2×2 rule and neighbor reasoning start to matter. Islands interact more. |
| Hard | 10×10 | Sea connectivity forcing enters the picture. You need to think about the sea as a whole. |
| Expert | 12×12 | Multi-step connectivity chains and expansion logic across 144 cells. |
| Einstein | 15×15 | Tightly interleaved islands on a 225-cell grid. Deep inference, no guessing required. |
Key strategies at a glance
The first technique most people learn is island isolation: a cell numbered “1” is an entire island by itself, so every orthogonal neighbor must be sea. From there, look for unreachable cells — any cell that sits too far from every clue to be part of any island has to be sea.
On medium and harder puzzles, the 2×2 avoidance rule becomes a reliable source of forced white cells: if three black cells form an L-shape, the fourth corner must be white. And connectivity forcing is where the real depth lives — if marking a cell as island would split the sea into separate pieces, that cell has to stay sea, and vice versa.
Frequently asked questions
What is nurikabe?
A grid logic puzzle from Nikoli. Shade cells to form a connected sea around numbered islands. Each island's size must match its clue number, and no 2×2 block can be all sea.
Is nurikabe harder than sudoku?
On small grids, Nurikabe is simpler. On 12×12 or 15×15, the connectivity and island adjacency constraints make it at least as hard. The two puzzles test different skills — Sudoku is about candidate tracking; Nurikabe is about spatial connectivity.
What does “nurikabe” mean?
It means “painted wall” in Japanese. In folklore, the nurikabe is a yokai that appears as an invisible wall blocking travelers at night. The puzzle name references the sea of black cells walling off the islands.
What is the 2×2 rule?
No 2×2 block of cells can be entirely black. This prevents the sea from forming wide pools, forcing it to flow in narrow channels between islands. It's one of the most useful constraints for making deductions.
How is nurikabe different from hitori?
Both are Nikoli grid-shading puzzles, but the mechanics are opposites. Hitori starts with a full grid and removes duplicates; black cells can't touch. Nurikabe starts mostly empty; black cells must all connect. Hitori is about elimination, Nurikabe is about construction.
Related puzzles
If you enjoy Nurikabe, try these:
Hitori
Nurikabe's closest sibling. Same three-state cell cycle, opposite shading rules. Both from Nikoli.
Binairo
Another binary-state puzzle. Fill cells with 0s and 1s using counting and adjacency rules.
Slitherlink
A Nikoli sibling. Draw a single loop along grid edges using number clues about edge counts.
Minesweeper
Grid deduction with adjacency logic. If you like reasoning about neighbor constraints, this is a natural fit.