How to Play Nurikabe

Nurikabe is a shading puzzle. The grid has numbered cells called “clues”. Your task: shade some cells black to form a connected “sea”, while the remaining unshaded cells form “islands” — one island per clue, with the size of each island matching its clue's number.

The Puzzle

A 5×5 puzzle with 5 clues. The numbers are island sizes. Cells without numbers may end up sea or island.

The Four Rules

Rule 1: Each clue is part of exactly one island, and that island has the clue's size

A “2” means an island of two unshaded cells (the clue cell itself plus one orthogonally adjacent neighbor). A “4” means four cells, all orthogonally connected, including the clue.

Rule 2: Islands cannot touch each other orthogonally

Two islands may share a corner (touch diagonally), but they may not share an edge. So any cell sandwiched directly between two different clue cells must be sea — otherwise it would belong to both islands.

Rule 3: All sea cells form one connected region

From any sea cell you must be able to walk to any other sea cell by moving up/down/left/right through sea cells only.

Rule 4: The sea may not contain a 2×2 block

Any four sea cells forming a 2×2 square is forbidden. This rule keeps the sea “thin” — it can't form fat blocks anywhere.

Walkthrough

Step 1 — The “1” clue is a singleton island

A clue of 1 means an island of one cell — just the clue cell itself. Since islands can't touch, all four orthogonal neighbors of a 1-clue must be sea.

Step 2 — A cell between two clues must be sea

In the bottom row, the clues 2 (column 2) and 3 (column 4) sit one cell apart. The cell between them — column 3 — would touch both clues if it were an island. That would merge two distinct islands. So column 3 must be sea.

Step 3 — Continue applying clue separation and 2×2 prevention

The same logic — clue separation, plus the 2×2 prevention rule — propagates through the rest of the grid:

• The clue 2 at (1,1) and the clue 4 at (1,5) need their islands separated. The path between them runs through row 1 / row 2 — careful inspection forces several cells to sea.
• The clue 2 at (5,2) is one cell away from the “1” island at (3,3); the cell between them (4,2) must be sea.
• Whenever 3 of 4 cells in a 2×2 are sea, the 4th must be island. Conversely, if shading a cell would create a 2×2 sea block, that cell must instead be island.

Combining all four rules eventually pins down each cell. The unique solution:

Tips for Beginners

• Start with 1-clues. They're always isolated — all 4 orthogonal neighbors are sea. Free shading.
• Then look at clue separations. A cell directly between two clues (in the same row or column) must be sea.
• Watch the 2×2 rule. Whenever a 2×2 block has 3 sea cells, the 4th must be island. This often forces unshaded cells near other shaded clusters.
• Track island reachability. Each island has a fixed size and a fixed clue. The clue's neighbors are candidates for the next island cell — but only the ones that don't touch a different clue.
• Keep the sea connected. If a candidate shading would isolate one part of the sea from another, it's wrong.

Ready to try one yourself? Hit the button below to play your first Nurikabe.