Nurikabe: rules and strategy

Updated 2026-10-30

Nurikabe is a shading puzzle: you decide which cells are land (islands) and which are water (the sea), using the numbers in the grid as island sizes. The rules fit in a few lines, and a handful of techniques will take you from your first grid to confident solving.

The Nurikabe rules

You start with a rectangular grid. Some cells contain numbers; the rest are empty. Your job is to shade some cells so that the finished grid follows these rules:

1. Each number is an island. A numbered cell belongs to an island of unshaded cells, and that island contains exactly as many cells as the number says. A 3 means an island of exactly three cells.
2. One number per island. Every island contains exactly one numbered cell. There are no unnumbered islands floating around.
3. Islands don’t touch. Two different islands may never share an edge. They can touch diagonally, but not orthogonally (left, right, up or down).
4. The sea is connected. All shaded cells together form one connected area, linked through shared edges.
5. No pools. The sea may not contain a 2×2 block of shaded cells anywhere.

Rules 1 and 2 decide where land goes; rules 4 and 5 shape the water. Most deductions come from playing these two sides against each other. A well-made Nurikabe has exactly one solution, so you never need to guess.

How to mark the grid

Use two kinds of marks: shading for sea cells and a dot for cells you know are land. Dotting is just as important as shading, because a dotted cell tells you an island must reach it, and that limits which number can claim it. When an island reaches its full size, surround it with sea right away.

Opening techniques

1-islands

A 1 is a complete island on its own. Every cell orthogonally next to a 1 must be sea. This is often the very first move in a puzzle.

Cells between two numbers

If an empty cell sits directly between two numbered cells (in a row or column), it must be sea: if it were land, it would join two numbers into one island. The same goes for two numbers placed diagonally from each other: the two cells that touch both of them must be sea. In general, any empty cell that is orthogonally adjacent to two different islands is always sea.

Unreachable cells

Every land cell must belong to some numbered island, so ask of each empty cell: can any number reach it? A 3 can extend at most two steps from its clue (counting along a path, not in a straight line through other islands). If no number has enough remaining size to get to a cell, that cell is sea. In early stages this often shades large parts of the grid in one sweep.

Middle-game strategy

Avoiding 2×2 pools

Whenever you see three shaded cells forming an L inside a 2×2 square, the fourth cell must be land. Then ask which island can reach it. Often only one number is close enough, which tells you the direction that island grows. This rule is one of the most productive in the whole puzzle, so scan for near-pools after every few moves.

Keeping the sea connected

A shaded region that is boxed in by land and the grid edge, with only one possible exit, must escape through that exit. So the exit cell is sea. Similarly, if completing an island in a particular way would cut the sea into two parts, that option is impossible. Thinking about where the sea needs to flow is the key to harder grids.

Islands with one way out

If an incomplete island has only one empty neighbour it can still grow into, it must grow there. A 2 next to a wall and a shaded cell, for example, has only one direction left. Combined with the pool rule, this kind of forced growth often cascades across the grid.

Counting cells

Add up all the clue numbers and subtract from the total number of cells, and you know exactly how many sea cells the solution has. Experienced solvers rarely need this, but it can confirm the last few decisions in a tight corner.

A simple solving routine

• Shade around every 1 and between neighbouring numbers.
• Shade every cell no island can reach.
• Look for forced island growth and close off finished islands.
• Check every 2×2 area with three shaded cells.
• Check that each pocket of sea still has a way to join the rest.
• Repeat until the grid is done.

Nurikabe and other grid puzzles

If you enjoy Nurikabe, you will probably like other puzzles where you shade or place things under strict adjacency rules. In Star Battle you place stars so that each row, column and region holds a fixed number of them and no two stars touch, even diagonally. The “no touching” logic feels familiar after Nurikabe’s island separation. Nonograms are another shading puzzle, driven by number clues on the edges instead of inside the grid. For a wider list, see our guide to puzzles like Sudoku, or simply play a free Star Battle puzzle now.