How to Solve Nurikabe: Islands, Walls and the 2x2 Rule

In Nurikabe some squares carry a number. That number is the size of an island: a connected group of that many squares, containing exactly one number. You shade every remaining square as the wall, under three rules:

1. Each island holds one number, and its size equals that number.
2. The wall is a single connected piece.
3. The wall never contains a 2x2 block (no "pools").

Deduction 1: seal the ones

A 1 is an island of a single square, so all of its neighbors are walls (direct neighbors, not diagonal). Shade these first.

Deduction 2: squares between two islands

If a square touches two different numbers, or sits between two so that it would join their islands, it must be wall. The same goes for the two squares shared by numbered ones that touch diagonally.

Deduction 3: unreachable squares

Any square too far from every number to be part of its island is wall. Count steps: for example, a 3 can reach squares at most two steps away.

Deduction 4: no pools

If three squares of a 2x2 block are already wall, the fourth must belong to an island, since a 2x2 block all squares is against the rules. This is the rule that grows islands in open space.

Deduction 5: keep the wall connected

If the wall squares would be cut into two groups, an island must not close the gap. Whenever a single square is the only path between two wall regions, it is wall, because all wall squares must be connected.

Try a real one

This 6x6 has one solution only. Start with the 1 in the top right corner (Deduction 1), then see how far Deduction 4 takes you in the open middle.

Show the solution

Where beginners slip

Finishing the islands before the wall is checked is the usual mistake. If the wall is not continuous or contains a pool (2x2 block), an island is the wrong shape. Fix the island, not the wall.

The Nurikabe app teaches these steps with a guided tutorial and ends on a puzzle with hints if you get stuck.