Nurikabe: rules and how to play

Goal of the game

Separate numbered white islands with a connected black sea. Every cell must end up as island or sea.

Rules

• Every white island is connected through sides, contains exactly one clue and has exactly as many cells as its clue, including the clue cell.
• Numbered cells stay white. Distinct islands cannot share a side, though corner contact is allowed. No unnumbered white island may remain.
• All black sea cells form one side-connected area. The sea may touch the edge; it need not form a loop.
• No 2×2 square may be entirely black. Unlike Hitori, neighbouring black sea cells are allowed and often necessary.

A small example, step by step

1. A clue 1 is an island already complete in its own cell.
2. Every existing side-neighbour of that cell must therefore be black. Do not automatically blacken diagonal neighbours.
3. Elsewhere, suppose three cells of a 2×2 block are already black.
4. The fourth must be white to prevent an all-black square. That white cell must eventually join exactly one numbered island of the right size.

Beginner deductions

• Surround a completed island with black cells along its sides.
• A cell between two different islands must be black if making it white would join them.
• If an unfinished island has only one possible expansion cell, that cell is white. If a sea component has only one possible exit to the rest of the sea, keep that exit black. Check island sizes and the 2×2 rule after each deduction.

Common mistakes

• Do not count only the added cells: the clue cell is part of its island’s size.
• Do not join two clues into one island or leave a white group without a clue.
• Do not connect the sea only diagonally, and do not apply Hitori’s ban on adjacent black cells.