Click the Grid two kinds of grid logic

How to play Nurikabe

Nurikabe is a logic puzzle about separating islands with water. Numbered cells belong to islands; shaded cells form the sea. Every cell must eventually be either island or water.

The rules

  1. Each island contains exactly one numbered cell.
  2. The number is the island’s exact size, including the numbered cell.
  3. Cells in an island connect horizontally or vertically. Diagonal contact does not connect two cells.
  4. All water cells form one horizontally or vertically connected sea.
  5. Water may not fill all four cells of any 2×2 square.

Different islands may touch diagonally, but never along an edge. Water can wind between islands, but it cannot split into separate lakes.

A complete example

The numbered islands at the start

Solved: one connected sea

The completed board has five separate islands of sizes 1, 1, 1, 4, and 3. All dark water cells connect horizontally or vertically, and no 2×2 area is entirely water.

Notice that island shapes do not need to be straight or rectangular. Only their size, numbered cell, and edge-to-edge connection matter.

The best places to start

Three patterns produce immediate deductions in many puzzles:

A completed island of 1

Its four orthogonal neighbours must be water.

Diagonal numbered islands

The shared neighbours are water, or the islands would join.

Avoid a 2×2 pool

With three water cells placed, the last cell must be land.

Surround completed islands

An island cannot grow after it reaches its number. Shade every cell touching the completed island along an edge. This applies to islands of any size, not only islands numbered 1.

Keep numbered islands separate

A land cell may never connect two different numbers. Use nearby clues and existing water to eliminate directions in which an island cannot grow. If only one route remains, those cells must belong to the island.

Watch every 2×2 square

Whenever three cells in a 2×2 square are water, mark the fourth as island. Do this before extending nearby water; otherwise it is easy to create an illegal pool without noticing.

Find cells no island can reach

For an unknown cell to be land, some numbered island must be able to reach it without crossing water, touching another island, or growing beyond its number. If no island can reach the cell, shade it as water.

Preserve one connected sea

Do not seal off a pocket of water. A narrow unknown cell that is the only possible connection between two water areas must itself become water. This connectivity reasoning is especially useful near the end of a puzzle.

A reliable solving routine

  1. Surround every island numbered 1.
  2. Mark the shared neighbours of diagonal numbered cells as water.
  3. Scan for 2×2 squares containing three water cells.
  4. Grow islands only where their remaining size allows.
  5. Surround each island as soon as it is complete.
  6. Recheck unreachable cells and the connectivity of the sea.

Each new cell can trigger several rules, so repeat the scan rather than trying to finish one island in isolation.

Controls

Choose Water or Island, then click or drag across the grid. Unshaded cells count as island cells when the finished puzzle is checked. The Island tool adds a small dot so you can record land cells you have proved; it is optional for completion. Click a cell again with the active tool to clear it.

Numbered cells cannot be changed. Your unfinished board and completed islands are saved automatically in this browser.

Every campaign puzzle is checked to have exactly one solution. Ready to try the rules? Start the first Nurikabe puzzle →