A step-by-step guide to solving a medium Nurikabe puzzle

Nurikabe is a shading puzzle built around a simple contrast: islands must remain separate and correctly sized, while the sea must form one connected region. Each numbered cell belongs to an island containing exactly that many squares. Every unnumbered square becomes either land or water.

A medium-level grid usually gives you enough information to make several deductions at once, but not enough to solve it by counting alone. The key is to alternate between island growth, water placement, and checks for forbidden patterns.

For this walkthrough, use a pencil, an online grid, or two clear symbols: a circle for confirmed land and a cross for water. Work from definite moves first. Guessing too early often creates a convincing-looking island that later breaks the sea rules.

The same habits are useful whether you are solving during a train trip in Sydney, at a Melbourne library, or over a coffee in Brisbane. Australian solvers often call the afternoon an “arvo”, but a few quiet minutes with a Nurikabe board can be productive at any time.

Read the board before making moves

Begin by circling every clue and checking its immediate neighbours. A clue showing 1 is already complete, so all orthogonally adjacent cells must be water. This is usually the first reliable source of crosses on a medium puzzle.

Next, look for clues that are close together. Two clues cannot belong to the same island, even if their numbers would permit it, because every island must contain exactly one number. If two clues sit in adjacent cells, the shared boundary is automatically water; the same applies to any cell that would connect their growing regions.

Do not treat diagonal contact as a problem. Islands may touch at corners, so a diagonal pair of land cells does not merge two islands. This distinction prevents many unnecessary water marks.

Mark the cells that must be land

Every clue cell is confirmed land. Some numbered cells also have neighbours that are forced land because their island has no other route to reach its required size. For example, a 3 clue in a narrow corner may have only two available cells beside it; both must join the island.

Use the clue’s number as a capacity, not as a target to approach casually. A 4 island can occupy four cells in several shapes, but it cannot expand into a region that would make one of those shapes impossible. When an island has reached its number, mark every orthogonally adjacent undecided square as water.

A useful medium-level pattern is the “closed end”. If a clue has one possible extension in a direction and all other directions are blocked by water or another clue, that extension is forced. Record it immediately, then recount the island from its clue rather than from the newest cell.

Grow each island without merging clues

Suppose a 2 clue has one confirmed land cell beside it. Its island is complete, so the remaining neighbours of both land cells become water. If a 3 clue has two connected land cells, it is also complete and must be sealed in the same way.

When an island still needs cells, examine its frontier. A candidate square is impossible if adding it would touch another clue or completed island orthogonally. It is also impossible if the addition would leave the clue unable to reach its required size.

A practical approach is to trace each possible path outward from the clue. If every legal path to the remaining number of cells passes through one square, that square is forced land. This “common route” deduction is especially helpful in medium puzzles where a clue is boxed in by earlier water marks.

Use neighbouring clues to create barriers

Two nearby clues often control the cells between them. Imagine a 2 clue and a 3 clue separated by a narrow strip. If the 2 island can expand only towards that strip, the cells required by the 3 island may become unavailable, forcing the 2 island to grow elsewhere.

Mark water between islands as soon as their separation is certain. A completed island must have water around it, but even an unfinished island may have water beside it when that square would create contact with a different clue.

This is where diagonal relationships become useful. Two islands can sit diagonally across a water corner, so do not fill every corner between them. Instead, ask whether the square creates an orthogonal connection. That simple check keeps the diagram accurate.

Watch for forbidden two-by-two water blocks

The sea may be connected, but it can never contain a solid 2-by-2 block of water. Whenever three cells in such a block are marked water, the fourth must be land. This is one of the strongest deductions in a medium Nurikabe.

The forced land square may belong to a nearby clue, or it may reveal which island must pass through that area. Check every new cross against surrounding 2-by-2 windows. A scan across rows and columns is faster than inspecting the whole board randomly.

Water connectivity provides a second check. If a proposed water mark would cut off a separate sea region from the rest of the water, it cannot be water. Sometimes this means a square must remain land even though no island number appears nearby.

Combine island and sea deductions

Nurikabe becomes easier when you switch viewpoints. After extending an island, inspect the surrounding water. After creating water, revisit the islands that border it. A new cross may reduce a clue to one possible route, while a new land cell may prevent a dangerous water loop.

Digital solving tools can help with notation and error checking, particularly for competitors comparing techniques between practice sessions; discussion of digital solving tools also shows why software works best as support rather than a replacement for deduction.

If you practise on a phone during a Sydney commute, zoom in before placing marks. On paper at a local puzzle gathering, keep the original clue visible and avoid overwriting it. Clear notation matters because Nurikabe deductions often depend on seeing several rows at once.

Finish with a complete rule check

When the board appears solved, count every island from its clue. Each must be orthogonally connected, contain exactly one number, and match that number precisely. A region that looks like an island but contains two clues is invalid, even if its total size is correct.

Then inspect the water separately. All water cells should form one orthogonally connected sea, with no isolated pocket and no 2-by-2 water square anywhere. Check edges and corners carefully, since mistakes often hide where fewer neighbouring cells are available.

Finally, review the last few deductions in reverse order. If the completed grid fails, the error is usually a missed island separation, an overlooked two-by-two pattern, or a cell marked water before its sea connection was established. This final audit turns a plausible medium puzzle solution into a properly verified one.