Nurikabe

Nurikabe is a Japanese shading puzzle in which you shade cells into one connected sea so the unshaded cells split into separate islands, each numbered cell marking an island of exactly that size. It is played on a square grid dotted with a few numbered cells. Your job is to shade some cells to form the sea — also called the wall — so that the cells you leave unshaded break up into separate islands. The numbers are the only clues you start with, and from them the entire layout is fixed.

What are the four rules of Nurikabe?

Four rules govern every board, and they work together. First, each island contains exactly one number, and that number is the island’s size in cells — a 3 marks an island of three unshaded cells, no more and no less. Second, no two islands may touch edge-to-edge; if two unshaded cells from different islands met along a side they would merge into one, so a wall of sea must always run between them (touching only at a diagonal corner is fine). Third, the sea is a single connected piece — you must be able to travel from any shaded cell to any other through shaded cells, moving up, down, left or right. Fourth, the sea may never contain a 2×2 block of shaded cells; four shaded cells in a square is forbidden anywhere on the grid.

Why is Nurikabe solvable by logic alone?

These rules pull in opposite directions, and that tension is exactly what makes Nurikabe solvable by logic alone. Islands want to grow to reach their number, but they must not touch their neighbours; the sea wants to flood freely, but it must stay connected and never form a fat 2×2 square. At every step there is a cell whose state is forced — it must be sea, or it must belong to an island — and following those forced moves leads to the one and only answer. Every puzzle here has exactly one solution, reachable by reasoning, never by guessing.

To solve Nurikabe, spot the cells that are forced before shading anything on a hunch — start with the 1s, the gaps between clues and the unreachable corners. Good solving is about reading which cells are forced, and these are the core nurikabe tips, roughly in the order you will reach for them:

• A “1” is a finished island. A clue of 1 is already complete on its own cell, so every cell next to it must be sea. This is the fastest opening move on any board — shade all four neighbours of every 1.
• Separate clues with sea. Two numbered cells can never share an island, so any cell that sits between two clues — diagonally adjacent numbers especially — must be sea to keep their islands apart. The cell touching two different clues is forced dark.
• Use the no-2×2 rule in reverse. If three cells of a 2×2 square are already sea, the fourth can never be shaded, so it must belong to an island. This single rule turns “empty” cells into island cells again and again.
• Shade the unreachable cells. Count how far each number can stretch. A cell that no island could possibly reach — too far from every clue for that clue’s size to cover it — must be sea. Marking these unreachable cells often opens up half the grid.
• Keep the sea connected. Because all the water is one piece, watch for a sea cell that is about to be walled off. If shading would leave a patch of sea with only one way out, that escape cell is forced to stay sea so the water can join up.

Which Nurikabe moves should you make first?

Build the habit of taking the forced moves first — the 1s, the gaps between clues, the unreachable corners — and only then look at the harder deductions. After each cell you shade, re-scan its neighbourhood for a new 2×2 threat or a newly trapped island. And never guess: on a proper Nurikabe there is always a next cell you can prove.

A few deductions recur on almost every Nurikabe board — learn to see these shapes and the grid opens up quickly.

• The completed 2×2 corner. Whenever three cells of any 2×2 square are sea, the fourth is instantly an island cell — the no-2×2 rule forbids the alternative. Scan for L-shaped clumps of three sea cells and fill in the missing corner as land.
• Walling an island to size. Once an island has grown to exactly its number, it is finished, so every cell around its whole border must be sea. A 2 that has claimed its second cell, for instance, gets a complete ring of water — which in turn often completes a 2×2 corner nearby.
• Don’t cut the sea in two. Before shading a cell, picture the water as one body. If a move would split the sea into two pieces that can no longer reach each other — or strand a single sea cell — it is illegal, which usually means a different cell was forced to be land instead. Keeping the sea whole is as much a clue as any number.

A small corner shows how Nurikabe deductions chain together — a single clue of 1 forces a cross of sea, and from there each move hands you the next. Suppose a cell carries the clue 1. A 1 is a whole island by itself, so straight away its up, down, left and right neighbours are all sea — four shaded cells placed with total certainty, no counting required. That little cross of water is often the very first thing to go on the board.

How does a clue of 2 complete itself?

Now look at a clue of 2 a short way off. Its island needs exactly two cells, so it reaches into just one neighbour. The moment you can tell which neighbour it must take — because the others are already sea, or would touch another island — the island is complete at size two. And once it is complete, every cell around both of its cells must be sea, since nothing may touch a finished island. You have turned one clue into a whole ring of water.

How does one move lead to the next?

That ring frequently hands you the next move for free. Lay it down and you may find three cells of some 2×2 square are now shaded — which forces the fourth to be land — or that a far-off cell can no longer be reached by any number, so it too must be sea. Take the forced move, re-scan, repeat. Cell by cell, the sea and the islands resolve into the single solution, without a guess anywhere.

This page offers two sizes: a 5×5 grid as the friendly introduction, and a 6×6 grid for a meatier solve with more clues and longer chains of deduction. The 5×5 is small enough to hold in your head, and the perfect place to get a feel for how the 1s, the gaps between clues and the no-2×2 rule force your hand. The 6×6 brings more clues, longer chains of deduction, and more chances for the “keep the sea connected” rule to bite.

What makes a Nurikabe board hard?

Difficulty in Nurikabe is not really about size — it is about how much each clue constrains its surroundings. A board sprinkled with small islands and tight 2×2 threats almost unravels itself, while one with larger islands and more open water asks you to look further ahead, counting reach and watching the sea stay whole before any cell is certain. As the grid grows, those interactions multiply and the satisfying chains get longer.

How do New, Daily and Reset work?

Tap New for a fresh board at either size whenever you like, or play the shared Daily to solve the very same puzzle as everyone else that day — a small, repeatable ritual and an easy way to compare notes. Reset clears your shading back to the starting clues if you want a clean run at it.

Nurikabe is the name everyone uses, though in English it has occasionally appeared as “Islands in the Stream” or, more drily, “Cell Structure” — the same puzzle under different labels. The first is a neat picture of the unshaded islands surrounded by flowing sea.

However it is named, Nurikabe belongs to the family of shading puzzles: you solve it by darkening cells, not by writing numbers into them. That makes it a close cousin of the other shading puzzles in this collection. Try Nonogram, where row and column clues tell you which cells to shade to reveal a hidden picture, or Hitori, where you shade out duplicate numbers under rules that — just like Nurikabe — keep the unshaded cells connected and the shaded cells apart. All three reward the same patient, cell-by-cell reasoning.

A handful of words turn up in every Nurikabe guide — sea, island, clue, the 2×2 rule and reach.

• Sea (wall) — the shaded cells, taken all together. They must form one single connected group, and they must never make a 2×2 block.
• Island — a connected group of unshaded cells, fenced off from every other island by sea. Each island holds exactly one clue.
• Clue (number) — a numbered cell. It can never be shaded, and its number is the exact size, in cells, of the island it sits on.
• 2×2 rule — the ban on four shaded cells forming a square anywhere on the grid; on this site a 2×2 shaded block turns red to warn you.
• Reach — how far an island could extend from its clue given its size; cells beyond the reach of every clue must be sea.

You do not need the vocabulary to play — just tap to shade — but it makes any strategy guide far easier to follow.

Most Nurikabe tangles come from a few avoidable habits: letting islands touch, building a 2×2 sea, cutting the sea in two, mis-sizing an island, or guessing.

• Letting two islands touch. Islands may meet at a diagonal corner but never along an edge. If two numbered regions sit side by side, the cell between them is sea — forgetting this merges them and breaks the board.
• Creating a 2×2 sea. It is easy to flood a corner and accidentally shade four cells in a square. Watch every cell you darken for the square it might complete — that is why the block turns red here.
• Cutting the sea in two. The water is one body. Shading a cell that strands part of the sea, or seals off a lone water cell with no neighbours, is illegal even if every island looks right.
• Over- or under-sizing an island. An island must match its number exactly. Stopping one cell short, or letting it sprawl one cell too far, is the most common slip — count as you go.
• Guessing. Shading on a hunch can quietly contradict a rule far away and corrupt the grid for many moves. There is always a forced cell — find it instead.

Nurikabe scratches a particular itch — the pleasure of carving a plain grid into tidy islands and one sweeping sea until every number is satisfied and the water flows as a single piece. It is calm, visual and entirely logical: no arithmetic beyond counting an island’s size, no luck, and a single answer always waiting to be deduced. A quick 5×5 is a relaxing few minutes; a 6×6 is a more involved spatial workout for when you have time to think.

It is also a genuine little workout for the mind. Because you must hold several constraints at once — island sizes, the gaps between them, the connected sea, the forbidden 2×2 — Nurikabe trains logical reasoning, spatial planning and working memory, and rewards patience over speed. Every board is fair by design, so progress always feels earned, and the shared Daily gives you the same grid as everyone else to compare against. If you enjoy turning a few numbers into one exact layout, try Nonogram for picture-shading or Hitori for more connected-region reasoning next.

Nurikabe is a Japanese pencil puzzle popularised by the celebrated publisher Nikoli — the same house that brought Sudoku and many other classics to a wide audience. Like much of the Nikoli catalogue, it is designed to be solved by pure deduction, with a single logical answer to every grid.

Its name comes from Japanese folklore. The nurikabe is a yōkai — a creature of legend — said to take the form of an invisible wall that blocks travellers walking at night, an unseen barrier you cannot get around no matter which way you turn. The puzzle borrows the image beautifully: the sea you shade is exactly that wall, hemming in the islands and barring every path between them.

In English the puzzle has occasionally been printed as “Islands in the Stream” or “Cell Structure”, but the original name has travelled the world intact — a small piece of Japanese folklore living on in a grid of shaded cells.