How to Solve Nurikabe Puzzles

How to play on this site

Tap a square to shade it. Tap again and it shows a dot, which is a note meaning "this square is island". A third tap clears it. Dots count as unshaded, but you never have to place them: the puzzle is solved the moment the shaded squares are right. Drag across several squares to paint them all the same way, and use a long press or a right click to drop a dot straight away. If you would rather tap for a dot first, switch on Dot first in the options.

Show mistakes rings problems in red: a 2 × 2 block of wall, an island that has grown too big or has two numbers, an island sealed off with no number, and a piece of wall cut off from the rest. Mark finished islands colours an island green once it has its full size and is walled in. Easy grids are 7 × 7, medium 8 × 8 and hard 10 × 10. On a keyboard, the arrow keys move, Space cycles a square, S shades, D places a dot, Backspace clears and U undoes.

Start with the 1s and the gaps

A 1 is already a finished island, so every square beside it, above, below, left and right, is wall. Shade them first; on most grids this gives the wall its first shape.

Next look for squares that sit between two numbers. If an empty square touches two different numbered squares, or two islands that already hold a number each, it cannot be island: it would join the two islands into one with two numbers. So it is wall.

Two numbers two squares apart in a row or column always have a wall square between them, for the same reason. These first moves are quick and safe, and they cut the grid into smaller pieces that are easier to think about.

Squares no island can reach

Every island grows out of its number, and it can only grow as far as its size allows. A 3 can reach squares at most two steps away; a 2 only its direct neighbours. Any square that no number can reach, counting steps around the wall you already have, must be wall.

This is the most useful idea in Nurikabe, and it gets stronger as the puzzle goes on. When an island is finished, its squares stop reaching anything, and when walls appear they make the paths longer. Late in a grid, whole corners often turn out to be out of reach of every number, and you can shade them in one drag.

Remember to count steps around walls and around other islands, not through them. An island cannot pass next to another numbered island either, so a narrow gap beside a different number is often a dead end.

Finishing islands

An island with its full number of squares is finished: every empty square beside it becomes wall. Shade the border as soon as you finish one, or watch for the green colour when Mark finished islands is on.

An island that still needs squares and has only one way to grow must grow that way. A 2 with three of its four sides already blocked takes the fourth square. Even a bigger island with a single open side has to step through that square first, and the square after it may then be forced too. Islands next to the edge of the grid run out of room quickly, so look along the edges first.

Dots are useful here. A square that must be island but has no number yet belongs to some island; place a dot on it and work out which number can reach it. Often only one can, and that tells you how the island bends.

The 2 × 2 rule

Wherever three squares of a 2 × 2 block are wall, the fourth must be island. That square then needs a number that can reach it. Usually there is only one, and the path from that number to the square is forced too, which can finish the island in one go.

The rule also works in advance. A long straight stretch of wall two squares wide is impossible, so wide open areas far from the numbers cannot all be wall. Some island has to reach into them, and the islands that can are often fewer than you would think.

Keep the wall in one piece

The wall must join up. A piece of wall with only one empty square next to it must grow through that square, or it would be cut off forever. So that square is wall too. This often decides the last squares around a finished island, and it is the reason the wall never quite runs into a dead end.

Watch for a square whose shading would split the open area so that a piece of wall can no longer reach the rest: that square must be island. Show mistakes rings a cut off piece of wall in red, so a slip here never goes unnoticed.

Counting the wall

The numbers tell you how many squares are island: add them up and subtract from the size of the grid, and you have the number of wall squares. The counter above the grid shows how many you have shaded against that total. When only a few wall squares are left to place, it is easy to check which open squares can still take them.

Common mistakes

• Joining two islands. A single shaded square between two numbers is easy to forget. Two numbers in one island is the most common reason a grid will not come out.
• A 2 × 2 block of wall. Shading fast around finished islands sometimes closes a block. Show mistakes catches it at once.
• Islands without numbers. Every unshaded square must belong to an island with a number. A dot walled in on its own is a mistake.

Scoring and the leaderboard

Nurikabe is ranked by time, fastest first; fewer changes break ties. Every tap is a change, so painting long runs of wall with a drag is quicker than tapping square by square. The grid is checked the moment the wall is right, and only a member's first solve of the day is ranked. If you like this kind of reasoning, try Tents & Trees and Bridges next.