Nurikabe Puzzle

Nurikabe is a shading puzzle. Numbers sit on white islands in a sea of black cells. You shade the sea so every island matches its number, islands never touch, and the black cells stay one connected piece with no 2×2 pool. You can play grids from 5×5 up to 10×10, and every puzzle here has exactly one solution that you can reach by logic, with no guessing.

Rules

The board is a grid of cells, and some cells hold a positive number. Those numbers are given and stay white.

Shade some empty cells black. The black cells are the sea. They form one connected group across and down, never diagonally, and no 2×2 block is all black.

Every white region is an island. An island contains exactly one number, and it has exactly as many cells as that number. Two islands never share an edge across or down; they may touch at a corner.

A numbered cell is never shaded. Empty cells you leave white belong to some island; empty cells you shade belong to the sea.

The puzzle is solved when every cell is sea or white, the sea is one connected piece with no 2×2 pool, and every island matches its number.

Controls

Tap a cell to shade it sea. Drag along a row or a column to shade several cells in one go; the stroke snaps to the line you started in. Tap a shaded cell again to clear it.

Dots mark cells you know must stay white. Pick Dot under the board, and taps and drags place dots instead. With a mouse you don’t need to switch: left-drag uses the pad tool and right-drag uses the other one. The first cell of a stroke decides what it does, so a stroke that starts on a cell that already has that mark clears instead.

On a keyboard, the arrow keys move around the grid. Space, Enter, S or 1 shades the cell sea, X, D or 2 places a dot, and Backspace, Delete or 0 clears it. Pressing a mark key on a cell that already has that mark clears it.

A 2×2 of sea, a white block with no number, two numbers in one white block, a number whose white block is the wrong size, a sea that is not one piece, and a numbered cell painted sea all turn red at once. Press Hint for one cell the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the board. When the colouring is right, the sea turns teal.

Techniques

Easy grids follow from the numbers and the local sea rules:

• A finished 1: a clue of 1 is already a full island, so every neighbour is sea.
• A full island: once a white block has as many cells as its number, every open neighbour is sea.
• Separation: a cell that touches two different islands must be sea, or the islands would join.
• No 2×2 pool: if three cells of a 2×2 are already sea, the fourth stays white.
• Unreachable cells: a cell no unfinished island can still reach is sea.
• Only one way out: an island that needs one more cell and has only one open neighbour takes that neighbour.

Medium grids also need you to count the room an island has left:

• Exact pocket: if an unfinished island has exactly as many reachable cells left as it still needs, those cells stay white.
• Every shape uses this cell: when you list every way to finish a small island, a cell that appears in every listing stays white, and a candidate none of them use is sea.

Hard grids need a wider view of the sea:

• Keep the sea connected: a cell that cannot join the sea stays white, and a cell whose removal would split the sea must be black.
• One-cell try: if painting a cell white (or black) breaks the sea or the island rules, that cell takes the other colour.

Beginner tips

Start with the 1s. Each 1 is a finished island, so shade every neighbour sea. Those blacks often force the next 2×2 corner to stay white.

When an island is full, wall it off with sea on every open side. Two islands that would touch must have sea between them.

Use dots freely. A white mark stops you shading a cell you already know is part of an island, and it makes the next pool or separation step easier to see. If you are new to Nurikabe, start with the daily 6×6 easy grid, and move up once finished islands and 2×2 pools feel natural.