How to play

Nurikabe is played on a grid with some numbered cells. You decide which cells are shaded sea and which are unshaded island.

1. Each number is part of an island: a group of unshaded cells joined side to side. The island's size, in cells, equals its number.
2. Each island holds exactly one number.
3. Two islands may not touch side to side. Touching only at a corner is fine.
4. All shaded cells form one connected sea, joined side to side.
5. The sea may not contain a 2×2 block of shaded cells anywhere.

Numbered cells are never shaded. The puzzle is solved when the shading matches the one solution. Dots are optional notes for cells you know are island.

Strategy

Wall off the 1s. A 1 is a complete island, so every cell directly beside it is sea.

Separate neighbouring clues. A cell that touches two different numbers, or sits directly between two numbers one cell apart, would join two islands. Shade it.

Shade what no island can reach. Count how far each number can grow. A cell further away from every number than its island size allows must be sea. On big grids this clears a lot of space early.

Close finished islands. When an island reaches its full size, shade every unshaded cell around its edge.

Follow the only way out. If an unfinished island has just one free cell beside it, the island must grow into that cell. The same goes for the sea: a group of shaded cells with one exit must continue through it, because the sea has to stay connected.

Break up 2×2 blocks. When three cells of a 2×2 square are sea, the fourth must be island. Then ask which number can reach it.

Keep the sea in one piece. Before you close an island, check that you are not cutting a shaded area off from the rest of the sea.

Try a cell both ways. On hard grids, imagine a cell as sea and follow the results a few steps. If that breaks a rule, the cell is island, and the other way round.

History

Nurikabe is a pencil puzzle published by Nikoli, the Japanese puzzle company that also named and popularised sudoku. The name comes from Japanese folklore, where a nurikabe is a spirit that appears as an invisible wall and blocks travellers at night.

Playing on Summoned Games

Choose 5×5 easy, 7×7 medium, 10×10 hard or 12×12 expert. Puzzles are generated as you play, so they never run out, and each one is checked by a solver to have exactly one solution, reachable by logic. The daily puzzle is a 7×7 grid, the same for everyone that day, and a streak counts the days in a row you solve it.

Rule breaks light up as you play: a fully shaded 2×2 block, two islands that touch, or an island that has grown bigger than its number. The Check button compares your shaded and dotted cells with the solution, flashes any that are wrong, and adds each wrong cell to the mistake counter. Undo steps back through your moves.

Solved puzzles count toward the puzzles solved leaderboard, and 10×10 grids have a leaderboard for the fastest time. Your best time per size shows in the menu. Your game saves as you play, so you can close the tab and resume later.

On a phone, tap a cell to cycle it from empty to shaded to dot and back, or switch the Shade button to Dot mode so taps only place dots. On desktop, left click shades and right click places a dot. Keyboard players use the arrow keys, Space to shade, X or Enter to place a dot, Delete to clear, D to switch modes, C to check and U to undo.

FAQ

Do I need to dot every island cell? No. Dots are notes for you. The puzzle is solved once the shaded cells are exactly right.

Does every puzzle have one solution? Yes. Each puzzle is checked by a solver before you see it, and it can be solved by step-by-step deduction, without guessing.

Can islands touch diagonally? Yes. Islands only need to be kept apart along their sides. Corner contact is allowed.

Why can the sea not have a 2×2 block? It is part of the classic rules. It keeps the sea thin, so the islands have to spread out across the grid, and it gives you one more way to rule cells in or out.

Is it free? Yes. It runs in the browser with no ads and no download.