What is Nurikabe?
Nurikabe is a shading puzzle where you leave islands exactly as large as their numbers and paint every other cell as sea. A numbered cell belongs to an island with that many cells, and each island contains exactly one number. Islands never touch each other by a side, the sea must be a single connected body, and the puzzle is solved when no 2×2 block of sea remains.
The name comes from a wall spirit in Japanese folklore, and the puzzle is also called Islands in the Stream or simply the islands puzzle. Unlike Sudoku you never write numbers; you find shapes by shading, so reading space and careful deduction matter more than arithmetic.
| Rule | Meaning |
|---|---|
| Number | How many cells its island has, counting the numbered cell itself |
| Island | A group of unshaded cells joined by sides. Exactly one number per island |
| Islands apart | Different islands may not share a side. Touching diagonally at a corner is fine |
| Sea | All shaded cells must join into one sea through shared sides |
| No 2×2 | Four sea cells may never form a 2×2 square |
| Dot | A note for cells you know are island. Only sea cells are checked for the answer |
Where should I start shading?
The classic openings are the cells around a 1, any cell squeezed between two numbers, and cells no island can reach. Those three alone reveal the skeleton of the sea, and from there the 2×2 rule and the connected sea rule narrow down each island's shape.
| Opening | What to shade |
|---|---|
| Around a 1 | A 1 is already a finished island, so all four neighbours are sea |
| Between two numbers | A cell with numbers on both sides is sea, or the two islands would merge with two numbers |
| Diagonal numbers | When two numbers touch at a corner, the two cells next to both are sea |
| Unreachable cells | A cell no number can reach within its remaining size is sea |
| Full island | Once an island has as many cells as its number, every cell around it is sea |
| No 2×2 | If three cells of a 2×2 square are sea, the fourth is island |
| Sea exit | If a patch of sea has only one way out, that cell is sea too |
| Island exit | If an unfinished island can only grow through one cell, that cell is island |
Does every puzzle have one answer? How are boards made?
Yes. Every board is generated fresh and only released after a solver confirms there is no other answer. The daily puzzle is built from a number fixed by the date, so everyone gets the same board.
The generator starts with an all sea grid, plants island seeds and grows them, then breaks any leftover 2×2 pools with island cells to form the solution. Each island gets its size written in one of its cells, and the solver looks for another answer. If it finds one, the number moves to a solution island cell that was sea in that rival answer, or a new 1 cell island is added, and this repeats until no rival answer remains. Easy boards fall to the basic openings alone, Normal boards need a one cell what if step, and Hard boards are unique 10×10 grids. All of this takes less than a second.
How do the difficulty levels differ?
Easy is 5×5, Normal is 7×7 and Hard is 10×10. Bigger boards hold more islands, and settling their shapes takes looking further ahead.
| Level | Size | What to expect |
|---|---|---|
| Easy | 5×5 | Basic openings such as around a 1 and between two numbers carry you to the end |
| Normal | 7×7 | Some cells need a one cell what if step to spot a contradiction |
| Hard | 10×10 | More than ten scattered islands, solved by weighing the connected sea and the 2×2 rule together |
| Daily | 7×7 | One board a day, the same for everyone, recorded in your daily streak |
What help is there when I get stuck?
Hint paints one correct cell with a gold outline, and Check shows every broken rule in red. Undo takes back your last action, and a whole drag comes back in one step.
- Hint: fixes a wrongly painted cell first; otherwise it fills a cell that follows by deduction from your current board. The count appears on the result screen.
- Check: marks 2×2 pools, sea cut off by islands, islands closed at the wrong size or larger than their number, and islands holding two numbers.
- Number colour: green when the island closes at exactly its size, red when it overflows or merges with another island.
Tips for solving Nurikabe faster
- Count the sea in advance. Total cells minus the sum of all numbers is the number of sea cells, handy for the last few decisions.
- Use dots freely. Dotting cells you know are island shows where each island can still grow, and lets you seal finished islands with sea at once.
- Never trap the sea. If a patch of sea has a single way out, that way must be sea. Corners trap sea most often.
- Hunt for 2×2 squares. The last cell of a square with three sea cells is island, and some nearby number must reach it.
- Save big numbers for later. Small islands like 1 and 2 finish first; large islands only take shape once the sea around them is fixed.
Controls
- Phone and tablet: tap once for sea, again for a dot, again to clear. Keep your finger down and drag to give every cell you pass the same state.
- Mouse: left click cycles sea, dot and clear; right click toggles a dot. Hold and drag to paint many cells at once.
- Keyboard: arrows or WASD pick a cell and Space cycles it. F for sea, E for dot, Backspace to clear, Z undo, H hint, C check.
Best times and the daily record are stored only in this browser and never sent to a server. When you finish, palm trees sprout on every island and sailboats appear on the sea, and Share result turns your time and level into a picture card.