How to solve a Nurikabe puzzle step by step
Nurikabe is a logic puzzle built on a simple contrast: white cells form numbered islands, while black cells form one continuous sea. The challenge is to determine which cells belong to each island without creating illegal shapes or disconnected water.
Each puzzle begins with a grid containing clue numbers. A number tells you the total size of the island connected to that clue, including the clue cell itself. Islands connect horizontally and vertically, never diagonally. Black sea cells also connect horizontally and vertically, and the sea may not contain a solid 2×2 block.
The best way to solve Nurikabe is to make small deductions and record them clearly. Use one mark for confirmed island cells and another for confirmed sea cells. This prevents repeated guesses and makes larger patterns easier to see.
Learn the rules before marking cells
Every island must contain exactly one clue. Two different clues may be diagonally adjacent, but their island cells cannot touch across an edge. If two clue cells are already next to each other horizontally or vertically, they must belong to separate regions, so the surrounding cells often become important.
The black sea must form a single orthogonally connected area. Diagonal contact does not connect sea cells. At the same time, four black cells may never occupy the corners of a 2×2 square, because that would create a forbidden pool.
A clue is also a size limit. A 1-clue is complete immediately, while a larger clue still needs enough connected white cells to reach its stated total. These basic facts drive nearly every useful deduction.
Start with forced cells
The easiest opening move comes from clues with very little room to grow. A clue marked 1 cannot expand, so all orthogonally adjacent cells must be black. Marking these cells may create new 2×2 restrictions or force a route for the sea.
When a clue has already reached its required size, surround its island with sea marks wherever an adjacent cell is not part of another confirmed island. For example, a completed 4-island cannot accept a fifth white cell, so every available edge around it is water.
Look for cells that are forced to be land because a clue has too few reachable spaces remaining. If a 5-clue can reach only five cells through currently available paths, all of those cells must be white. This is often called a reachability or maximum-area deduction.
Use local patterns and the 2×2 rule
The 2×2 rule is one of the strongest Nurikabe techniques. If three cells in a 2×2 area are already black, the fourth must be white. Otherwise the puzzle would contain an illegal black square. The same idea works when one possible black cell would complete a 2×2 block.
A black cell can also be forced when making it white would isolate sea regions or create an impossible island shape. Scan around marked areas after every placement rather than waiting until the grid is crowded. Nurikabe rewards frequent rechecking because one mark can affect several nearby squares.
Be careful not to treat every blank cell as undecided in the same way. Some blanks are restricted by island size, some by sea connectivity, and some by both. Write marks only when a rule supports them.
Separate islands without guessing
Two clues cannot belong to the same island. If a proposed expansion would connect them orthogonally, that expansion is forbidden. Mark the cell as sea or search for another route that keeps the islands apart.
A completed island also creates a boundary. Its neighboring cells must be black unless they are already confirmed parts of another island, and those neighboring island cells must still respect the no-touch rule. This can produce a chain of forced sea cells between two growing regions.
Diagonal relationships deserve special attention. Islands may touch at a corner, so diagonal contact alone is not a violation. However, a diagonal arrangement can limit the cells available around both clues and often reveals where the sea must pass.
| Situation | What it tells you | Typical action |
|---|---|---|
| Clue has value 1 | The island is complete | Mark orthogonal neighbors as sea |
| Island has reached its clue size | It cannot expand further | Mark suitable neighboring cells black |
| Three black cells occupy a 2×2 area | The fourth cell cannot be black | Mark the fourth cell white |
| Two islands would touch by an edge | The connection is illegal | Mark the separating cell as sea |
| Sea regions have only one connecting route | That route is necessary | Keep the route black and protect it |
| A clue has only enough reachable cells to finish | All reachable cells are required | Mark the necessary cells as island |
Protect sea connectivity
A sea region cannot be cut into separate parts. If a black cell is the only possible bridge between two sea areas, it must remain black. Conversely, if marking a cell black would seal off a section of sea that has no other connection, that cell must be white.
This deduction becomes clearer when you imagine the grid as a network of corridors. Narrow passages, edge channels, and one-cell gaps deserve close inspection. A black route may need to curve around an island, but it cannot pass through a confirmed white region.
Do not connect separate sea areas too early if doing so would create a 2×2 block. Connectivity is required, but it must be achieved legally. Check both conditions together whenever a large black region develops.
Finish through systematic scanning
When the grid is nearly solved, inspect every clue and count its confirmed island cells. Then count the remaining cells it can legally reach. A clue with no remaining capacity forces all adjacent undecided cells to sea; a clue with exactly enough space forces those spaces to land.
Scan rows, columns, and 2×2 windows in a fixed order. This reduces missed deductions and helps reveal patterns that are difficult to notice during random inspection. If progress stops, return to the earliest uncertain area rather than making a guess.
Guessing should be a last resort in a well-constructed logic puzzle. If a hypothetical mark is necessary, follow both possibilities until one produces a contradiction such as a 2×2 sea block, an oversized island, two clues in one island, or disconnected water.
Build a reliable solving routine
Use the following habits for faster and cleaner solving:
- Mark every immediate 1-clue deduction before examining advanced patterns.
- Recount an island whenever a new white cell is added.
- Inspect every nearby 2×2 square after placing a black cell.
- Trace the sea in sections to identify bridges and possible dead ends.
- Avoid guessing until local size, separation, and connectivity rules are exhausted.
With practice, these steps become a natural rhythm: identify forced cells, expand islands carefully, prevent illegal sea shapes, and verify global connectivity. Nurikabe puzzles then shift from trial and error to a sequence of precise deductions.
Open a fresh grid, use clear island and sea marks, and work through each rule patiently. Regular practice with logic-puzzle resources and tournament-style grids on Zagonetka.net can help turn these techniques into confident solving habits.