Nurikabe techniques
18 techniques, all of which the solver can spot for you as a Smart Hint. Each one has a worked example.
Easy
Island of One
A clue of 1 is already a complete island — every cell touching it is sea.
Diagonal Clues
Two clues that touch only at a corner can never share a cell — the two squares between them are sea.
Separated by One
Two clues with exactly one empty cell between them in a row or column — that cell is sea.
Completed Island
Once an island's marked cells already equal its clue, every cell around it is sea.
Forced Expansion
An island blocked everywhere but one open cell has no other way to grow — that cell joins it.
No 2×2 Sea
Three sea cells already forming an L would complete a 2×2 block — the last cell must be land.
Out of Reach
A cell too far from every remaining clue for that island to ever stretch to — it's sea.
Medium
Between Two Islands
A cell touching two different islands can't belong to either one — it's sea.
Landlocked Cell
A cell fully surrounded by land can't be a lone patch of sea with nowhere to connect — it's land.
Counting the Sea
The sea's total size is fixed from the start: the whole board minus the sum of every clue. Once the marked sea reaches that count, every remaining blank is land — and once marked land reaches the clue total, every blank is sea.
Sea Connectivity
If a cell becoming land would cut the sea into two pieces that could never reconnect, it stays sea.
Hard
Completion Pool
A cell touching an island one short of its size would finish it — and a finished island is walled in sea on every side. If that walling would trap a 2×2 pool, the cell can't be land after all: it's sea.
Chokepoint
If the only way to reach a cell is through one narrow gap, and going that way traps a 2×2 sea block, the gap must be land.
Expert
Contested Space
If sending a cell to sea would leave a different island with nowhere left to grow, that cell is land instead.
Land Squeeze
The mirror image of Contested Space: if making a cell land would break the board right away — overflowing the island it would join, or walling a different island off from the room it still needs — then it's sea.
Pocket Overflow
If making a cell land would seal an empty cell in on all four sides, that trapped cell can't be a lone piece of sea with nowhere to connect — so it has to be land too. If taking both together overflows an island or joins two islands, the first cell was never land: it's sea.
Extreme
Every Path Agrees
List every way an island could still fill out its size — then cross off any that would break the water around it, like trapping a 2×2 pool. A cell in all the survivors is land; a cell in none of them is sea. Often the checks leave just one way, which settles the whole pocket at once.
Contradiction
When nothing else advances the puzzle, try one cell's two possible states and follow the consequences a few steps ahead. If one choice breaks a rule, the other is forced.