Nurikabe

How to play Nurikabe

Nurikabe is a logic puzzle played on a grid of numbered and blank cells. Some cells start with a number; every other cell is undetermined. Your job is to shade the undetermined cells so the grid splits cleanly into two kinds of territory: white “islands” — one per number, each island the exact size of its number — and a single black “sea” that connects every shaded cell into one unbroken region. The name comes from a mischievous wall-building spirit in Japanese folklore, and the puzzle plays like a spatial tug-of-war: shade too aggressively and you wall off part of the sea into two pieces; shade too little and clumps of unclaimed cells create forbidden solid blocks. Every puzzle on this page is generated fresh and machine-checked to have exactly one valid arrangement of islands and sea, so if you ever feel stuck, there is always a logical deduction waiting — you never have to guess. Three sizes are available, from a gentle 6×6 up to a full 10×10 challenge, each timed and scored.

The goal

Shade every cell that is not part of an island black, so the finished grid obeys three rules at once: each numbered cell anchors an island of white cells whose total count equals that number, no two islands touch each other, and every shaded cell is part of one single connected sea with no 2×2 block of four shaded cells anywhere on the board. The numbered clue cells are fixed from the start — they can never be shaded and always belong to their own island. Once every cell is either correctly shaded or correctly left as part of its island, the puzzle is solved and a win banner appears immediately.

The three rules

• Every island has exactly one number, and its size. A numbered cell is the seed of an “island” — a connected group of unshaded cells reached by moving up, down, left or right (never diagonally). The island containing that number must contain exactly that many cells, no more and no fewer, and it must contain that one number and no other. Two different islands can touch at a corner, but never share an edge — if they did, they would really be one bigger island with two numbers, which is not allowed.
• All shaded cells form a single connected sea. Every cell you shade black must be reachable from every other shaded cell by a path of shaded cells moving only up, down, left or right. If your shading ever splits into two or more separate patches, the board is invalid — the sea must always stay in one piece.
• No 2×2 block is ever entirely shaded. Look at any square formed by four cells arranged in a 2×2 pattern anywhere on the grid: at least one of those four cells must NOT be shaded. A fully shaded 2×2 block is forbidden even if the rest of the sea is perfectly connected.
• Every puzzle has one and only one solution. You never have to guess: if a cell cannot be logically determined, the puzzle would have more than one valid answer, which this game never generates. Every grid is verified by a dedicated counting solver before it is ever shown to you, checking there is exactly one arrangement of islands and sea that satisfies every rule above.

Making moves

Tap any cell that is not a number to cycle it through three states: the first tap shades it black (sea), a second tap marks it white (your claim that the cell belongs to an island), and a third tap clears it back to blank/undetermined. Right-click on desktop, or press and hold on a touchscreen, jumps straight to the white island mark as a shortcut — tap it again (or right-click again) to clear it. Numbered clue cells are locked: they are always part of their own island and cannot be tapped. As you mark cells, the board watches for two live mistakes and rings the offending cells in red: a 2×2 block you have shaded entirely black, and a white-marked patch that has grown to touch two different numbers or has outgrown its own number. Full sea-connectivity is only checked once the whole board is filled, at the moment you win, since checking it continuously would be needlessly expensive. A timer and mistake counter sit above the board, and you can start a fresh puzzle, switch grid size, or jump to today’s shared daily puzzle at any time.

Solving techniques (no guessing needed)

• Measure the reach of every clue. An island of size N can never stretch more than N−1 steps (moving up, down, left or right) away from its number, because a connected group of N cells simply cannot be any further apart than that. Any cell that sits beyond every clue’s maximum reach can never join an island, so it must be shaded black immediately — this single idea eliminates huge stretches of open board before you place a single guess.
• Wall off satisfied islands. The moment an island’s cell count already equals its number, every cell touching that island from the outside must be sea, because adding one more cell would overshoot the number and because islands may never touch each other. Shading that entire border in one sweep often triggers a chain reaction of further deductions nearby.
• Watch the gaps between clues. When two numbered cells are close enough that their potential islands could otherwise grow into each other, the cells directly between them are usually forced to be sea — an island can never touch a second island, so any cell that would let two different numbers’ territory merge must be shaded to keep them apart.
• Use the no-2×2-block rule proactively. Whenever three cells of a would-be 2×2 square are already shaded, the fourth is forced to stay unshaded (and therefore part of some island) even before you know exactly which number it belongs to. This rule alone frequently completes the last few stubborn cells of a nearly finished grid.

Winning and scoring

You win the instant every cell on the board is either correctly shaded black or correctly marked white as part of a completed island, matching the puzzle’s one true solution exactly — the app checks this automatically the moment your last move lands. Your score rewards speed and accuracy: it starts at a base of 10,000 points, loses 5 points for every second you spend solving, and loses 200 points for every mistake, a mark that disagrees with the true solution. In formula terms, score = max(1, min(99999, 10000 − seconds × 5 − mistakes × 200)), so the result always fits the leaderboard between 1 and 99,999. Higher is always better, so a fast, careful solve beats frantic tapping. Sign in to save your best score per size to the online leaderboard; otherwise your best result stays on this device.

Strategy tips

• Start from the largest and most isolated numbers. A big island in an open corner has fewer possible shapes to consider, and pinning it down early shades a large amount of guaranteed sea around it, which then feeds smaller deductions everywhere else on the board.
• Keep a running tally of each island’s remaining cells. Every time you white-mark a cell, mentally subtract one from that clue’s target; once an island reaches its number, immediately seal its entire border with black — this is the single most repeated winning move.
• Save ambiguous middle cells for last. Cells equidistant from two clues, or sitting in wide-open sea territory, are usually the hardest to pin down early. Clear the forced cells near numbers and along 2×2 blocks first; by the time you reach the open middle of the board, far fewer possibilities remain.

Frequently asked questions