How to play
Nurikabe is played on a square grid in which some squares hold numbers. You decide which squares are wall (shaded) and which belong to islands (white). The puzzle is solved when all three rules hold:
- every island, a group of white squares joined side by side, contains exactly one number, and that number is the size of the island;
- all shaded squares form one wall connected side by side (touching corners do not count);
- no 2×2 block of the grid is completely shaded.
Numbered squares are always white. Pick a level and a size, then press Start: Easy uses 5×5 and 7×7 grids, Medium 8×8 and 10×10, Hard 10×10 and 12×12. The clock stops while you pause or switch to another tab. The puzzle is solved as soon as your shading matches the solution; dots are only notes for you and never need to be complete.
Controls
- Mouse: click a square to switch it from empty to shaded, to a dot (your note that it is white) and back to empty. Right-click places or removes a dot. Drag to paint: a drag from an empty square shades the empty squares it passes, a drag from a shaded square erases, a drag from a number places dots.
- Touch: tap and drag the same way; the page does not scroll while your finger is on the board. The Shade and Dot buttons choose what a tap does. When squares are small (8×8 and larger on most phones), a Zoom button appears; while zoomed, move the board with two fingers or pinch.
- Keyboard: arrows move a cursor (the first press only shows it). Space cycles, S shades, D places a dot, Backspace clears, Shift+arrow copies the square you leave. Ctrl+Z / Ctrl+Y undo and redo; H hint, C check, P pause, N new puzzle, F full screen.
Mistakes you can see without the solution are marked at once: a red exclamation mark in a 2×2 block that is all shaded, and red stripes with a ring around the number for an island with two numbers, with too many squares, or closed in by shading before it is big enough. A number turns grey when its island is closed in at exactly the right size.
Check marks shaded squares that should be white with a red cross and dots on wall squares with a red ring, until your next move. Hint first removes a wrong mark if there is one. Otherwise it fills in one square that follows from the Easy or Medium steps and says why; if none does, it reveals one square of the solution.
How it works
Because every island holds exactly one number, the numbers add up to the count of white squares, so the size of the wall on an n×n grid is known before you start:
On a 7×7 grid whose numbers add up to 18, exactly 49 − 18 = 31 squares are shaded. Most steps come from a few facts. A square touching two different numbered islands is wall, or it would join them. A complete island is surrounded by wall. A square that no number can reach with the squares it has to spare is wall. Three shaded squares of a 2×2 block make the fourth white. And because the wall is one piece, a piece of wall with only one free side must continue there.
All 770 puzzles were made by computer: a random connected wall without 2×2 blocks, its white area split into islands, one number per island. A solver that knows only the steps of the chosen level then had to finish each puzzle without guessing. Easy needs only the facts above. Medium also needs the possible shapes of an island, or the insight that white in a square would cut the wall apart or leave an island no room. Hard sometimes needs a test: if this square were wall, the other steps would hit a contradiction, so it is white (or the other way round). A second, independent program that tries every possible island shape confirmed that each puzzle has exactly one solution. Each puzzle can also appear rotated or mirrored in eight ways.
Nurikabe first appeared in a Japanese puzzle magazine in March 1991. The name comes from a spirit of Japanese folklore that appears as an invisible wall blocking travellers at night. Deciding whether a Nurikabe grid has a solution is NP-complete in general, so no fast method is known for every puzzle.
Tips and strategy
- Start with the 1s: each is a complete island, so shade its neighbours.
- When two numbers are diagonal neighbours, the two squares touching both are wall.
- When three squares of a 2×2 block are shaded, dot the fourth.
- Keep the wall in one piece: shading boxed in by islands must escape through its last free side.
- Dot squares you know are white. Dots that join two numbers turn red at once.
Frequently asked questions
Why is the 12×12 board small on my phone?
Twelve squares across a phone screen leave about 29 pixels each. Use the Zoom button or pinch with two fingers.
Sources
- Holzer M, Klein A, Kutrib M, Ruepp O (2011). Computational Complexity of NURIKABE. Fundamenta Informaticae 110(1–4), 159–174. The proof that Nurikabe is NP-complete, even when the only numbers are 1 and 2.
- Wikipedia contributors (accessed 25 October 2026). Nurikabe (puzzle). Wikipedia. Encyclopedia article. The first publication in March 1991 and the wall of Japanese folklore the puzzle is named after.