What is Kurotto?
Kurotto is a shading puzzle from Nikoli, the Tokyo publisher that gave the world Sudoku, Slitherlink and Masyu. You start with a grid scattered with numbered circles and you paint cells black. The black cells clump together into connected groups - think of them as clouds drifting across the grid - and every numbered circle is a weather station that reports on the clouds around it. The number on a circle is the total area of all the clouds that press right up against it.
Said precisely: a circle's number equals the combined size of every group of black cells that is orthogonally adjacent to the circle. A circle that touches one cloud of three cells and another of two reads 5. A circle with nothing black beside it reads 0. Circles themselves are never painted, and that single counting rule - clouds, summed by area, once each - is the whole game. There is no second rule waiting to trip you up, which is exactly why Kurotto feels so clean once it clicks.
- Shade any number of cells black; connected black cells form a single cloud.
- Cells with a circle are clues and are never shaded.
- A circle's number is the total size of all clouds orthogonally touching it.
- Each cloud is counted once, by its whole size - even if it hugs the circle along two edges.
- A 0 means no black cell may touch that circle.
- Clouds may be any shape; white cells do not need to connect.
How to play Kurotto online
Click or tap an empty cell to paint it black; tap again to drop a small white dot, marking a cell you have decided stays empty; a third tap clears it. Right-click toggles the white dot directly. The white dots are only for you - they keep your reasoning visible without changing the puzzle - so use them freely to record every cell a circle has ruled out.
Check looks at your board and flags any cell that disagrees with the unique solution, without telling you which way to fix it. Hint fills in one correct cell - usually a black one, because watching a cloud take shape teaches the logic faster than being told a blank. Undo steps back, Erase clears your white dots in one go, and Solution paints the finished grid when you would rather study a completed board than build it.
- Tap a cell to cycle empty, black, then white dot.
- Right-click sets a white dot straight away.
- Check highlights cells that conflict with the solution.
- Hint reveals one correct cell; Undo steps back one move.
- New puzzle builds a fresh board for the chosen size and difficulty.
Start with the zeros, then the small numbers
Every Kurotto board hands you a free opening: the zeros. A circle marked 0 is touching no cloud at all, so all four of its neighbours are white. Sweep the grid, dot every cell next to a 0, and you have carved out the empty channels before doing any real thinking. Boards with a scatter of zeros almost unzip themselves once you trust them.
After the zeros, hunt for the small numbers, because small numbers are the least flexible. A 1 is the loudest clue in the game: exactly one black cell touches it, and that cell must be a cloud of size one - a lone pixel with no black neighbours of its own. So a 1 not only places a single black cell, it also forbids black on every cell around that cell. A 2 is nearly as rigid: the clouds touching it must total two, which means either a single domino-shaped cloud laid against the circle, or two separate lone cells on opposite sides of it - and nothing more, since any extra black would push the total past 2. Reading what a number cannot be is often faster than reading what it can.
- A 0 whites out all four of its neighbours immediately.
- A 1 is a single isolated black cell touching the circle - and nothing black around that cell.
- Small numbers have few shapes; list the impossibilities first.
- Mark forced white cells with dots so each deduction stays on the board.
- Clear the zeros and ones before reaching for anything subtle.
The 'counted once' rule is where Kurotto bites
The rule that catches everyone is this: a cloud touching a circle is counted once, by its entire size, no matter how much of it hugs the circle. Picture an L-shaped cloud of four cells wrapping around the corner of a circle so that two of its cells sit against the circle's edges. It still contributes 4, not 8 and not 2. The circle is not counting contact points; it is counting the area of the clouds it can see.
This changes how you read a number near a big blank region. A circle reading 6 beside open space does not mean six separate black neighbours - it usually means one large cloud of six, most of which curls away from the circle into the empty cells beyond. Your job is to figure out how a cloud of the right total area can be anchored against the circle. That is why Kurotto rewards thinking in whole shapes rather than cell by cell: the number describes a region, and you are reverse-engineering the region from its reported area.
- A clue counts cloud area, not the number of cells touching it.
- One cloud against a circle contributes its full size, once.
- Large numbers usually mean a single cloud reaching away into open space.
- Sketch candidate cloud shapes, then test their total against the number.
- Two cells of the same cloud touching a circle never double-count.
Shared cells: let neighbouring circles argue
The richest deductions come from circles that look at the same cells. When two circles are a knight's move or a short hop apart, they often share one or two cells that either could see. Their numbers then constrain each other. Suppose a circle reading 1 and a circle reading 4 both border the same empty cell. If that cell were black and stood alone, it would give the 1 its single cell - but it would also feed the 4, and the rest of the 4's cloud would have to avoid the 1, which usually pins the cloud's direction precisely.
A reliable habit is to ask, for each candidate cell, 'which circles would this cell report to, and can all of them still hit their numbers?' A black cell is shared by every circle it or its cloud touches, so painting one cell can satisfy two clues at once - or break one while completing another. When a board stalls, look for a cell that two numbers disagree about: one of them almost always forces it, and that single resolved cell tends to cascade.
- Cells between two circles are constrained by both numbers.
- A cloud can be counted by several circles, so one cell may serve many.
- Test a contested cell against every circle that could see it.
- A cell that would overflow any neighbouring circle must stay white.
- Resolved shared cells usually cascade into the next deduction.
Counting the ceiling: what a circle can possibly see
Alongside the floor (a clue's number must be met) there is a ceiling (it cannot be exceeded), and the ceiling is a quietly powerful tool. Look at a circle and count the cells its clouds could ever reach - the empty cells around it and the open space those connect to. If a circle reads 5 but only four cells could possibly be marshalled into clouds touching it, the board is wrong; more usefully, if exactly five cells could reach it and it reads 5, then every one of those cells is black. Equality at the ceiling forces the whole region.
This bound is sharpest in corners and along edges, where circles simply have fewer cells to draw on. A high number jammed into a corner has almost no freedom - the cloud is nearly drawn for you. Conversely, a circle reading 0 sets the ceiling to zero for its neighbours, and those whited-out cells lower the ceilings of nearby circles in turn. Floors push cells black, ceilings push cells white, and good solving is a conversation between the two.
- Count the cells a circle's clouds could ever include - that is its ceiling.
- Number equals available cells means every available cell is black.
- Corners and edges give circles small ceilings and strong forcing.
- Whited-out cells lower the ceilings of neighbouring circles.
- Alternate between 'must reach' and 'cannot exceed' to break a board open.
Where the name comes from
Kurotto wears its subject on its sleeve. 'Kuro' is Japanese for black, and the puzzle is, at heart, an exercise in measuring blackness - how much of it gathers beside each circle. Nikoli has a long habit of naming its puzzles with a wink, from Masyu ('evil influence', born of a misread) to Yajilin (arrow + link), and Kurotto sits comfortably in that family of plainly descriptive, slightly playful titles.
Like most of Nikoli's catalogue, Kurotto grew up in the pages of the quarterly Puzzle Communication Nikoli, where reader-submitted ideas are refined into genres. It has never been as famous as Sudoku or Slitherlink, which is part of the pleasure of meeting it now: it is a fully-formed, elegantly minimal puzzle that many solvers have simply never tried. If you like the area-counting flavour here, you are tasting the same design instinct that produced Nurikabe and Fillomino - Nikoli's fondness for puzzles that are really about the sizes of regions.
Kurotto vs Nurikabe, Kuromasu and Mosaic
Kurotto belongs to the shading family, but it counts something none of its cousins do. Nurikabe also builds numbered regions, yet there the numbers label white islands, the black 'sea' must be one connected piece, and no 2x2 block may be fully black - a thicket of structural rules. Kuromasu numbers tell you how many white cells are visible in a straight line through the clue, so its logic runs along rows and columns rather than through blobs. Mosaic (Fill-a-Pix) numbers count black cells in the 3x3 square around the clue, a fixed little window. Kurotto is the only one of the four whose clue measures the area of arbitrary connected groups touching it.
That gives Kurotto a distinctive feel: no connectivity rule, no forbidden shapes, no line of sight - just clouds and their sizes. The freedom is liberating and occasionally dizzying, because a number can be satisfied by clouds that wander far from the circle. If you enjoy that, Nurikabe is the natural next step for more region logic with added structure, while Kuromasu and Mosaic scratch the same shading itch with tighter, more local clues.
- Nurikabe: numbers mark white islands; the sea is connected and 2x2-free.
- Kuromasu: numbers count white cells visible in a straight line.
- Mosaic: numbers count black cells in the fixed 3x3 around the clue.
- Kurotto: numbers sum the area of the connected black clouds touching the clue.
- Only Kurotto measures arbitrary group sizes, with no shape or connectivity rules.
Grid sizes and difficulty levels
The 6x6 boards are the place to learn the reflexes: zeros and ones do most of the work, clouds stay small, and you can usually hold the whole deduction in your head. On 8x8 the clouds grow and the 'counted once' rule starts to matter, because larger numbers invite larger shapes that reach across the board. The 10x10 boards are proper Kurotto - sparser circles, bigger clouds, and long chains of reasoning where a ceiling argument in one corner finally settles a floor argument in another.
Difficulty changes how much the numbers hand you. Easy boards are densely circled, so almost every cell is pinned by a nearby clue and progress is steady. Medium thins the circles and lets clouds spread, so you lean more on shared-cell and ceiling deductions. Hard keeps circles sparse and clouds generous, demanding longer combinations before a cell is certain. Whatever you choose, a solver checks every board before you see it and trims circles only as far as a single solution survives - so even the sparsest hard puzzle is solvable by pure logic, never by guessing.
- 6x6 - learn the zeros, ones and small clouds.
- 8x8 - bigger numbers and the first real 'counted once' shapes.
- 10x10 - sparse circles and long cross-board chains.
- Easy, medium and hard change circle density and cloud size.
- Every puzzle is verified to have exactly one solution.