What is Ripple Effect?
Ripple Effect - known in Japan as Hakyuu - is a number-placement puzzle from the same Nikoli stable as Sudoku. The grid is divided by bold walls into rooms of different shapes and sizes, like a patchwork of little cages. A room made of N cells has to be filled with the numbers 1 to N, each used exactly once, so a single cell is always a 1, a two-cell room holds a 1 and a 2, a five-cell room holds 1 through 5, and so on.
The twist - the part that gives the puzzle its name - is the spacing rule. If the same number appears twice in one row or one column, the two copies must be separated by at least that many cells. Two 1s in a line can never sit next to each other; two 3s need at least three empty cells between them; a pair of 5s must be five cells apart or more. A number sends a 'ripple' down its row and column that pushes its twins away, and the whole puzzle is the interplay between filling each room and keeping those ripples clear.
- Bold walls divide the grid into rooms of various sizes.
- A room of N cells holds the numbers 1 to N, each once.
- Equal numbers in the same row or column must be spaced out.
- Two of the same number n need at least n cells between them.
- So two 1s can't touch, and two 4s need four cells between them.
- Every cell holds a number, and each puzzle has one solution.
How to play Ripple Effect online
Click or tap an empty cell to select it, then choose a number from the pad below the board or type it on your keyboard - only the numbers that fit the selected cell's room are offered, so a three-cell room shows 1, 2 and 3. Click the same number again, or use Erase, to clear a cell. The given numbers are printed in a darker, fixed style and cannot be changed; everything else is yours to fill in. The counter shows how many cells you have completed out of the whole grid.
Check looks over your grid and flags any number that disagrees with the unique solution, without telling you the right value. Hint drops one correct number into place, Undo steps back through your moves, Reset clears everything you have entered, and Solution fills the finished grid when you would rather study it than solve it. New puzzle builds a fresh board for the size and difficulty you have chosen.
- Tap a cell, then pick a number from the pad or type it.
- Only the values that fit that cell's room are offered.
- Tap the same number again, or use Erase, to clear a cell.
- Given numbers are fixed; Check flags entries that conflict.
- Hint, Undo, Reset and Solution help when you get stuck.
Two rules, pulling on each other
Everything in Ripple Effect comes from two rules pulling in different directions. The room rule is local and tidy: this cage needs a 1, a 2 and a 3, end of story. The ripple rule is long-range and disruptive: a number you place here reaches out along its row and column and forbids the same number from appearing too close. Solving the puzzle means satisfying both at once - choosing, for each room, the arrangement of 1 to N that also keeps every repeat the right distance away.
Because the two rules see the board differently, the best moves usually come from playing them against each other. A room might allow a 2 in two of its cells, but the ripple from a 2 elsewhere in the row rules one of them out, leaving only one place for it. Or a row might be desperate for its last 1 while every empty cell but one is too close to a 1 already down. Learning to switch between 'what does this room still need?' and 'what does this line forbid?' is the whole craft of the puzzle.
- The room rule fixes which numbers a cage must contain.
- The ripple rule controls how close repeats may sit.
- A repeat banned by ripple can leave a room only one home for a value.
- A line short of a number can force it into the one legal cell.
- Alternate between room logic and line logic on every pass.
Start with the small rooms and the 1s
The easiest footholds are the smallest rooms. A one-cell room is a free gift: it can only be a 1, so write it in before anything else. Two-cell and three-cell rooms are nearly as friendly - their numbers are small, so their ripples are short and easy to track, and a given inside them often pins the rest immediately. Sweep the board for these little cages first and you will have a scattering of fixed numbers to build from.
The number 1 deserves special attention because it appears in every single room and its ripple is the gentlest - just 'no two 1s adjacent in a line'. That makes 1s both common and easy to reason about: in many rows and columns you can quickly see where the 1s can and cannot go, and each 1 you fix removes a candidate from its neighbours. Pinning down the 1s early often cracks a stubborn region open, because once a cell can't be a 1 it must be something larger, and larger numbers have their own, longer ripples to obey.
- A one-cell room is always a 1 - fill every one first.
- Small rooms have small numbers and short, easy ripples.
- The number 1 is in every room and only bans adjacent 1s.
- Fixing the 1s removes candidates all along their lines.
- Ruling a cell out as a 1 forces a larger, longer-reaching number.
Counting the gaps the big numbers leave
The larger a number, the louder its ripple, and that is exactly what makes the big numbers so useful. A 4 forbids another 4 within three cells on either side along its row and column; a 5 clears five cells; on a small grid a single big number can rule itself out of most of a line at a stroke. When a room contains a 4 or a 5, finding the one or two places that number can legally live is often the key that unlocks the rest of the cage.
Counting along the line is the core skill. Pick a number, look at where its copies already sit, and shade out every cell within its ripple distance; whatever survives is where the next copy may go. Often a row has room for only one more 3, or a column has exactly one legal home left for its 2. These forced placements then feed back into the rooms, and the rooms feed back into the lines, until the grid settles. On the hardest boards you will be juggling several of these counts at once, but every step is pure logic - the puzzles here are checked to have a single solution, so you never need to guess.
- A number n blocks its twin within n cells along the row and column.
- Bigger numbers clear more of a line, so they pin down faster.
- Shade each repeat's reach; the surviving cells are its only homes.
- A line with one legal home left for a value forces it there.
- Forced line placements feed the rooms, and rooms feed the lines.
Where Ripple Effect comes from
Ripple Effect is one of the original puzzles published by Nikoli, the Japanese company that named Sudoku and built a whole catalogue of logic puzzles around simple, elegant rules. Its Japanese name, Hakyuu, means exactly what the English title says - a ripple effect, the way a disturbance spreads outward in rings - and it captures the puzzle's central image perfectly: each number you write sends a wave along its row and column that decides where its repeats can land.
Like the best Nikoli puzzles, Ripple Effect gets enormous depth from very little. There is no arithmetic and only one extra idea beyond 'fill each region with 1 to N' - the spacing rule - yet that single addition turns a gentle filling exercise into a genuine deduction puzzle. It has travelled under several names in English, including Ripple Effect and simply Hakyuu, and it has become a staple of puzzle apps and collections for solvers who enjoy Sudoku-style placement with a geometric twist.
Ripple Effect vs Suguru, Sudoku and other number puzzles
Ripple Effect sits in the same family as Suguru and Sudoku but mixes the ingredients differently. Like Suguru, it fills irregular rooms with 1 to N rather than using fixed rows of 1 to 9, so reading the partition is half the battle. Where Suguru bans equal numbers from touching - including diagonally - in a tight one-cell radius, Ripple Effect uses a distance that grows with the number, so a 1 behaves almost like Suguru while a 5 reaches far down the line. That single change shifts the whole feel of the solve toward counting along rows and columns.
Compared with classic Sudoku, Ripple Effect drops the 'each digit once per row and column' constraint entirely - a row can hold two 2s, as long as they are far enough apart - and replaces it with the spacing rule and the room sizes. The result is friendlier in some ways (rooms are small, the numbers rarely climb past five) and trickier in others (the same number can legally repeat, so you reason about distance rather than uniqueness). If you like Suguru, Kakuro or jigsaw Sudoku, Ripple Effect is an easy and rewarding next step.
- Suguru: fill rooms with 1 to N; equal numbers may not touch at all.
- Sudoku: each digit once per row, column and box - no repeats.
- Ripple Effect: fill rooms with 1 to N; repeats allowed if spaced by the number.
- The ripple distance grows with the value, unlike Suguru's fixed radius.
- Reasoning is about distance along lines, not strict uniqueness.
Grid sizes and difficulty levels
The 6x6 boards are the place to learn the rhythm: a handful of small rooms, numbers that rarely pass four or five, and ripples short enough to picture at a glance. On 7x7 the grid carries more rooms and more repeated numbers per line, so the spacing rule starts to do real work and you lean harder on counting the gaps. The 8x8 boards are full puzzles - many rooms, long rows and columns where the same number appears several times, and deductions that travel right across the grid before a cell is certain.
Difficulty changes how many numbers you are given to start. Easy boards hand you plenty, so most cells fall to a single short deduction and progress is steady. Medium thins the givens, leaving more cells that only the ripple counting can resolve. Hard strips the board back to a lean set of starting numbers, so you work the room logic and the line logic against each other for longer before the grid gives way. Whichever you choose, every board is checked by a solver before you see it and only those with exactly one solution are kept - so each puzzle is always solvable by pure logic, never by guessing.
- 6x6 - small rooms and short ripples to learn the two rules.
- 7x7 - more repeats per line and real work for the spacing rule.
- 8x8 - long lines and deductions that cross the whole grid.
- Easy, medium and hard change how many numbers are given.
- Every puzzle is verified to have exactly one solution.