Play Ripple Effect Online

Big numbers can barely appear at all

The rule that makes Ripple Effect its own puzzle is the spacing one: if the same number appears twice in a row or column, there must be at least that many squares between them. Two 1s need one square between; two 5s need five.

Read that as a constraint on the whole line rather than on a pair, and something useful falls out. A line only has so much room.

The general form is worth memorising: in a line of width w, a value v can appear at most ⌈w ÷ (v+1)⌉ times. Which gives this:

On a 6×6 board — the middle size here — a 5 or a 6 can appear only once in any row or column, and a 4 at most twice. That turns every large number into something close to a Sudoku digit: place it, and its entire row and column are done with that value.

Small numbers are the opposite. A 1 can appear three times in a six-wide line, so spotting a 1 tells you very little. Work the big numbers first — they are the ones that eliminate.

Room size decides which numbers exist

Each room holds the numbers 1 up to its own size, once each. A room of two squares holds a 1 and a 2 and nothing else; a room of six holds 1 to 6.

Two consequences follow immediately, and both are free information at the start:

A one-square room is always a 1. There is nothing else it can hold. Boards here carry one or two of these on average, and they are the cheapest openings available.

Only large rooms can hold large numbers. If you are looking for where the 6 in a row goes, only the six-square rooms are candidates — and combined with the spacing rule, that often leaves a single square.

A worked board

Here is a 6×6 from the generator on this page: nine rooms, ranging from one square up to six, and twelve numbers given.

The way in is the small rooms and the big numbers. Fill the one-square rooms with their 1s, then ask where each six-square room puts its 6 — on a six-wide grid that 6 forbids its entire row and column, which usually settles the neighbouring rooms too.

Where solvers get stuck

Counting the gap wrong. Two 3s need three squares between them, so the nearest legal pair sits four apart. Off-by-one here quietly invalidates everything built on top of it.

Applying the spacing rule diagonally. It only works along rows and columns. Two equal numbers may sit diagonally adjacent quite legally, which looks wrong and is not.

Starting with the 1s. They are the most common numbers and the least constrained. The large numbers are where the eliminations are.

Two things you can check about these puzzles

Every puzzle has exactly one solution, and it is confirmed before you see it. A room layout is generated, solved, and clues are removed from that solution; the result is then handed to a solver, and the board is only offered if exactly one filling satisfies it. When no layout passes within the time allowed, the generator starts over rather than shipping a board it could not confirm.

Difficulty is the number of givens, and it drops steeply. On a 6×6 the generator leaves about 17 numbers on Easy, 12 on Medium and just 7 on Hard, out of 36 squares. The rooms themselves stay much the same — around nine or ten per board at every setting — so a hard board is not a more tangled layout, only a quieter one.

Ripple Effect, Hakyuu and Seismic

Ripple Effect is the English name used by Nikoli, which first published it. In Japanese it is Hakyuu Kouka — often shortened to Hakyuu — and you will also see it as Seismic.

It sits alongside Suguru as a room-based number puzzle: both divide a grid into small regions that each hold 1 up to their size. What separates them is the second rule. Suguru forbids equal numbers from touching, including diagonally; Ripple Effect lets them touch diagonally but pushes them apart along rows and columns by a distance that depends on the number itself.

More room puzzles

If you like grids divided into small regions, try these: