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Sudoku
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Sudoku tips & strategy
Scan by number, not by cell
Pick a digit that already appears five or six times and look for the rows, columns and boxes where it is still missing. The remaining candidates for that digit are usually one or two cells. Cell-by-cell scanning is slower because every empty cell has up to nine possibilities to weigh.
Finish the crowded boxes first
A 3×3 box with six or seven digits placed has only two or three openings, and each of them is cross-checked by a full row and column. Those cells solve fastest and each one unlocks a neighbour, so start where the grid is densest, not at the top-left.
Use pencil marks only when scanning stalls
Marking every candidate in every cell buries the useful information under noise. Write candidates only for cells that are down to two options, then look for a naked pair: two cells in the same unit sharing the same two candidates remove those digits from every other cell in the unit.
Never guess — every puzzle has one solution
A guess that happens to work teaches nothing and a guess that fails costs the whole grid. If you are stuck, re-scan the digit you placed most recently: a new placement always removes candidates somewhere else, and that is where the next move is.
Check the row and column before you commit
Most wrong entries come from checking the box and forgetting the line, or the other way round. Before placing a digit, confirm the three units — row, column, box — and make sure the digit is absent from all three. The two seconds it costs save a five-minute unwind.
Frequently asked questions
Is Sudoku free to play online here?
Yes. Sudoku is completely free in your browser on ArcadeWhale. No download, no sign-up and no payment — pick a difficulty and start.
Do I need to be good at maths to play Sudoku?
No. Sudoku involves no arithmetic at all. The digits 1 to 9 are simply nine distinct symbols, and the puzzle is pure logic about where each symbol can and cannot go. The game would work identically with nine colours or nine shapes.
What are the rules of Sudoku?
Fill the nine-by-nine grid so that every row, every column and every three-by-three box contains each digit from 1 to 9 exactly once. Some digits are given at the start, and the rest are deduced from them.
What is notes mode for?
Notes mode lets you write small pencil marks into a cell listing the digits that could still go there. It is how most people solve harder puzzles, because it takes the candidate list out of your head and puts it on the grid where patterns become visible.
What is the Journey?
The Journey is 48 puzzles in four worlds, ordered so that each world introduces one new solving technique: box scanning and the last candidate, then row and column scanning, then pencil-mark elimination, then pairs. Each puzzle earns up to three stars (three for no mistakes and no hints), and you can replay any puzzle you have reached from the level list.
Does every Sudoku puzzle have only one solution?
A properly constructed Sudoku has exactly one solution, and every puzzle here is built that way. That is what makes guessing unnecessary — any cell you cannot yet fill can be worked out by logic from the cells you can.
