Sudoku Rules – How to Play

What Is Sudoku?

Sudoku is a globally popular logic puzzle that requires neither mathematics nor luck — only logical thinking. The puzzle was invented in the 1970s in the United States under the name "Number Place" and gained worldwide fame through a Japanese magazine in the 1980s, which named it "Sudoku" (short for "Suji wa dokushin ni kagiru" — "the digits must remain single"). Since the 2000s, Sudoku has become one of the most widely played puzzles in the world.

The Grid

A Sudoku puzzle is played on a square grid of 9 rows and 9 columns — 81 cells in total. The grid is further divided into nine 3×3 boxes (also called "regions" or "blocks") that together cover the entire 9×9 field. At the start, some cells are pre-filled with digits from 1 to 9; these are called "givens" or "clues." All other cells are empty and must be filled in by the player.

The Three Core Rules

The goal of Sudoku is straightforward: fill every empty cell with a digit from 1 to 9 so that all three rules are satisfied simultaneously:

  1. Every row contains each digit from 1 to 9 exactly once — no repetition.
  2. Every column contains each digit from 1 to 9 exactly once — no repetition.
  3. Every 3×3 box contains each digit from 1 to 9 exactly once — no repetition.

These three rules sound simple but together create a dense web of dependencies. Each digit you place affects up to 20 other cells (8 in the row + 8 in the column + 4 in the box, minus overlaps). That interdependency is the heart of the puzzle.

No Arithmetic, Pure Logic

Important: Sudoku involves no arithmetic. The digits 1–9 are merely symbols; the puzzle would work equally well with letters or colours. The only thing that matters is which symbols are already placed and which are missing. No addition, no multiplication — anyone who understands the three rules can start playing immediately.

How to Solve a Sudoku

The core idea when solving: look for cells where only one digit can legally go. For any empty cell, check which digits are already present in its row, column, and box. Every digit that already appears in those three units cannot go in that cell. If only one option remains, the cell is solved.

Step-by-step approach for beginners:

  1. Look at each 3×3 box: which digits are missing? Is there a digit that can only go in one cell of that box?
  2. Repeat the same check for each row and each column.
  3. When you find a cell with only one possible digit, fill it in immediately.
  4. Every newly placed digit may unlock further cells — revisit the affected rows, columns, and boxes after each entry.

For harder puzzles, this basic approach may not be enough. That is when advanced techniques come in — you can find them in our strategies guide.

Tips for Beginners

  • Use the notes mode: Write all possible candidate digits as small pencil marks in empty cells (our game has a built-in notes mode). This gives you an overview and makes patterns much easier to spot.
  • Start with busy boxes: Boxes with many given digits are usually the easiest entry point because the options are most constrained there.
  • Chase one digit at a time: Go through all nine digits one by one and check in which boxes each digit is still missing and where it can go.
  • Never guess: A well-formed Sudoku can always be solved without guessing. If you are stuck, there is a logical move you have not spotted yet — not a lucky break waiting to happen.
  • Use the hint feature: Our game shows you the next logical step on demand — ideal for staying unblocked while learning new techniques at the same time.

Unique Solution Guarantee

Every puzzle on this site has exactly one valid solution. This is guaranteed by our puzzle generator, which uses Dancing Links (Algorithm X) — one of the fastest known algorithms for exact cover problems. You can be confident: there is always precisely one arrangement of digits that satisfies all three rules.