Sudoku is a logic-based number puzzle played on a 9x9 grid. The goal is simple: fill every cell with a digit from 1 to 9, following three fundamental rules. No math is required — only logic and deduction.
A standard Sudoku grid contains 81 cells arranged in 9 rows and 9 columns. The grid is further divided into nine 3x3 regions called boxes (also known as blocks or squares). When you start a puzzle, some cells already contain digits — these are called givens or clues.
Every valid Sudoku solution must satisfy three constraints simultaneously:
A properly constructed Sudoku puzzle has exactly one valid solution.
Begin by scanning the grid for cells where only one digit is possible. Look for rows, columns, or boxes that already have many digits filled in — the more digits present, the fewer options remain for empty cells.
For each empty cell, ask yourself: which digits are already present in this cell’s row, column, and box? Whatever is left over is a candidate for that cell. When only one candidate remains, you have found the answer.
As puzzles get harder, you will encounter cells with multiple candidates. Pencil marks (also called notes) let you write small candidate digits inside a cell to track possibilities. This is essential for intermediate and advanced techniques.
On Sudoku69, toggle notes mode using the pencil icon in the controls. Click a number to add or remove it as a note in the selected cell.
A completed puzzle is valid when no digit repeats in any row, column, or box. If you place a digit that conflicts with an existing one, Sudoku69 highlights the error so you can correct it immediately.
Start with Very Easy difficulty to learn the basics, then gradually increase the challenge as your skills improve.