Sudoku Rules: a simple guide for beginners

The rules of Sudoku are simple, and that is one of the reasons why the game is so famous. Even someone who has never solved a grid before can understand how it works in just a few minutes.

The difficulty is not in remembering many different rules, but in applying them carefully. Every number you place must respect the row, column, and block at the same time. When these three conditions are met, the grid can be completed step by step.

In this guide, we will look at the basic rules of classic 9×9 Sudoku, with examples and tips to avoid the most common mistakes.

Classic Sudoku is played on a grid made up of 9 rows and 9 columns. In total, there are 81 cells.

At the start of the game, some cells already contain a number. These starting numbers are often called “clues” or “givens”. They cannot be changed, because they represent the starting point of the grid.

The empty cells must be filled with the numbers from 1 to 9. Every number you place must follow all Sudoku rules. It is not enough for it to seem correct in one part of the grid: it must be valid in every direction.

To understand Sudoku, you need to know three fundamental elements: rows, columns, and blocks.

A row is a horizontal line made up of 9 cells. A column is a vertical line made up of 9 cells. A block is a 3×3 area made up of 9 cells.

The 9×9 grid contains 9 rows, 9 columns, and 9 blocks. Every cell always belongs to one row, one column, and one block at the same time.

This is the core of the game: when you want to place a number in a cell, you must check all three of these elements.

The most important rule of Sudoku is this: in every row, every column, and every block, the numbers from 1 to 9 must appear without repetitions. When the grid is complete, each of these areas must contain all numbers from 1 to 9.

This means that a row cannot contain the number 5 twice. In the same way, you cannot have two 5s in the same column or in the same 3×3 block.

This rule lets you eliminate many possibilities. If a cell is in a row where 7 is already present, then that cell cannot contain 7. If 3 is already present in its column, it cannot contain 3. If 9 is already present in its block, it cannot contain 9.

Sudoku is solved by continuing to eliminate what is impossible, until you find what is certain.

Completing a Sudoku does not simply mean filling all the cells. The grid is correct only if all rows, all columns, and all blocks contain the numbers from 1 to 9 without repetitions.

A full grid with a duplicated number is not solved. Even a single mistake can make it impossible to complete the rest of the game correctly.

That is why it is important not to proceed randomly. When you place a number, you should always have a clear reason. If you are only trying things out, you risk building a wrong solution that will reveal itself much later.

Imagine a row that already contains these numbers: 1, 2, 3, 4, 5, 6, 7, and 8. Only one number is missing: 9.

In this case, the empty cell in the row must contain 9: it is the only missing number. Before placing it, however, it is always a good habit to also check the cell’s column and block, so you can confirm that the move respects all the rules.

This is a very simple example, but the principle is the same even in more complex situations: you observe what is missing, check what is possible, and place only what is certain.

Beginners often tend to focus on only one part of the grid. For example, they check the row but forget the column, or they look at the block without verifying whether the number is already present horizontally.

Another common mistake is placing a number because it “seems to fit”, without having real logical confirmation. This may work for a few moves, but sooner or later it leads to contradictions.

The best advice is to proceed slowly, especially at the beginning. Before confirming a number, always check the row, column, and block.

The rules of Sudoku are few:

• each row must contain the numbers from 1 to 9 without repetitions;
• each column must contain the numbers from 1 to 9 without repetitions;
• each 3×3 block must contain the numbers from 1 to 9 without repetitions.

Everything else comes from these rules. The more you learn to apply them in an orderly way, the easier it becomes to recognize correct moves and solve increasingly difficult grids.