How to solve a Sudoku: basic step-by-step method

Solving a Sudoku does not mean filling cells at random until something works. A good Sudoku is solved with logic, one move at a time.

At first, it can be hard to understand where to start. The grid has many empty cells, and the numbers already present may seem scattered without a clear order. In reality, there is a simple method for getting started: observe, eliminate impossible possibilities, and look for cells where only one valid choice remains.

In this guide, we will look at a basic method for solving a Sudoku step by step, designed for anyone who is just starting out or wants to play in a more organized way.

The first thing to do is look at the grid as a whole. You do not need to place a number immediately. First, it is better to understand which rows, columns, or blocks are already fairly full.

Areas with many numbers already present are often easier to complete, because they have fewer possibilities left. If a row already contains seven or eight numbers, it could be a good starting point. The same applies to a column or a 3×3 block.

This first observation helps you avoid getting lost. Instead of looking at the whole grid at the same time, you can focus on the parts where more information is available.

Another useful method is to choose a number and look for it across the whole grid. For example, you can observe where the 5s already present in the grid are located.

If a number is already present in many rows, columns, or blocks, it may be easier to understand where it is missing. This approach works especially well with simple Sudoku puzzles, where some numbers have almost forced positions.

The reasoning is this: if a 5 is already present in a row, no other cell in that row can contain a 5. If it is already present in a column, no other cell in that column can contain it. By combining this information, often only one possible position remains.

Every Sudoku cell must satisfy three constraints at the same time: the row, the column, and the block.

When you want to understand what can go in a cell, you need to ask yourself:

• which numbers are already present in the row?
• which numbers are already present in the column?
• which numbers are already present in the block?

All numbers already present in these three areas cannot be placed in the cell. The ones that remain are the possible candidates.

At the beginning, you can do this reasoning mentally. When grids become more difficult, however, it is very useful to mark candidates in the cells.

A forced cell is a cell where only one number can go. This happens when all other numbers are excluded by the row, column, or block.

For example, if a cell could theoretically contain the numbers from 1 to 9, but the row eliminates 1, 2, and 3, the column eliminates 4, 5, and 6, and the block eliminates 7 and 8, only 9 remains. That cell must be a 9.

This idea is the basis of one of the simplest Sudoku techniques: the Naked Single, which is the situation where a cell has only one possible candidate. Even if you do not know the name of the technique yet, you can already apply the reasoning.

One of the most common mistakes is trying a number “to see if it works”. This approach may seem fast, but it often creates problems.

Sudoku is designed to be solved through logical steps. If you place a number without certainty, you may notice the mistake much later, when the grid is already compromised.

In a good solve, every move should have an explanation. It does not need to be complicated: it can simply be “this row is missing only 4” or “this cell cannot contain any other number”. The important thing is not to proceed randomly.

Getting stuck is normal, especially when moving from easy Sudoku puzzles to medium ones. When it happens, it is not helpful to place random numbers. It is better to recheck the grid in an orderly way.

You can try to:

• look for rows, columns, or blocks that are almost complete;
• update the candidates;
• check whether there are cells with only one candidate;
• look for numbers that can go in only one position within a block;
• review your latest moves, checking that they have not created repetitions in rows, columns, or blocks.

If you are using Sudoku Arena, a hint can help you understand which technique was available at that moment. Used well, a hint does not interrupt learning: it makes it clearer.

Imagine starting a grid and noticing that the top-left block already contains six numbers. Only 2, 5, and 8 are missing.

You look at the three empty cells. In the first cell, the column already contains 2 and the row already contains 5. Among the missing numbers in the block, therefore, neither 2 nor 5 can go there. Only 8 remains.

At this point, you can place the 8. After doing so, the situation in the block changes: now only 2 and 5 are missing. It may become easier to complete the other cells too.

This is the correct way to proceed: one logical move opens the way to the next.

To solve a Sudoku, it is useful to follow a simple method:

1. observe the grid;
2. start from the most complete areas;
3. check rows, columns, and blocks;
4. eliminate impossible numbers;
5. place only certain moves;
6. use candidates when the grid becomes more complex.

With this approach, you can tackle basic Sudoku puzzles without guessing and prepare for more advanced solving techniques.