Sudoku Solver

Easy

Naked Single

A cell where only one number is possible because all others are eliminated.

The Naked Single (also known as a "Sole Candidate" or "Singleton") is the most basic and common strategy in Sudoku. It occurs when a specific cell has only one possible candidate left.

In other words, for a given empty square, eight of the nine possible numbers (1-9) are already present in its row, column, or 3x3 box. Since Sudoku rules state that numbers cannot repeat in these units, only one number remains as a legal option.

Interactive Example

1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
8
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
3
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
8
1 2 3 4 5 6 7 8 9
3
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
3
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9

Click "Apply Logic" to see the strategy in action.

Real Example Explanation

In the grid above, verify the candidates for the highlighted cell.

  1. Look at the Row: Does it contain numbers that eliminate possibilities?
  2. Look at the Column: Are there other numbers blocking candidates?
  3. Look at the Box: What numbers are already placed in the 3x3 area?

When you combine all these restrictions, you will find that for this specific cell, only the number 3 is allowed. Every other number strikes out.

How to Spot It

  1. Scan for crowded areas: Look at rows, columns, or boxes that are almost full. The empty cells there are more likely to have few possibilities.
  2. Use Pencil Marks: If you mark all possible candidates for a cell (based on row/col/box constraints) and find only one number written down, that's a Naked Single.
  3. Cross-Hatching: Pick a specific number (like 5) and check every box to see where it can go. Sometimes this reveals that a cell must be 5, but often this technique leads to finding Hidden Singles. For Naked Singles, it's better to focus on a specific cell and ask "what can go here?".

Why "Naked"?

The term "Naked" is used in Sudoku terminology to distinguish it from "Hidden" strategies. * Naked: The fact is visible just by looking at the candidates in that cell. The answer is "nakedly" obvious if you write down candidates. * Hidden: The fact is hidden among other candidates. You have to look at the entire unit (row/col/box) to realize that a number has nowhere else to go.

Mastering the Naked Single is the first step to becoming a Sudoku solver. The step-by-step solver relies on finding these continuously to make progress.