Minimal Sudoku
Naked Single technique in Sudoku
Learn Section

Naked Single

The Most Fundamental Sudoku Technique

By Minimal Sudoku TeamLast updated:

The Naked Single (also called "Singleton" or "Lone Number") is the most fundamental Sudoku solving technique. When a cell has only one possible candidate left, that candidate must be the answer.

Quick Summary
  • What: A cell where 8 of 9 digits are eliminated by row, column, or box
  • Result: The remaining digit is the only possibility — place it!
  • Difficulty: Beginner — the first technique every solver learns
  • Also called: Singleton, Lone Number, Forced Digit

The Concept

Every empty cell in a Sudoku puzzle will eventually contain a digit from 1 to 9. The Naked Single technique asks a simple question: which digits could possibly go in this cell?

To answer this, we check:

If we can eliminate 8 of the 9 possible digits, only one remains — and that's our answer.

The Golden Rule

1️⃣ Naked Single Rule
When a cell has only one possible candidate remaining (all other digits 1-9 are eliminated by the row, column, or box), that candidate is the solution for that cell.

How to Find Naked Singles

1

Pick an empty cell

Choose any empty cell to analyze. Cells with many filled neighbors are often easier to solve.
2

Check the row

Look across the row and note which digits (1-9) are already placed. These are eliminated as candidates for your cell.
3

Check the column

Look down the column and eliminate any additional digits you find.
4

Check the 3×3 box

Look at the box containing your cell. Eliminate any remaining digits that appear there.
5

Count remaining candidates

If only one digit remains, you've found a Naked Single! Place it in the cell.
Speed Tip
Focus on cells that have many filled neighbors. A cell at the intersection of a nearly-complete row, column, and box is much more likely to be a Naked Single.

Practical Example

Finding a Naked Single

Naked Single example at R5C5

Look at the cell at R5C5 (row 5, column 5) — the center of the grid.

Let's check which digits are eliminated:

  • Row 5: Contains 1, 2, 3, 4, 6, 7, 8
  • Column 5: Contains 9
  • Box 5: No additional digits

Combined, we've eliminated: 1, 2, 3, 4, 6, 7, 8, 9

Solution
Only 5 remains as a candidate. R5C5 = 5!

Naked Single vs Hidden Single

Beginners often confuse Naked Singles with Hidden Singles. Here's the difference:

Naked Single

Only one candidate remains in a cell.

"This cell can only be 5."

Hidden Single

A candidate can only go in one cell within a row, column, or box.

"5 can only go here in this row."

Both techniques result in placing a digit, but they approach the problem from different angles. Master both to solve puzzles efficiently!

Hidden SingleBeginner

Hidden Single

Learn the complementary technique: finding where a digit can only go in one place.

Tips for Beginners

Use Pencil Marks
Write small candidate numbers in cells to track possibilities. When a cell shows only one candidate, it's a Naked Single. Learn about pencil marks.
Scan After Each Placement
After placing a digit, immediately check if it creates new Naked Singles in the same row, column, or box. One placement often leads to another!
Combine with Scanning
Use scanning to quickly identify promising cells. Naked Singles often appear where rows, columns, and boxes intersect with many filled cells.

Related Techniques

Beginner Techniques

🔎Hidden Single📍Last Remaining Cell👀Scanning Technique

Next Steps

👥Naked Pairs🔍Hidden Pairs✏️Using Pencil Marks