How to Find Naked Singles in Sudoku

By Avi Israeli · Reviewed by Ninely Engine Logic Council · Updated November 2026

What is a Naked Single?

A Naked Single (often called a Sole Candidate) occurs when a single empty cell can only contain one possible digit because the other eight digits (1 through 9) are already present in its peers—that is, in its row, its column, or its 3×3 box.

Unlike a Hidden Single (where a number can only go in one spot in an entire house), a Naked Single is identified by looking directly at the cell itself and ticking off all digits blocked by its intersecting neighbors.

Worked Example: Cell Row 5, Column 5

Consider the central cell at Row 5, Column 5 (R5C5) in the classic 30-clue puzzle below. Let us examine every intersecting unit that shares a peer relationship with R5C5:

  • Row 5 contains: 1, 3, 4, 8 (at R5C9, R5C6, R5C1, and R5C4)
  • Column 5 contains: 1, 2, 6, 7, 8, 9 (at R8C5, R6C5, R4C5, R1C5, R9C5, and R2C5)
  • Center 3×3 Box (Box 5) contains: 2, 3, 6, 8 (at R6C5, R5C6, R4C5, and R5C4)

Now check each candidate digit from 1 to 9 for R5C5:

1 (blocked by Row 5 & Col 5), 2 (blocked by Col 5 & Box 5), 3 (blocked by Row 5 & Box 5), 4 (blocked by Row 5), 6 (blocked by Col 5 & Box 5), 7 (blocked by Col 5), 8 (blocked by Row 5, Col 5 & Box 5), 9 (blocked by Col 5).

The only surviving candidate is 5!

Before (Peer Elimination at R5C5)
123 456 789
➔
After (Resolved Naked Single)
5
5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
5
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 3 of 3: Digit 5 confirmed

⚠️ The Tempting Wrong Move: Guessing 7 in R5C5

A player scanning only Row 5 (`4, 8, 3, 1`) might notice that `7` is missing from Row 5 and Box 5, and hurriedly place `7` in the center cell (R5C5).

Why this fails: Column 5 already contains a given `7` at the top (`R1C5`)! Placing `7` in R5C5 creates an immediate collision in Column 5. In Ninely, Smart Hints check all three intersecting units simultaneously so you never overlook a vertical blocker.

💡 Solving Tip: The "Rule of 8"

Whenever a row and column cross at a cell that shares a crowded 3×3 box, count the distinct numbers across all three units. If they cover 8 distinct digits, that intersection is guaranteed to be a Naked Single!

Put It Into Practice

Try spotting naked singles in real time on today's Ninely daily board:

Play Today's Daily Warmup ➔