Guide

Naked Singles vs Hidden Singles: The Two Techniques Every Solver Needs

Naked singles and hidden singles are the foundation of Sudoku. Learn exactly how each works, how to spot them fast, and how they differ — with clear examples.

Updated August 2, 2026 · 5 min read

Almost every Sudoku is cracked open with two simple techniques: the naked single and the hidden single. They sound similar and beginners often confuse them, but they're mirror images. Master both and you'll finish every easy puzzle and most mediums.

What is a naked single?

A naked single is about one cell. You focus on a single empty cell and check its row, its column, and its 3×3 box. If those together already contain eight different digits, only one digit is left for that cell — so it must go there.

  • You're looking at a cell and asking: 'what can go here?'
  • The answer is forced when exactly one candidate remains.
  • Naked singles are easiest to find in nearly-complete rows, columns, and boxes.
123467895
A naked single. The highlighted cell's row, column, and box (shaded and outlined) already contain eight different digits — 1, 2, 3, 4, 6, 7, 8, 9 — so the only value left for it is 5.

Tip

A row, column, or box with a single empty cell is an instant naked single — the missing digit is whatever isn't already there.

What is a hidden single?

A hidden single is about one digit inside one unit (a row, column, or box). Instead of asking what fits a cell, you pick a digit and ask where it can go in that unit. If there's only one legal spot, the digit belongs there — even if that cell could also hold other numbers.

It's called 'hidden' because the cell might have several candidates, so the placement isn't obvious until you hunt for that specific digit.

44444
A hidden single for 4. Rows 1–2 and columns 1–2 already contain a 4, blocking it from every cell in the top-left box except the bottom-right one — so 4 goes there, even though that cell could also hold other digits.

The key difference

  1. 1.Naked single: fix a CELL, find the only digit that fits it.
  2. 2.Hidden single: fix a DIGIT, find the only cell it fits in a unit.

Both give you a guaranteed placement. Strong solvers constantly switch between the two viewpoints — scanning cells for naked singles, then scanning digits for hidden singles — because whichever you're not looking for is often the one that breaks the puzzle open.

Which is more common, naked or hidden singles?
Hidden singles usually appear more often on real puzzles, especially early on, because most empty cells still have several candidates. Naked singles become plentiful as the grid fills up.
Do I need pencil marks to find singles?
Not for easy puzzles — you can spot both by scanning. On harder grids, pencil marks make hidden singles much easier to see.

Try it on a real puzzle

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