Single Candidate in Sudoku: How to Spot It
Learn the Single Candidate Sudoku technique: check one cell's row, column, and box until only one legal number remains.
Guided example
See the Single Candidate on this board
Read this as one move: focus on the blue target cell, then use its row, column, and 3×3 box to cross off impossible digits until only one number remains.
Quick answer
A Single Candidate in Sudoku is one empty cell with exactly one legal number left. Check the cell's row, column, and 3×3 box; if those three areas rule out eight digits, the remaining digit must be placed in that cell.
How to spot it
Follow the board in order
- 1 Start with one empty cell that has many filled neighbors in its row, column, and 3×3 box.
- 2 Check the row and cross off any digits already used there.
- 3 Check the column and cross off any digits already used there.
- 4 Check the 3×3 box and cross off any digits already used there.
- 5 If exactly one digit from 1–9 is not crossed off, place that digit in the cell.
What is a Single Candidate?
A Single Candidate appears when one empty cell has only one possible number left. Instead of asking where a number can go, you ask a simpler question: what can go in this cell?
To answer it, check the cell’s row, column, and 3×3 box. Any digit already used in one of those three places cannot go in the target cell. If eight digits are blocked, the ninth digit is forced.
When to look for it
Look for Single Candidates early and often. They are the foundation of Sudoku solving and usually appear before harder techniques are needed.
Start with crowded cells: empty cells in rows, columns, or boxes that already contain many numbers. A crowded cell gives you more evidence, so it is faster to prove whether only one option remains.
Example walkthrough
This walkthrough now uses a real bundled Sudoku Coach app puzzle tagged Naked Single. Focus on r7c9. Its row, column, and bottom-right box remove every digit except 5, so r7c9 = 5. The important habit is to test one cell against all three constraints before placing the number.
Why the placement works
Sudoku allows each number once per row, column, and box. If a number is already present in any of those three areas, it cannot also appear in the target cell. Once every digit except one is impossible, the remaining digit is not a guess — it is forced.
Single Candidate vs. Hidden Single
A Single Candidate focuses on one cell: “What number can go here?”
A Hidden Single focuses on one number inside a unit: “Where can this number go in this row, column, or box?”
In short: Single Candidate is cell-first. Hidden Single is number-first.
Common mistake
Do not check only the row. A Single Candidate is only confirmed after the row, column, and box all agree that every other number is impossible.
FAQ
What is a Single Candidate in Sudoku?
A Single Candidate is an empty cell with only one legal number left after its row, column, and 3×3 box are checked.
Is Single Candidate the same as Naked Single?
Yes. Many Sudoku guides call this technique a Naked Single because the cell's candidate list has only one visible option.
When should beginners use Single Candidate?
Use it early and often, especially on empty cells surrounded by many filled numbers. Those cells are more likely to have eight digits blocked already.
Does Single Candidate require candidate notes?
Candidate notes help, but they are not required. You can also test the digits 1 through 9 manually against the cell's row, column, and box.
In this lesson
- Board example: see the pattern first.
- Walkthrough: connect each highlight to the rule.
- FAQ: check edge cases after the move is clear.
Practice this technique in Sudoku Coach
Read the pattern, then practice it step by step with guided hints that explain why the move works.
Related Sudoku techniques
Hidden Single in Sudoku: How to Spot It
Learn the Hidden Single Sudoku technique: when a number has only one possible position in a row, column, or box.
Candidate Notes in Sudoku: How to Use Them
Learn how to write and maintain Sudoku candidate notes so every later technique has reliable evidence.
Naked Pair in Sudoku: How to Spot It
Learn how Naked Pairs work in Sudoku, when to look for them, and how to remove candidates safely.