If you have ever opened a Sudoku guide and seen the word candidate, the meaning is simple: a candidate is a digit that could still legally go in an unsolved cell. It is not a guess and it is not a final answer yet. It is one of the remaining possibilities after you check the row, column, and 3×3 box.
Understanding what is a candidate in Sudoku makes every other technique easier to learn. Hidden singles, naked pairs, locked candidates, X-Wing, and many other strategies all depend on reading and eliminating candidates correctly. Once you understand candidates, tougher puzzles stop feeling random.
What Is a Candidate in Sudoku?
A candidate in Sudoku is any number that still fits an empty cell without breaking the three core rules:
- the number cannot repeat in the same row
- the number cannot repeat in the same column
- the number cannot repeat in the same 3×3 box
If a cell cannot be a 1, 2, 3, 5, 6, 8, or 9 because those digits already appear in its row, column, or box, then the remaining legal digits are that cell’s candidates. Sometimes there is only one candidate left. Sometimes there are several.
Short answer for featured snippet intent: a Sudoku candidate is a possible digit for an empty cell that has not been ruled out by the row, column, and box rules.
Candidate vs Clue vs Guess
Beginners often mix up a few important Sudoku terms, so it helps to separate them clearly.
Clue
A clue, also called a given, is a number that was already in the puzzle before you started solving.
Candidate
A candidate is a possible number for an unsolved cell. It stays a candidate until logic proves it must be placed or eliminated.
Guess
A guess is when you place a number without enough logic to prove it belongs there. Strong Sudoku solving tries to avoid guessing whenever possible. Candidates help you solve with evidence instead of instinct.
How to Find Candidates in Sudoku
The basic method is consistent on every puzzle. Look at one empty cell and test digits 1 through 9 against the row, column, and box that contain that cell.
- Check the row and cross off digits already used there.
- Check the column and cross off digits already used there.
- Check the 3×3 box and cross off digits already used there.
- The digits that remain are the candidates for that cell.
This sounds small, but it is the foundation of almost every solving routine.
Quick Example
Imagine an empty cell in row 4, column 7.
- Row 4 already contains 1, 2, 5, 7, and 9.
- Column 7 already contains 3, 4, and 8.
- The box already contains 6.
That removes every digit except one. If 1, 2, 3, 4, 5, 6, 7, 8, and 9 are all blocked except 4, then 4 is the only candidate and the cell is solved. If two or three digits remain, you keep them as candidates until more information appears.
When to Write Candidates as Pencil Marks
On paper, candidates are usually written as small notes inside the cell. In apps, they are often called pencil marks, notes, or candidate mode. They all serve the same purpose: showing the remaining possibilities without committing to one final number too early.
Pencil marks help most when:
- you have moved past the obvious singles
- several cells in a unit have two or three valid options
- you want to compare patterns across a row, column, or box
- you are trying to solve without guessing
Easy puzzles can often be solved with very light note-taking. Medium and hard puzzles usually become much clearer once clean candidates are visible.
How Candidates Lead to Real Solving Progress
Many Sudoku techniques are just different ways of reading candidates.
Naked Single
If one cell has only one candidate left, place it immediately.
Hidden Single
If a digit appears as a candidate in only one cell within a row, column, or box, that cell must take the digit even if it still shows several notes.
Naked Pair
If two cells in the same unit share the exact same two candidates, those two digits can be removed from other cells in that unit.
Locked Candidates
If a candidate is confined to one row or one column inside a box, you can eliminate that candidate from the rest of that row or column outside the box.
That is why candidate reading matters so much. Advanced techniques are not random tricks. They are organized ways to eliminate impossible candidates until the real placements appear.
Common Candidate Mistakes Beginners Make
Writing notes but never updating them
Outdated candidates are one of the fastest ways to get stuck. Every confirmed placement changes nearby cells, so notes need to stay current.
Treating candidates like guesses
If a cell has candidates 2, 4, and 7, that does not mean you should pick the one that feels best. It means the puzzle still needs more logic.
Adding candidates to every cell too early
On easier boards, that can create visual clutter. Start with obvious scans first, then add notes where they actually help.
Forgetting that candidates are local, not global
A digit can be a candidate in several different cells across the grid. What matters is whether it remains legal in one specific row, column, or box.
Should You Use Candidate Mode on Every Sudoku?
Not always. The best approach depends on difficulty and on how comfortable you are with scanning.
- Easy Sudoku: often no, or only for a few tricky cells
- Medium Sudoku: usually yes once easy singles dry up
- Hard and expert Sudoku: almost always yes if you want a clean logical solve
If your goal is speed on easy puzzles, excessive note-taking can slow you down. If your goal is accuracy on harder puzzles, candidates usually save time because they reduce repeated checking.
Best Habit for Reading Candidates Faster
Do not scan the whole board randomly. Use a repeatable routine.
- Scan for solved cells and singles first.
- Then review one row, one column, or one box at a time.
- Compare candidate positions, not just candidate counts.
- After every placement, immediately rescan the affected units.
This keeps the puzzle organized and helps you notice patterns sooner.
FAQ
What is a candidate in Sudoku in simple words?
A candidate is a number that might still fit in an empty cell because it has not been ruled out by the row, column, or box.
Are candidates the same as pencil marks?
No. Candidates are the possible digits themselves. Pencil marks are the way you write those candidates down on paper or in an app.
Should beginners use candidates?
Yes, especially once easy scans stop working. Candidates help beginners solve more accurately and understand why a move is valid.
Can you solve Sudoku without candidates?
Many easy puzzles can be solved with minimal notes, but harder puzzles are usually much easier once candidates are tracked clearly.
Conclusion
If you wanted a clear answer to what is a candidate in Sudoku, here it is: a candidate is a legal possible digit for an empty cell, not a guess. That one idea sits underneath almost every Sudoku strategy you will learn next.
If you want to improve faster, practice building candidates carefully, updating them after every placement, and using them to spot singles, pairs, and eliminations. Then move on to related guides like How to Scan Sudoku Faster, Common Sudoku Mistakes for Beginners, and Sudoku Methods Explained.
For hands-on practice, open a fresh puzzle on Pure Sudoku and turn on notes only when they help. Clean candidates lead to cleaner solves.