Most of any sudoku is solved with two deductions: the naked single and the hidden single. They place more digits than every advanced pattern combined, so learning to spot them quickly is the single biggest step up in solving speed. This guide defines both, shows how each one is found, and explains how to tell them apart. For the wider set of methods, see our overview of sudoku techniques; if the board itself is new to you, start with how to play sudoku.
One rule drives both deductions: each digit from 1 to 9 must appear exactly once in every row, every column, and every 3×3 box. A “unit” is one of those groups. A candidate is a digit that could still legally go in a cell. Keep those two words in mind and the rest follows.
Naked singles: reading one cell
A naked single is a cell with only one candidate left. To find one, look at an empty cell and cross off every digit that already appears in its row, its column, or its box. If a single digit survives, that digit is the answer, because the cell has no other legal option. Nothing outside the cell is needed to confirm it, which is why the deduction is called “naked” — the answer is fully exposed once you clear the used digits away.
Worked example: a cell sees 1, 2, 3, 4, and 5 across its row and column, and its box already contains 6 and 8. That removes seven of the nine digits, leaving 7 and 9; if the box or the lines also account for a 9, only 7 remains, and the cell must be 7. Naked singles appear constantly as a grid fills in, so it pays to recheck the neighbours of every digit you place — a new entry often strips the last candidate from a nearby cell.
Hidden singles: reading one unit
A hidden single works the other way around. Instead of asking what a cell can hold, you take one digit and one unit and ask where that digit can go. If the digit can legally land in only one cell of that row, column, or box, it belongs there — even if the cell still lists several other candidates. The digit is “hidden” among those extra candidates, but it has nowhere else to sit in the unit, so it is forced.
Worked example: in a given row, the digit 4 is blocked from every cell but one, because a 4 already sits in the crossing columns or boxes of the others. That last cell might still show candidates like 2, 4, and 9, yet the 4 must go there, since no other cell in the row can take it. Hidden singles are easy to miss precisely because the answer is buried, so scan each unit digit by digit and count the legal homes for each one.
Telling the two apart
The difference is the starting point. A naked single is found by looking at a cell and running out of options for it. A hidden single is found by looking at a unit and running out of homes for a digit. Both give a certain placement, and both are pure logic — you never guess. In practice you will spot naked singles almost automatically, while hidden singles reward a deliberate pass: pick a digit, check each row, column, and box, and place it wherever it has only one seat left.
How to use them in a solve
Open every puzzle by scanning for these two first, before writing any pencil marks. Place all the singles you can find, and each placement often reveals another, so keep circling back. Only when singles dry up should you write full candidate lists and move on to pairs and locked candidates. The habit is what matters: a fast, accurate singles pass clears easy puzzles outright and does most of the work on hard ones.
The best way to build the habit is to solve regularly. Try the daily board on Microsoft Sudoku or a fresh daily sudoku, and on each puzzle name the singles as you place them. Browse the full set of boards in our sudoku catalog when you want more practice.
Frequently asked questions
What is the difference between a naked single and a hidden single?
A naked single is found by looking at one cell: after removing every digit used in its row, column, and box, only one candidate remains, so that digit is the answer. A hidden single is found by looking at one unit: a digit that can legally go in only one cell of that row, column, or box, even if the cell still lists other candidates.
Which should I look for first?
Scan for both before anything else, but naked singles are usually quicker to see because the cell itself is nearly empty of options. Hidden singles reward a separate, deliberate pass where you take one digit at a time and count how many cells in each unit can still hold it.
Do I need pencil marks to find singles?
No. Both naked and hidden singles can be found by scanning alone, which is why they come first, before you write any candidate lists. Pencil marks help once the easy placements run out and you move to pairs, locked candidates, and beyond.
Can a cell be both a naked single and a hidden single?
Yes. If a cell has only one candidate, that candidate is also, by definition, the only place its digit can go in each of the cell’s units. The two names simply describe two ways of arriving at the same certain placement.
