Hidden Singles in Sudoku

What Is a Hidden Single?

A hidden single occurs when a particular number can only go in one cell within a row, column, or box — even though that cell might have several other candidates. The number is "hidden" because the cell does not look special at first glance. It might have three or four pencil marks, but only one of those numbers is unique to that position in the group.

Compare this to a naked single, where a cell has only one candidate total. Naked singles are obvious once you have filled in pencil marks. Hidden singles require you to think about the group (row, column, or box) rather than the individual cell.

Why Hidden Singles Matter

Hidden singles are the most productive technique in Sudoku. On easy puzzles, scanning for hidden singles can fill the entire grid without any other strategy. On medium and hard puzzles, they remain your primary workhorse, solving 60–80% of cells before you need techniques like naked pairs or pointing pairs.

If you learn only one technique beyond basic scanning, make it this one.

How to Find Hidden Singles

There are two reliable methods, and experienced solvers use both interchangeably.

Method 1: Cross-Hatching (No Pencil Marks Needed)

Pick a number — say, 4. Look at every row that already contains a 4 and mentally draw a line through that row. Do the same for every column with a 4. Now look at each 3×3 box that does not yet have a 4. The lines you drew eliminate certain cells. If only one cell in the box is left uncrossed, the 4 must go there. That is a hidden single.

Repeat for every number, 1 through 9. This is the same "scanning" technique described in our tips page, and it is specifically targeting hidden singles.

Method 2: Candidate Checking (With Pencil Marks)

If you have pencil marks filled in, look at a row, column, or box and check each missing number. Count how many cells in that group list that number as a candidate. If the count is exactly one, you have found a hidden single — place the number in that cell and erase it from the other candidates.

For example, suppose Box 5 (the center box) is missing the numbers 2, 5, 7, and 9. You check each: the number 2 appears as a candidate in three cells, 5 appears in two cells, 7 appears in two cells, but 9 appears in only one cell. That cell is a hidden single for 9.

Worked Example

Consider Row 6 with these candidates in its empty cells:

R6C2: {1, 3, 8} — R6C4: {1, 3} — R6C6: {3, 5, 8} — R6C9: {1, 5}

Now check which numbers appear how many times across these four cells. The number 1 appears in three cells (C2, C4, C9). The number 3 appears in three cells (C2, C4, C6). The number 5 appears in two cells (C6, C9). The number 8 appears in two cells (C2, C6).

No hidden single in this row — every number appears in multiple cells. But now check Box 4 (which contains R6C2). If no other cell in Box 4 has 8 as a candidate, then R6C2 is a hidden single for 8 within that box — even though the row alone did not reveal it.

This is why you need to check all three groups (row, column, box) for every cell. A hidden single that is invisible in the row might be obvious in the box.

Hidden Singles vs. Naked Singles

A naked single asks: "Does this cell have only one candidate?" It focuses on the cell.

A hidden single asks: "Does this number have only one possible cell in this group?" It focuses on the group.

Both result in a placed number. Both are "free" moves that require no trial-and-error. The difference is perspective. Good solvers check both angles simultaneously — looking at the cell and looking at the group.

Common Mistakes

Checking only one group. A cell belongs to three groups: a row, a column, and a box. A number might not be a hidden single in the row but is one in the box. Always check all three.

Stale pencil marks. If you placed a number recently but forgot to erase it from candidates in the same row, column, and box, your hidden single count will be wrong. Update marks immediately after every placement.

Confusing hidden singles with hidden pairs. A hidden single is one number in one cell. A hidden pair is two numbers restricted to two cells. They are related concepts but different techniques.

Skipping the cross-hatch scan. Beginners sometimes jump straight to pencil marks for the whole grid. Cross-hatching is faster and finds the same hidden singles with less setup. Try scanning first.

Stopping too early. After placing a hidden single, re-scan its row, column, and box immediately. The new placement often creates another hidden single nearby.

Overlooking boxes. Rows and columns get most of the attention, but boxes are equally important. Many hidden singles are only visible at the box level.

Practice

Hidden singles appear at every difficulty, but easy and medium puzzles are the best training ground because the logic is cleaner. Play the Daily Sudoku and try to solve it using only scanning (cross-hatching for hidden singles) before touching pencil marks. You will be surprised how far it gets you.

For paper practice, print an easy pack or medium pack and challenge yourself to solve each puzzle without writing any pencil marks at all. If you can do it, your hidden-single instincts are sharp.

Frequently Asked Questions