Hidden Single in Sudoku: Finding the Hidden Number

The Hidden Single is one of the most important techniques for solving easy and medium Sudoku puzzles. It is a fundamental step for anyone who wants to go beyond the most immediate moves.

Unlike the Naked Single, the Hidden Single is not always recognized by looking at a single cell. The correct number may be “hidden” among the cell's other candidates, but it becomes forced because, in a row, column, or block, it can appear in only one position.

This technique teaches you to observe the grid more broadly. You do not only look at what a cell can contain: you look at where a certain number can go within an area.

In this guide, we look at what a Hidden Single is, how to recognize it, and how to apply it correctly.

A Hidden Single is a number that, within a row, column, or block, can be placed in only one cell.

The cell where it appears may also have several candidates. That is why the technique is called “hidden”: the correct number is not necessarily the only candidate in the cell, but it is the only possible place for that number in the area being analyzed.

For example, imagine a 3×3 block where the number 5 has not been placed yet. If, among all the empty cells in the block, 5 can go in only one cell, then that cell must be a 5.

It does not matter if that cell also contains other candidates. The 5 is forced because it has no other available positions in the block.

The number is “hidden” because it does not appear as the cell's only possibility.

In a Naked Single, a cell has only one candidate, so the move is very obvious. In a Hidden Single, instead, the cell may have two, three, or more candidates. If you look only at that cell, you might not notice the solution.

To find a Hidden Single, you need to change perspective. Instead of asking yourself “which number can go in this cell?”, you need to ask “where can this number go in this row, column, or block?”.

This change in perspective is one of the first real improvements in the way you solve Sudoku.

A Hidden Single in a row occurs when a number can be placed in only one cell of that row.

Imagine a row where the number 8 has not been placed yet. You look at all the empty cells in the row and check their related columns and blocks. If 8 is excluded from every position except one, then it must go in that single cell.

Even if that cell has other candidates, the reasoning on the row makes 8 forced.

This type of check is very useful in almost complete rows, but it can also work in rows with more empty cells if the column and block constraints are strong enough.

The reasoning is identical for columns too.

Choose a column and observe which numbers are missing. Then, for each missing number, check which empty cells it could be placed in.

If a number can go in only one cell of the column, you have found a Hidden Single.

For example, if 3 has not been placed in a column yet and all empty cells except one exclude it because of the corresponding blocks or rows, then 3 must be placed in the only available cell.

This method requires attention because columns are often less easy to read than rows. Using highlights or notes can help a lot.

The Hidden Single in 3×3 blocks is one of the most frequent cases.

In classic Sudoku, every block must contain all numbers from 1 to 9 without repetitions. If a number is missing from the block, you need to understand which empty cells it could be placed in.

Often, the rows and columns outside the block eliminate many possibilities. If, in the end, only one possible cell remains for that number, that cell is a Hidden Single.

This is a very powerful reasoning pattern because blocks are compact areas and easy to analyze visually. In beginner Sudoku puzzles, a lot of progress comes precisely from observing blocks carefully.

The difference between Hidden Single and Naked Single is very important.

In a Naked Single, a cell has only one possible candidate. The solution is found by looking directly at the cell.

In a Hidden Single, instead, a number has only one possible position within a row, column, or block. The cell may also have other candidates, but that number is still forced.

A simple way to remember it is this: the Naked Single answers the question “what can go in this cell?”. The Hidden Single answers the question “where can this number go in this area?”.

Both techniques are fundamental and often alternate during solving.

Imagine a 3×3 block where the number 6 is missing.

There are four empty cells in the block. The first cell cannot contain 6 because there is already a 6 in the same row. The second cannot contain it because there is already a 6 in the same column. The third cannot contain it because its row also already contains a 6. Only the fourth cell remains.

Even if that fourth cell might have other candidates, 6 must go there. It is the only possible position for 6 in the block.

This is a Hidden Single.

The important thing is to notice that you did not choose the cell because it had only one candidate. You chose it because the number 6 had no other available positions in the area.

The Hidden Single is a basic but very powerful technique. It occurs when a number can be placed in only one cell of a row, column, or block.

To find it, you need to observe an area of the grid and ask yourself where a specific number can go. Even if the cell has multiple candidates, that number can be forced because there are no alternatives in the area.

Learning to recognize Hidden Singles helps you solve many Sudoku puzzles without guessing and prepares you for intermediate techniques.