Candidates in Sudoku: what they are and how to use them

Candidates are one of the most important tools for improving at Sudoku. When grids become more difficult, it is no longer enough to look only at the numbers already present: you also need to keep track of the numbers that could go in the empty cells.

That is exactly what candidates are for. They are the possibilities still valid for a cell, based on the numbers already present in the same row, column, and block.

Learning to use them well allows you to reason more clearly, avoid mistakes, and recognize techniques that would otherwise be difficult to see.

A candidate is a number that could be placed in a cell without breaking the rules of Sudoku.

For example, if a cell cannot contain 1, 2, 3, 5, 7, and 9 because these numbers are already present in its row, column, or block, then only 4, 6, and 8 may remain as candidates.

This does not mean that all three are correct solutions. It only means that, based on the information available at that moment, none of the three has been excluded yet. As you continue solving the grid, some candidates will be eliminated and others will become certain numbers.

Without candidates, you have to keep too much information in mind. This can work in an easy Sudoku, but it becomes complicated as soon as the grid requires more steps.

Candidates turn the grid into a map of possibilities. Instead of asking yourself every time “what can go here?”, you can immediately see which numbers are still available.

This makes it easier to find cells with only one candidate, pairs of cells that share the same candidates, candidates aligned within a block, and other useful situations for solving techniques.

In practice, candidates do not solve the Sudoku for you, but they help you see the hidden logic in the grid more clearly.

In paper Sudoku, candidates are often written small inside the cells. In online Sudoku, however, you can use note mode or an automatic feature.

The most orderly method is to mark only the numbers that follow the rules for the cell. For each empty cell, you check the row, column, and block, then note the numbers that are still possible.

At first, it can seem slow, but with a little practice it becomes natural. Also, marking candidates well saves time later on, because it avoids repeating the same checks over and over.

Every time you place a number in the grid, some candidates must be removed from the connected cells.

If you place a 6 in a cell, that 6 can no longer appear as a candidate in the other cells of the same row, the same column, and the same block.

Updating notes is essential. If you leave candidates that are no longer valid, you risk reasoning with wrong information. Many mistakes in medium and hard Sudoku puzzles come precisely from candidates that were not updated.

For this reason, if you use manual notes, it is a good habit to update them immediately after every number you place.

In online Sudoku, you can find two main modes: manual candidates and automatic candidates.

With manual candidates, you decide which numbers to note. This method requires more attention, but it helps learning a lot, because it forces you to reason about every cell.

With automatic candidates, instead, the site updates the basic possibilities for you, removing numbers that cannot be in a cell because of the row, column, or block. It is a convenient feature, especially when you want to focus on techniques rather than manual note management.

Both approaches make sense. If you are a beginner, it can be useful to start with automatic candidates to understand the mechanism. If you really want to train your analysis skills, also try using manual notes.

The first mistake is marking too many candidates without checking them. A wrong note can confuse the whole solve.

The second mistake is forgetting to update candidates after placing a number. Even if the move is correct, old notes can lead you off track.

The third mistake is deleting candidates without a logical reason. Every elimination should be justified by a rule or a technique. If you eliminate a candidate only because “it seems unlikely”, you are turning Sudoku into a guessing game.

Good candidate management must be precise, orderly, and always based on the rules.

Imagine an empty cell. Its row already contains 1, 3, and 9; its column contains 2, 4, and 7; its block contains 5 and 8.

At this point, which numbers remain available? The numbers from 1 to 9 are all excluded except 6. So 6 is the only possible candidate and can be placed with certainty.

This is a simple example, but it clearly shows the value of candidates: instead of looking for the solution by intuition, you find it by eliminating everything that cannot work.

Candidates are the numbers still possible for a cell. Using them well allows you to read the grid better, avoid mistakes, and apply more advanced techniques.

To improve at Sudoku, learning to manage candidates is a fundamental step. Even if it may seem like a waste of time at first, it is actually one of the most useful tools for solving medium and hard grids without guessing.