How to solve Sudoku: techniques and examples

Know the rules but cannot always see the next move? Learn to find it with candidates. Start with simple elimination, then build up to pairs and interactions between boxes and lines.

By IRBIS GamesPublished October 4, 2026

Start with reliable candidates

In a classic 9×9 Sudoku, each row, column and 3×3 box contains the digits 1–9 without repeats. A candidate is a digit not yet ruled out by the placed digits in all three areas of a cell. A pencil mark records a possibility, not an answer.

Pick an empty cell. Start with 1–9 and remove the digits already present in its row, column and box. Note what remains. After placing a confirmed digit, update the notes in cells that share its row, column or box: stale candidates can hide straightforward moves.

In the examples, large digits are already placed and small digits are candidates. Count columns from left to right, from 1 to 9.

Four techniques, step by step

These are separate teaching fragments, not one complete puzzle. Assume the listed candidates have already been checked against their rows, columns and boxes.

Naked single: one candidate in a cell

When all checks leave a cell with just one possible digit, you can place it. The key is to combine the restrictions from all three areas.

The intersection of the three sets leaves one candidate: 5.

The row allows 2, 5 and 8. That gives three possibilities; it is not a reason to choose one yet.
The column allows 1, 5 and 6, while the box allows 3, 5 and 9. Find the digit shared by all three sets.
Only 5 survives. Place 5, then remove it from the candidates of other cells in the same row, column and box.

Check all three areas. A digit missing from the row may already appear in the column or box.

Hidden single: one place for a digit

Instead of looking for a cell with one candidate, look for a digit with only one possible position in a row, column or box.

Across the complete candidate lists for this row, 7 appears only in column 3, so that cell must be 7.

This row is missing 2, 5, 7 and 8. Inspect the candidates in all four empty cells.
Only the cell in column 3 contains candidate 7, even though it also contains notes for 2 and 5.
The row must contain a 7, so column 3 must be 7. Replace its other notes with the confirmed digit.

Inspect the whole area. Accidentally omitting an allowed 7 from another cell can create a false hidden single.

Naked pair

Two cells in one area that each have exactly the same two candidates reserve those digits. Their order is still unknown, but they already allow an elimination.

The pair {2, 7} in columns 1 and 4 removes 2 and 7 from column 8 of the same row, leaving 5.

Columns 1 and 4 of this row each contain only 2 and 7. One cell must be 2 and the other must be 7.
No other cell in this row can contain either digit. Remove 2 and 7 from column 8, whose candidates are {2, 5, 7}.
Column 8 now contains only 5. Place it, while leaving the pair unresolved.

Both cells must contain exactly those two candidates and share an area. {2, 7} and {2, 7, 8} do not form a naked pair.

Locked candidates: from a box into a row

When every possible position for a digit within a box lies in one row, remove that digit from the rest of that row outside the box.

Only candidate 9 is shown. Inside the left box it is confined to row 2; crossed-out 9s outside the box can be removed. Dots show positions without candidate 9 in this box.

In the top-left box, candidate 9 appears only in row 2, in columns 1 and 3.
One of those cells must contain 9. A second 9 elsewhere in row 2 would therefore create a repeat.
Remove candidate 9 wherever it is noted in columns 4–9 of row 2. The same reasoning works along a column.

This does not yet tell you which of the two cells contains 9. A correct elimination is not permission to guess its position.

Common mistakes and how to fix them

Treating a possible digit as a proven one

If both 2 and 7 fit, keep both notes. Ask why the other choice is impossible. Without a reason, the move is not yet proven.

Forgetting the third area

Check the row, column and box for every move. Avoiding a repeat in one row does not guarantee the correct solution.

Removing a candidate without a reason

Every elimination needs a placed digit or a valid deduction. If a cell has no candidates left, revisit your recent entries and deletions.

Calling two cells a pair despite an extra candidate

The pair {2, 7} works only when both cells contain exactly that set. An additional 8 prevents you from reserving the two digits.

Rushing without updating notes

After placing a digit, check its neighbours. If you get stuck, scan another area or take a break; speed develops with practice.

A practical routine for beginners

Start with an easy classic Sudoku. Choose a nearly complete row, column or box and list the missing digits.
Look for naked and hidden singles. After each placement, revisit the affected areas.
If no simple move appears, write complete candidates for an area and look for pairs or locked candidates.
Before entering a digit, explain the reason in a short sentence. Use a hint to understand a deduction, not simply to fill a cell.
Keep a comfortable pace and value the moves you understand, not just your finishing time. These four techniques will not solve every difficult Sudoku.

Check your understanding

1. For one cell, the row allows {1, 4, 7}, the column allows {3, 4, 8}, and the box allows {2, 4, 9}. Which digit can you place?

Show answer and explanation

4. It is the only digit shared by all three sets: a naked single.

2. Two cells in one row each have exactly {3, 8}. Another cell in that row has {3, 6, 8}. What remains in it?

Show answer and explanation

6. The naked pair reserves 3 and 8 for the first two cells, so both candidates can be removed from the third. The order of 3 and 8 in the pair is still unknown.

Try the techniques in a game

Choose a 9×9 grid and easy difficulty for your first practice, or open the daily puzzle. Return to an example whenever you want to check your reasoning.

Choose a game Daily Sudoku Basic Sudoku rules