A candidate is a number that could still fit in an empty cell. Solvers often write candidates small, in pencil, so they can tell possibilities apart from final answers. The notes do some of the remembering for you, leaving you free to compare what each row, column, and box allows.
For a basic candidate list, check all three areas that cross the cell: its row, its column, and its box. Rule out every number already placed in any of them. If you are still learning the rules, start with our small worked sudoku example.
Example: finding the candidates for cell A
This original 9×9 grid marks A in row 1, column 5. The thicker lines outline the 3×3 boxes. Before reading on, see which numbers you would keep.
Count rows from the top and columns from the left.
- Row 1: It already contains 4, 6, 7, 8, and 9. The missing numbers are 1, 2, 3, and 5.
- Column 5: It already contains 8, 7, 1, and 4. Cross out 1 from the row's list. That leaves 2, 3, and 5.
- The top-middle box: It contains 4, 6, 8, 9, 1, and 3. Cross out 3. The candidates for A are 2 and 5.
Write a small 2 and 5 in A. Neither conflicts with a number already in the cell's row, column, or box. That does not tell us which one is the answer; we need another deduction.
Now try cell B
B is in row 1, column 3. Start with the numbers missing from the row, then check the column and box. How many candidates remain?
Show B's candidates and deduction
The row allows 1, 2, 3, and 5. Column 3 already contains 1, 5, and 2, so all three are ruled out. Only 3 remains. Nothing in the top-left box rules it out. B must be 3, so you can write it as a full-size answer.
Keep your notes up to date
After placing a certain number, remove that candidate from other cells in the same row, column, and box. Do not erase it from the whole grid: it may still be possible elsewhere.
Once you place 3 in B, A's notes of 2 and 5 stay as they are. Sharing a row does not make a candidate disappear with every move; remove only the number you just placed, if it was noted in an affected cell.
Do you need to write every candidate?
You can begin with one area that is holding you up. Just distinguish a complete candidate list for a cell from a partial note about one number you are tracking. If you wrote down only 2 for convenience, seeing it alone does not prove it is the only possibility.
A consistent layout makes notes easier to read. Imagine a tiny 3×3 grid inside each cell, with 1 at the top left and 9 at the bottom right. That is not a sudoku rule; it is simply a way to put each candidate in a predictable spot.
If your notes contradict one another, pause and check whether you missed a possibility or erased one that belonged to a different area. Our guide to common beginner sudoku mistakes offers a way to find the error without wiping the whole grid.