Beginner
Last Possible Number
A core Sudoku solving technique for beginner players
What is the Last Possible Number technique?
The Last Possible Number technique focuses on a single empty cell. By looking at all digits already placed in the cell's row, column, and 3×3 box, you can often eliminate candidates until only one digit remains possible. That digit must be the answer.
This is also called cross-referencing or cross-hatching. It is distinct from Obvious Singles only in emphasis: here, you start with the cell; in Obvious Singles, you write candidates first and then look for cells with only one mark.
Step-by-step guide
- Select an empty cell.
- List the nine digits: 1 2 3 4 5 6 7 8 9.
- Scan the cell's row - Cross off every digit already present.
- Scan the cell's column - Cross off every digit already present.
- Scan the cell's 3×3 box - Cross off every digit already present.
- If exactly one digit remains, place it in the cell.
Number of candidates remaining by difficulty
| Difficulty | Avg candidates per cell | Primary technique needed |
|---|---|---|
| Easy | 1–2 | Last Possible Number, Last Free Cell |
| Medium | 2–4 | Obvious Singles, Hidden Singles |
| Hard | 3–5 | Obvious Pairs, Hidden Pairs |
| Expert+ | 4–6 | X-Wing, Swordfish |
Pro tip: When Last Possible Number doesn't find a single answer immediately, write down the remaining candidates as pencil marks. These notes become the foundation for more advanced techniques.