When you have eight numbers in a row, column, or 3x3 box, the ninth is obvious. This technique is where everyone starts.
What is it?
A row, column, or 3x3 box in Sudoku must contain all numbers from 1 to 9. When eight of those spots are filled, only one number is missing. That empty cell can only hold that missing number.
Numbers 1-8 are placed. The empty cell must be 8.
How to use it
Look for rows, columns, or boxes that are almost complete. Count which number is missing and fill it in.
- Find a row, column, or box with only one empty cell.
- Count from 1 to 9 and note which number is absent.
- Write that number in the empty cell.
Works for rows and columns too
The same logic applies to full rows and columns. Eight numbers present means the ninth is determined.
This row has 1-7 and 9. The empty spot must be 8.
Quick tips
- Scan each 3x3 box first. They are easier to check at a glance.
- After placing a number, check if it creates another last free cell somewhere else.
- This technique often triggers a chain reaction of easy fills.
Practice makes it automatic
Start with easy puzzles. You will spot last free cells without consciously looking for them after a while. It becomes second nature.