Last Free Cell
The Last Free Cell technique is the most fundamental solving method in sudoku. It comes into play when a row, column or 3×3 box has all but one cell filled, and the missing digit is forced by the basic rules of the sudoku puzzle. This technique requires no guesswork — it is pure logical deduction.
What is Last Free Cell?
In a standard 9×9 sudoku grid, each row, each column and each 3×3 box must contain every digit from 1 to 9 exactly once. When you look at any of these 27 houses and see that eight cells already hold distinct digits, the ninth cell — the last free cell — has only one possible value. That value is simply the digit that has not yet appeared in that house.
How to spot it
Scan your sudoku grid row by row, column by column and box by box. Focus on almost-complete houses — those with seven or eight digits already filled in. Count the digits already present; the missing digit goes into the only empty cell. Easy sudoku puzzles often rely heavily on this technique, but it remains useful at every difficulty level. Even the hardest sudoku puzzles will eventually present last free cells after you apply more advanced strategies.
Why it works
Every sudoku solver starts with the Last Free Cell technique. It builds confidence because each placement is 100% certain — there is no ambiguity. Mastering this technique trains your eye to scan the sudoku board efficiently. As you progress to medium and hard sudoku, you will combine Last Free Cell with candidate notes, naked singles and cross-elimination patterns. The habit of scanning houses for completeness stays with you throughout your entire sudoku solving journey, making it the single most important foundation skill.