Sudoku solving techniques, from easy to expert
Learn the logic behind solving Sudoku: naked singles, hidden singles, locked candidates and when backtracking takes over.
Naked singles
A naked single is an empty cell whose row, column and box already use eight of the nine digits, leaving only one possible value. Filling these is the fastest way to make progress and is often all an easy puzzle needs.
Hidden singles
A hidden single is a digit that can legally go in only one cell within a unit, even if that cell still has other candidates. Scanning each row, column and box for a digit that fits in just one spot unlocks most medium puzzles.
Locked candidates
When a digit's only spots inside a box all fall on one row or column, that digit can be eliminated from the rest of that row or column. This pointing-and-claiming step is the bridge into harder puzzles.
When backtracking takes over
Expert grids reach a point where pure logic stalls and you must guess, then unwind the guess if it fails. The solver here automates that search efficiently, always trying the most constrained cell first.