The standard nine by nine logic puzzle in four difficulties, with pencil marks and optional mistake checking.
Controls
| Input | Action |
|---|---|
| Click a cell then a number | Place a digit |
| N | Toggle pencil mark mode |
| Backspace | Clear the selected cell |
How to play
Fill the grid so that every row, every column and every three by three box contains the digits one to nine exactly once. Every valid sudoku has exactly one solution, and that solution is always reachable by logic alone — you never need to guess.
Pencil marks let you write small candidate digits into a cell. On anything above easy difficulty they are not optional; trying to hold the candidates in your head is what makes people believe sudoku requires guessing.
Two techniques that cover most puzzles
The first is scanning for a naked single: a cell where eight of the nine digits already appear in its row, column or box, leaving exactly one possibility. Easy and medium puzzles are largely solvable with this alone.
The second is the hidden single, which is the one most people miss. Instead of asking what can go in this cell, ask where can this digit go in this box. Often a digit is excluded from eight of the nine cells by its row and column, leaving one home even though that cell has several candidates of its own. Learning to scan by digit rather than by cell is the single biggest jump in sudoku ability.
Frequently asked questions
Is guessing ever required?
No. A properly constructed sudoku with a unique solution can always be completed by logic. If you feel stuck enough to guess, there is a hidden single or a locked candidate you have not spotted.
Should I use the mistake checker?
While learning, yes — it tells you immediately when a deduction was wrong. Once you are comfortable, turn it off, because knowing an error will be caught changes how carefully you reason.