How to play Sudoku

A Sudoku is a 9×9 grid, divided into nine 3×3 boxes. Some cells already contain a number (the givens); the rest are empty. The goal is to fill every empty cell with a number from 1 to 9.

A candidate is a number that could still go in an empty cell, based on what you already know about the rest of the board. In Sudoku Rivals you can turn on Notes mode and pencil in several candidates in the same cell instead of a final value — very useful on Medium and Hard, where the obvious moves run out early.

A well-formed Sudoku always has exactly one possible solution, reachable purely by logic — you never need to guess or try your luck. If you think you need to guess, you've probably missed a technique that actually solves it; you can review them in Sudoku Techniques.

1. Look first for cells with a single possible candidate (Naked Single) and place them.
2. Check each row, column, and box for a number that can only go in one cell (Hidden Single), even if that cell has other candidates pencilled in.
3. Once the obvious moves run out, turn on Notes mode and pencil in the candidates of the most constrained cells to start using elimination techniques.
4. Repeat: every number placed reduces the candidates in its row, column, and box.

• Guessing a number "because it seems to fit" instead of confirming it with logic.
• Not pencilling in candidates and losing track of which numbers are still possible in each cell.
• Forgetting to check the 3×3 boxes, focusing only on rows and columns.
• Jumping to Hard before mastering Naked/Hidden Single on Easy.

Start with Easy Sudoku to apply these rules step by step, or try the Daily Challenge if you already know the rules and want to compete on time.