Learn Sudoku
How to Play Sudoku: A Step-by-Step Beginner Guide
Sudoku is a logic puzzle, not a math test. You never add, subtract, or calculate. Your goal is to place the digits 1 through 9 so that every row, every column, and every 3Ă—3 box contains each digit exactly once.
The three Sudoku rules
- Each row must contain the digits 1–9 with no repeats.
- Each column must contain the digits 1–9 with no repeats.
- Each 3×3 box must contain the digits 1–9 with no repeats.
The numbers already printed in the puzzle are called givens. They never move. Every number you add must obey all three rules at the same time.
Example: one missing number in a row
This row already contains eight different digits. The only missing digit is 6, so the outlined cell must be 6.
This is a naked single: only one candidate can fit the cell.
How to make your first move
Start with a row, column, or box that already has many givens. List its missing digits. For each empty cell, eliminate any missing digit that already appears in the cell’s row, column, or box. If only one candidate survives, place it.
- Choose a crowded row, column, or box.
- Identify which digits are missing.
- Check each empty cell against its other two units.
- Place a digit only when the logic forces it.
- Scan again—every placement changes the puzzle.
Hidden singles: one place for a digit
Sometimes a cell can hold several candidates, but a particular digit has only one possible location in a row, column, or box. That digit is a hidden single. The answer is “hidden” because the cell itself does not have just one candidate; the whole unit forces the placement.
Example: candidate 7 has one home
Imagine three empty cells in the same 3×3 box. Rows and columns rule out 7 in the first two cells. Even if the third cell could also contain 2 or 5, it is the only place left for 7—so it must be 7.
When and how to use candidate notes
If no digit is immediately forced, use Notes mode to record the digits that could legally fit each empty cell. Good notes show possibilities, not guesses. When a nearby number is placed, remove that candidate from cells sharing the same row, column, or box.
For example, if a cell can be 2 or 7, write both candidates. If another 7 is later placed in its row, 7 disappears from the note and the cell becomes a naked single: 2.
A beginner solving routine that works
- Scan the boxes. Look for a missing digit with only one possible cell.
- Scan rows and columns. Find units with many filled cells.
- Fill naked and hidden singles. Avoid more complicated techniques until these are exhausted.
- Add candidates. Record every legal possibility in unresolved cells.
- Repeat after every placement. A new number often creates a chain of easy moves elsewhere.
Common beginner mistakes
- Checking only the row. Every placement must also fit the column and box.
- Guessing when stuck. A well-made Sudoku has a logical next step; update candidates and scan again.
- Forgetting to update notes. Old candidates hide singles and create false patterns.
- Staring at one area too long. Switch from boxes to rows, columns, or one digit at a time.
How do you know the puzzle is complete?
A solved grid has no empty cells and no repeated digit in any row, column, or box. If every unit contains 1 through 9 exactly once, the puzzle is complete. On CalmSudoku, a correct completion automatically opens the result screen.
Practice your first moves
Start with Easy Sudoku and use Notes whenever more than one candidate remains.
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