Puzzle guide · By Platon Games
How to play Sudoku: rules and a first logical move
Learn the three placement rules, work through one justified move and build a simple routine for your next classic Sudoku puzzle.
The three rules of classic Sudoku
A classic Sudoku has 81 cells arranged in nine rows, nine columns and nine 3 by 3 boxes. Fill every empty cell with a number from 1 to 9, keeping the starting numbers unchanged.
- Each row contains 1 to 9 once. A number cannot appear twice in the same horizontal row.
- Each column contains 1 to 9 once. Apply the same rule vertically.
- Each 3 by 3 box contains 1 to 9 once. The heavier grid lines mark these nine boxes.
The three rules apply together. A number that fits a row can still be wrong because it duplicates a number in the column or box. There is no adding or multiplying: the numbers identify nine distinct symbols.
A worked example: why this cell must be 5
Use the actual starting grid below. Look at the cell marked ?, in row 4, column 6. Start by asking which numbers its row, column and box still allow.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
|---|---|---|---|---|---|---|---|---|---|
| 1 | · | · | · | · | · | · | · | · | 1 |
| 2 | · | 4 | 6 | · | · | · | · | · | 8 |
| 3 | · | 8 | 7 | 9 | · | · | · | 2 | 4 |
| 4 | 2 | · | · | 8 | · | ? | 9 | 7 | 6 |
| 5 | · | · | · | · | 7 | 1 | 8 | · | · |
| 6 | 4 | · | · | 3 | · | · | · | 1 | · |
| 7 | 5 | · | · | · | · | 3 | 2 | · | · |
| 8 | · | · | · | · | · | 4 | · | · | · |
| 9 | · | 1 | 3 | · | · | · | · | · | · |
- 1. Check row 4
- The row already contains 2, 8, 9, 7 and 6. Its missing numbers are 1, 3, 4 and 5.
- 2. Check column 6
- This column already contains 1, 3 and 4. Cross those out of the row's possibilities. Only 5 remains.
- 3. Check the middle box
- The box spanning rows 4–6 and columns 4–6 contains 8, 7, 1 and 3. It does not contain 5, so 5 also fits the box.
Conclusion: row 4, column 6 is 5. Every other number is excluded by the row or column, and 5 fits all three units. This is a naked single: a cell with just one remaining candidate.
You have justified one move, rather than guessed a number. After placing it, check the same row, column and box again: the new information may make another move available. This starting grid comes from Platon Sudoku's existing puzzle content.
How pencil notes help
Pencil notes list the numbers that are still possible in an empty cell. For the worked cell, checking the row gives the temporary list 1, 3, 4, 5; checking the column reduces it to 5. A note is a possibility, not a placed number.
Keep notes current. Whenever you place a number, remove that number from other empty cells in its row, column and box. Stale notes can hide the next single. Automatic notes can save copying, but you still need to understand why a candidate is removed.
In Platon Sudoku for Android, editable auto notes are free. Smart Hints can explain a logical step; free hint balances and daily rewards apply, while unlimited Smart Hints are an optional Premium benefit.
A simple routine for your next puzzle
- Start with a crowded row, column or box. Fewer missing numbers give you a shorter list to check.
- Cross-check before placing. Every placed number must fit the cell's row, column and box together.
- Look for a single candidate. If a cell has only one possibility, you have a reason to place it.
- Look for a number with one possible home. A row, column or box may have only one legal cell for a missing number. This is often called a hidden single.
- Update notes and repeat. If you get stuck, recheck a different area or use an explanation. Harder puzzles can require techniques beyond these first steps.
Keep confirmed numbers separate from tentative notes. If a move needs several assumptions, pause and look for a smaller deduction you can explain. The next useful step is not necessarily in the area you last worked on.
Common beginner questions
Is Sudoku a maths test?
No arithmetic is needed for classic Sudoku. You use the three placement rules and logical elimination; the number values do not need to be added together.
Does an empty cell need all nine pencil notes?
No. Start with the numbers not already present in its row, column or box. You can note candidates only where they help, as long as you distinguish an incomplete note list from a fully checked one.
Can two numbers both fit a cell?
Yes, at an intermediate stage. Do not choose between them without another reason. A later placement elsewhere may remove one of the candidates.
Can I play Sudoku offline?
The rules and the worked example above can be followed on paper. Platon Sudoku's core puzzles, hints and replay also work offline; ads, purchases and some privacy services can need a connection.