Before you place a number
- Check the row for that digit.
- Check the column for that digit.
- Check the 3 × 3 box for that digit.
GridHearth Game · How to play
How to play Sudoku
Fill the 9 × 9 grid so every row, column, and 3 × 3 box contains digits 1–9 exactly once. Start with the given numbers already in the grid, then use logic—not guessing—to place each new number.
Sudoku rules
Three checks make every move valid
A digit is safe to place only when it does not already appear in the same row, the same column, or the same 3 × 3 box. A completed classic Sudoku uses each digit from 1 through 9 once in every one of those three groups.
Beginner terms
- Given
- A starting number already in the puzzle. It never changes.
- Candidate
- A digit that could legally fit in an empty cell.
- Single
- A cell or digit position with only one legal choice.
- r1c3
- Row 1, column 3. The same shorthand works for any cell.
Starter techniques
Find naked and hidden singles
Both techniques below come directly from the three checks above. A naked single leaves one candidate in a cell. A hidden single leaves one position for a digit within a row, column, or box.
Technique 01
Naked single
In row 1, the only digit missing is 4. After checking the column and box, cell r1c3 has just one legal candidate: 4.
Technique 02
Hidden single
In row 2, digit 6 can go in only one cell: r2c1. That cell could hold other candidates, so the single is hidden in the row rather than exposed in the cell.
These examples demonstrate the two starter techniques taught in the Easy book.
Next level · Medium
Remove candidates before placing a digit
When singles stop, write the legal candidates in the cells you are checking. A candidate elimination is useful only if the pattern is true across the entire row, column, or box involved. After removing candidates, scan for a new single.
Technique 03 · Medium
Pointing (locked candidate)
Look for: every remaining position for a digit in one 3 × 3 box lining up in a single row or column.
Then: That digit must be placed somewhere on that line inside the box. Remove it from the same row or column outside the box.
Pattern cells: r4c5, r4c6. Candidate changes: r4c1.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r4c5Pattern | 25 | 25 |
| r4c6Pattern | 57 | 57 |
| r4c1Change | 1 | 1 |
Removed: 5 from r4c1.
The elimination leaves a one-candidate cell. Placing that digit is the next move, so it is not filled in on these boards.
Technique 04 · Medium
Claiming (locked candidate)
Look for: every remaining position for a digit in one row or column falling in the same 3 × 3 box.
Then: That digit must be placed on the line inside the box. Remove it from the other cells of that box outside the line.
Pattern cells: r7c8, r7c9. Candidate changes: r8c7, r8c9, r9c7.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r7c8Pattern | 45 | 45 |
| r7c9Pattern | 45 | 45 |
| r8c7Change | 2 | 278 |
| r8c9Change | 2 | 278 |
| r9c7Change | 2 | 2 |
Removed: 4 from r8c7; 4 from r8c9; 4 from r9c7.
The elimination leaves a one-candidate cell. Placing that digit is the next move, so it is not filled in on these boards.
Technique 05 · Medium
Naked pair
Look for: two unsolved cells in one row, column, or box that each have exactly the same two candidates.
Then: Those two digits must fill those two cells in some order. Remove both digits from the other cells in that unit.
Pattern cells: r1c3, r2c1. Candidate changes: r1c1, r1c2, r3c1.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r1c3Pattern | 16 | 16 |
| r2c1Pattern | 16 | 16 |
| r1c1Change | 379 | |
| r1c2Change | 9 | |
| r3c1Change | 3 | 379 |
Removed: 1 from r1c1; 6 from r1c1; 6 from r1c2; 6 from r3c1.
The elimination leaves a one-candidate cell. Placing that digit is the next move, so it is not filled in on these boards.
Next level · Hard
Spot triples and X-Wings
Keep using singles, locked candidates, and pairs. The Hard book adds worked naked triples in rows and columns, then row-based and column-based X-Wings. Both patterns eliminate candidates; they do not by themselves guarantee a digit can be placed immediately.
Technique 07 · Hard
Naked triple in a row
Look for: three unsolved cells in one row whose combined candidate set contains exactly three digits.
Then: Those digits occupy the selected cells in some order. Remove them from other cells in that row.
Pattern cells: r9c1, r9c3, r9c5. Candidate changes: r9c4, r9c7.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r9c1Pattern | 369 | 369 |
| r9c3Pattern | 369 | 369 |
| r9c5Pattern | 36 | 36 |
| r9c4Change | 12 | 12 |
| r9c7Change | 124 | 124 |
Removed: 3 from r9c4; 6 from r9c4; 9 from r9c7.
Technique 08 · Hard
Naked triple in a column
Look for: three unsolved cells in one column whose combined candidate set contains exactly three digits.
Then: Those digits occupy the selected cells in some order. Remove them from other cells in that column.
Pattern cells: r1c1, r7c1, r9c1. Candidate changes: r2c1, r3c1, r5c1.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r1c1Pattern | 578 | 578 |
| r7c1Pattern | 578 | 578 |
| r9c1Pattern | 78 | 78 |
| r2c1Change | 24 | 246 |
| r3c1Change | 24 | 246 |
| r5c1Change | 24 | 24 |
Removed: 5 from r2c1; 8 from r2c1; 5 from r3c1; 5 from r5c1.
Technique 09 · Hard
Row-based X-Wing
Look for: two rows where one digit has exactly two candidate positions in each, aligned in the same two columns.
Then: The digit must occupy opposite corners of that rectangle. Remove it from other cells in those two columns outside the selected rows.
Pattern cells: r1c1, r1c7, r5c1, r5c7. Candidate changes: r2c7, r6c1, r8c7, r9c7.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r1c1Pattern | 156 | 156 |
| r1c7Pattern | 156 | 156 |
| r5c1Pattern | 57 | 57 |
| r5c7Pattern | 358 | 358 |
| r2c7Change | 13 | 136 |
| r6c1Change | 1 | 167 |
| r8c7Change | 3 | 3 |
| r9c7Change | 68 |
Removed: 5 from r2c7; 5 from r6c1; 5 from r8c7; 5 from r9c7.
The elimination leaves a one-candidate cell. Placing that digit is the next move, so it is not filled in on these boards.
Technique 10 · Hard
Column-based X-Wing
Look for: two columns where one digit has exactly two candidate positions in each, aligned in the same two rows.
Then: The digit must occupy opposite corners of that rectangle. Remove it from other cells in those two rows outside the selected columns.
Pattern cells: r4c2, r4c7, r9c2, r9c7. Candidate changes: r4c3, r9c1, r9c9.
Pattern cells Before · candidate to remove → After · candidate removed
The grids keep the same placed digits. The after state changes only the selected candidate sets listed below; struck-through digits are removed. These rows are a selection, not every candidate in the puzzle.
| Cell | Before candidates | After candidates |
|---|---|---|
| r4c2Pattern | 79 | 79 |
| r4c7Pattern | 47 | 47 |
| r9c2Pattern | 79 | 79 |
| r9c7Pattern | 47 | 47 |
| r4c3Change | 6 | 69 |
| r9c1Change | 46 | 469 |
| r9c9Change | 6 | 6 |
Removed: 7 from r4c3; 7 from r9c1; 7 from r9c9.
The elimination leaves a one-candidate cell. Placing that digit is the next move, so it is not filled in on these boards.
For shorter reminders after these worked examples, use the quick technique reference.
Easy book method
Start with the Easy five-step checklist
- Scan: look for digits missing from a row, column, and box.
- List: choose an empty cell and write candidates that do not conflict with all three units.
- Single cell: if that cell has one candidate left, place it as a naked single.
- Single place: if a digit has only one legal place in a row, column, or box, place it as a hidden single.
- Update and repeat: check what changed, update candidates, and scan again; if stuck, revisit the examples and check for an incorrect note.
At Medium and Hard, keep scanning and updating candidates after every elimination. Use the next pattern only when the simpler ones do not reveal a move.
Use the controls without losing your place
The game includes Notes, Erase, Undo, Redo, Check, Show a hint, Reveal solution, and Restart. Check marks incorrect entries without changing them. Check and Show a hint mark the attempt as assisted; a hint explains one logical next move and can fill one cell if you choose. Reveal asks for confirmation and ends that attempt. Restart returns to the starting numbers.
Practice an Easy SudokuBeginner questions
Common questions about Sudoku
Do I need mathematics to solve Sudoku?
No. The digits act as symbols. Classic Sudoku is solved by checking which symbols are missing and eliminating impossible positions.
Can the same digit repeat inside a 3 × 3 box?
No. Each 3 × 3 box must contain digits 1–9 exactly once, just like every row and column.
What should I do when I cannot see a move?
Scan each row, column, and box again, list legal candidates for a few empty cells, and look for a naked or hidden single. If neither appears, move through the Medium and Hard patterns above in order.
Last reviewed September 23, 2026.