How to Play Sudoku: Rules, Methods, and Beginner Strategies
How do you play Sudoku?
To play standard Sudoku, fill every empty cell of a 9 by 9 grid with a number from 1 to 9. Each number must appear exactly once in every row, every column, and every 3 by 3 box. The printed numbers are clues and cannot be changed. A valid puzzle has a logical route to one complete solution, so guessing is usually unnecessary.
Sudoku is not an arithmetic test. The numbers could be replaced by nine different symbols without changing the puzzle. What matters is recognizing where a value is allowed. For every empty cell, check its row, column, and box. If eight numbers are already excluded, the remaining number must go there.
The sections below take you from the basic rules to a repeatable solving routine.
1. Learn the three Sudoku rules
A standard grid contains nine rows, nine columns, and nine smaller boxes. Each group must contain 1 through 9 without repetition.
Consider an empty cell in a row that already contains 1, 2, 3, 4, 6, 7, 8, and 9. The missing number is 5, so you can enter 5 immediately. The same reasoning works in a column or box.
A placement is legal only when it satisfies all three groups at once. A 5 may be absent from a row but still be forbidden because the cell's column already contains a 5. Before confirming any entry, scan horizontally, vertically, and inside the surrounding box.
Most browser versions highlight related cells or duplicate entries. These aids are useful, but understanding the rule yourself makes it easier to switch between different interfaces and difficulty levels.
2. Begin with a systematic scan
Do not inspect random cells. Start by scanning every row for missing numbers, then every column, and finally every box. Repeat this cycle whenever you add several values, because each placement creates new information.
There are two basic questions:
- What numbers are missing from this group?
- In which empty cells can each missing number legally fit?
Suppose a box is missing 2, 6, and 8. One empty cell cannot contain 2 because its row has a 2, and it cannot contain 8 because its column has an 8. That cell must be 6. This is called a naked single because only one candidate remains in the cell.
Now imagine that several cells have multiple candidates, but only one cell in the box can accept 8. Place 8 there. This is a hidden single: the answer is forced by the group even though the cell has other apparent possibilities.
Scan boxes early. Their compact shape often makes restrictions easier to see than restrictions across a full row.
3. Use candidate notes carefully
When immediate answers run out, record possible numbers in empty cells. These small pencil marks are called candidates. To build a correct candidate list, remove every number already present in the cell's row, column, or box.
Candidates turn vague uncertainty into visible logic. A cell marked 3, 7 can contain only 3 or 7. If another placement later removes 7, the cell becomes 3.
Avoid filling every cell with nine marks before doing basic scans. Too many notes create visual noise and require more maintenance. Start with constrained areas, difficult cells, or a puzzle's automatic note feature. After placing a number, remove that candidate from all related cells.
Never treat a candidate as a confirmed value. Writing a small 4 means that 4 is possible, not certain. A frequent beginner error is to promote a candidate because it looks promising. Confirm a number only when the alternatives have been eliminated.
4. Find pairs and box-line interactions
Harder puzzles often require relationships between candidates. The most approachable pattern is a naked pair. If exactly two cells in one row contain only the candidates 2 and 7, those cells must eventually hold 2 and 7 in some order. Therefore, 2 and 7 can be removed from every other cell in that row. The same pattern works in columns and boxes.
A hidden pair is similar but viewed from the group. If 4 and 9 can occur in only the same two cells of a box, those two cells must contain 4 and 9. Other candidates can be erased from them.
Another useful technique is the box-line interaction. Suppose every possible 6 in the upper-left box lies in its top row. The box must place 6 somewhere in that row, so no cell outside the box on the same row can contain 6. Conversely, if all candidates for 3 in a row lie inside one box, remove 3 from the other cells of that box.
These eliminations may not place a number immediately. Their purpose is to create a single or another simpler pattern on the next scan.
5. Recover when you get stuck
Being stuck usually means that a useful restriction has been overlooked. It does not mean that you must guess. Pause and perform a clean rescan:
- Check every row for a missing number with only one legal position.
- Check every column in the same way.
- Inspect each box for singles, pairs, and candidates confined to one line.
- Update notes affected by your latest confirmed entries.
- Examine the areas with the most clues and fewest empty cells first.
You can also focus on one digit. Trace every 1 across the grid and identify where another 1 can fit in each box. Then repeat for 2 through 9. Digit scanning often reveals hidden singles that cell-by-cell inspection misses.
If nothing appears, verify your earlier entries. A contradiction such as a cell with no candidates or a duplicate in a group often points back to a mistaken placement. Use undo rather than building more deductions on an uncertain answer. In timed or competitive modes, accuracy is usually faster than repeatedly repairing guesses.
6. Finish and verify the grid
Near the end, the puzzle often accelerates. Each solved cell removes a candidate from its row, column, and box, producing a chain of forced placements. Continue checking all three groups instead of assuming that the remaining blanks are obvious.
A full grid is correct only if every row, column, and box contains 1 through 9 exactly once. Do not rely solely on the absence of empty cells. Check for duplicate values, especially if the game allows mistakes without warning.
For improvement, choose a difficulty that makes you pause without forcing constant guesses. Easy puzzles train scanning and singles. Medium puzzles add pairs and box-line interactions. Difficult puzzles may introduce more advanced candidate patterns, but the same foundation still applies.
Practical Sudoku tips
- Scan in a consistent order so that you do not repeatedly inspect the same area.
- Prioritize groups with many filled cells because they have fewer possibilities.
- Place only values supported by logic, and keep uncertain values as notes.
- Update candidates immediately after every confirmed placement.
- If you stall, switch from scanning cells to scanning one digit across all boxes.
- Use hints as lessons: before accepting one, try to explain why the suggested value is forced.
- Take a short break when the grid becomes visually confusing. Fresh eyes often spot a missed single.
- Increase difficulty gradually and aim for accuracy before speed.
The essential routine is simple: identify missing numbers, eliminate illegal positions, enter forced values, and scan again. With practice, candidate patterns become easier to recognize and even crowded grids feel manageable. All the Sudoku games above run free in your browser without downloads, so you can start with an easy board and practice immediately.