Step 1: Understand the Rules
Sudoku has exactly three rules. Every valid puzzle, every grid size, follows them without exception:
- Every row must contain each number from 1 to 9 exactly once — no repeats, no blanks.
- Every column must contain each number from 1 to 9 exactly once.
- Every 3×3 box (the nine sub-grids outlined in bold) must contain each number from 1 to 9 exactly once.
Step 2: Start With the Most Filled Rows, Columns or Boxes
Look for any row, column or box that already has 7 or 8 numbers placed. With so few empty cells, it's easy to determine what's missing by simple elimination. This is always your fastest entry point into any puzzle.
Example: if a row contains 1, 2, 3, 4, 5, 6, 7, 8, _ — the missing number is clearly 9. Fill it in immediately.
Step 3: Use Naked Singles
A "naked single" is the most fundamental solving technique. When a cell has only one possible valid number — because all other numbers already appear in its row, column or box — that number must go there.
How to find them: For each empty cell, list which numbers already appear in the same row, same column, and same box. If only one number from 1–9 is not yet used in any of those three, it's a naked single.
Example: A cell is in row 3, column 5, box 5 (centre box). Row 3 contains 1,2,4,5,6,7,8,9. Column 5 contains 3. Box 5 has no additional new numbers. The only unused number in row 3 is 3 — and that's the answer for this cell.
Step 4: Use Hidden Singles
A "hidden single" is when a number can only go in one cell within a row, column or box — even if that cell still has multiple possible candidates. The number is "hidden" among other candidates.
How to find them: For each number (1–9), scan each row to see if there's only one cell it could go in. Then scan columns, then boxes. When a number has only one valid position in a unit, place it.
Step 5: Enable Notes (Pencil Marks)
When simple elimination runs out, it's time to use pencil marks — small candidate numbers written in each empty cell. In SudokuMaster, click the Notes button or press N on your keyboard.
Fill in candidates for every empty cell. Then use these advanced techniques to eliminate candidates:
- Naked pair: Two cells in the same unit share the same two candidates. Remove those candidates from all other cells in that unit.
- Pointing pair: If a number in a box is restricted to one row or column, eliminate it from that row/column outside the box.
- Naked triple: Three cells collectively contain only three candidates. Eliminate those three from the rest of the unit.
Step 6: Advanced Techniques for Hard+ Difficulty
For Hard, Expert, Master and Extreme puzzles, you'll need:
- X-Wing: A number appears in exactly two cells in each of two rows, aligned in the same two columns. Remove that number from those columns elsewhere.
- Swordfish: A three-row version of X-Wing.
- XY-Wing: Three cells form a "pivot" pattern. Eliminates candidates from cells that see both "pincer" cells.
- Coloring: Trace chains of conjugate pairs and use colour logic to eliminate candidates.
Practice What You Learned
Apply these techniques with our free online Sudoku — hints, notes, and undo built in.
Start Easy Puzzle Full Techniques Guide