How to Do Sudoku Step by Step: A Complete Walkthrough

Step 1: The Initial Scan for Obvious Placements

Begin every puzzle with a quick scan across all rows, columns, and 3x3 blocks. Your goal is to find the easiest placements, known as Singles. There are two types to look for. A Naked Single is a cell where only one digit is possible because all other eight digits are already present in its row, column, and block combined. For example, if a cell's row contains 2, 4, and 6, its column contains 3, 5, and 7, and its block contains 8 and 9, then the only missing digit is 1, so that cell must be a 1.

Also look for Hidden Singles. This occurs when a digit has only one possible cell left in a specific row, column, or block, even though that cell may have other candidate numbers. Fill in every Single you can find. This first pass often solves several cells in easier puzzles and provides a crucial starting foundation for harder ones.

• Scan systematically: check all rows from top to bottom, then all columns left to right, then all blocks.
• Don't move on to pencil marks until you are confident no more singles are visible from the initial scan.

Step 2: Building Complete Pencil Marks

When the obvious placements dry up, the next critical step is to note all possible candidates for every empty cell. This process is called making pencil marks. For each unsolved cell, mentally check its row, column, and block. Write small numbers (candidates) in the cell for every digit from 1-9 that is not already present in those three units.

Accurate pencil marks are the foundation for all advanced logic. A clean, complete set of candidates transforms the puzzle into a logic problem you can solve visually. Many solving errors originate from incorrect or missed pencil marks, so take your time to be thorough. This step makes the hidden patterns and relationships between cells much clearer.

• Work in batches: fill pencil marks for one block or row at a time to maintain focus.
• If you prefer a cleaner grid, you can use a notation system like center marks (small digits in the cell center) or corner marks (tiny digits in the cell's corners).

Step 3: Applying Intermediate Solving Strategies

With a fully marked grid, you now use logical strategies to eliminate candidates and reveal new placements. Start with simpler pattern-based techniques. Look for 'Pointing Pairs' or 'Pointing Triples': if all candidates for a specific digit within a 3x3 block are aligned in a single row or column, you can eliminate that digit as a candidate from the rest of that row or column outside the block.

Next, examine interactions between cells. A common strategy is looking for 'Naked Pairs' or 'Naked Triples'—two or three cells in the same unit (row, column, or block) that share the exact same two or three candidates. When you find this, you can erase those candidates from all other cells in that unit. These eliminations will often create new Naked Singles or Hidden Singles, allowing you to fill in more numbers. The cycle continues: after each placement, update your pencil marks and scan for the next logical step.

Step 4: Final Cleanup and Verification

As you apply intermediate techniques, the puzzle will gradually resolve. The final stage involves cleaning up the last few stubborn cells. At this point, your pencil marks should be sparse. Look for any last Hidden Singles that may have appeared or small patterns like a final Naked Pair. Often, the last few digits fall into place quickly once the grid is mostly full.

After you place the last digit, perform a final verification. Check each row, column, and block to ensure it contains the digits 1 through 9 exactly once. This quick check confirms your logical process was flawless and catches any simple transcription errors.

• If you're completely stuck, walk away for a few minutes. A fresh look often reveals a pattern you missed.
• Double-check your most recent pencil mark updates. A single incorrect elimination can block progress.