Sudoku Tips & Strategies

Naked Singles

The most basic technique. When a cell has only one possible candidate number (all other numbers already appear in its row, column, or box), that candidate must be the answer. Scan each empty cell and count which numbers are already used in its row, column, and box. If only one number remains possible, fill it in.

Hidden Singles

A hidden single occurs when a number can only go in one cell within a row, column, or box — even if that cell has other candidates. For example, if the number 7 can only fit in one cell in a particular row (even though that cell could also hold 3 or 5), then 7 must go there. Always check each number 1–9 within each unit.

Scanning Technique

Scanning is the fastest way to find easy placements. It involves looking at rows and columns to eliminate possibilities within 3×3 boxes:

• Cross-hatching: Pick a number (e.g., 5). Look at which rows and columns already contain that number. In each 3×3 box missing that number, the intersecting constraints often leave only one possible cell.
• Row/Column scanning: For each row or column, identify which numbers are missing and check where each can go.
• Start with numbers that appear most frequently on the board — they have fewer remaining placements.

Using Pencil Marks Effectively

Pencil marks (candidate notation) become essential for medium and harder puzzles:

• Fill in all candidates for every empty cell. Then systematically eliminate candidates using the constraints.
• When a candidate is eliminated from a cell, check if any related cells now have naked or hidden singles.
• Keep pencil marks updated as you fill in cells — remove the placed number from all cells in the same row, column, and box.

Advanced Techniques

For hard and expert puzzles, you'll need these techniques:

• Pointing pairs/triples: When candidates for a number in a box are all in the same row or column, that number can be eliminated from other cells in that row/column outside the box.
• Naked pairs/triples: When two cells in a unit have the same two candidates, those numbers can be eliminated from all other cells in that unit.
• Box/line reduction: When a number in a row/column is confined to one box, eliminate it from other cells in that box.

Common Mistakes to Avoid

1. Guessing instead of using logic. Every Sudoku can be solved with pure deduction — if you're guessing, you're missing a technique.
2. Forgetting to update pencil marks after placing a number. Stale candidates lead to errors.
3. Focusing only on one area of the grid. Regularly scan the entire board for new opportunities.
4. Not starting with the easiest placements. Always look for naked and hidden singles before trying advanced techniques.