The Scanning Technique: Your Most Useful Sudoku Skill
Sudoku a Day Blog
If you learn one Sudoku technique well, make it scanning. It is the foundation of everything else, and it works at every difficulty level from easy to master.
What scanning actually means
Scanning is the process of looking across rows, columns, and boxes to find where a specific number must go. Instead of staring at one empty cell and wondering what fits, you pick a number and ask: where can this number still be placed?
The logic is simple. If a row already contains a 7, no other cell in that row can be 7. Same for columns and boxes. By checking all three constraints, you can often narrow a number down to a single possible cell.
How to scan effectively
Most beginners scan randomly, jumping between numbers and areas. That works, but it is slow. A systematic approach is faster:
1. Start with the most common numbers. Look at the grid and find which digit appears most often. If there are already six 3s placed, finding the remaining three is easier than finding a number that only appears twice.
2. Scan one number at a time. Pick 1 and check every row, column, and box. Where can 1 still go? If you find a placement, make it and move on. Then do the same for 2, then 3, and so on.
3. Focus on nearly complete units. A row with seven numbers filled in only has two gaps. A box with eight numbers has one. These are the easiest wins.
4. After each placement, re-scan nearby. Every number you place removes a candidate from its row, column, and box. That often creates new singles in nearby cells.
Scanning in action
Imagine row 4 has digits 1, 2, 4, 5, 7, 8, and 9 already placed. The missing numbers are 3 and 6. Now check the columns of the two empty cells. If one column already has a 3, the other cell must hold the 3, and the first gets the 6.
That is scanning. No advanced technique needed, just careful observation.
When scanning is not enough
On harder puzzles, scanning alone will not solve everything. You will reach a point where no number has an obvious single placement. That is when you need techniques like hidden singles and naked pairs.
But even expert solvers start every puzzle with scanning. It is always the first pass, and it typically solves 30 to 50 percent of easy and medium grids before any advanced work is needed.
Practice tip
Next time you solve a puzzle, try scanning all nine numbers in order (1 through 9) before doing anything else. You will be surprised how many cells you can fill with just this one technique.
Scanning across difficulty levels
On easy puzzles, scanning alone can solve 60 to 80 percent of the grid. On medium, it handles about 30 to 50 percent before you need additional techniques. On hard and above, scanning is your opening pass: it clears the low-hanging fruit so you can focus your advanced techniques on the genuinely tricky cells.
No matter what level you play, scanning is always step one. Master it, and everything else becomes easier.
A common scanning mistake
One thing to watch for: do not just scan rows. Beginners tend to check rows and forget columns, or check both but skip the box. All three constraints matter equally. Get in the habit of checking row, column, and box for every candidate, every time. It takes an extra second per check and prevents the majority of early-game errors.
Build the scanning habit now, and every future technique will be easier to learn. Try it on today's daily puzzle, or grab a printable easy pack for focused practice.
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