Scanning Technique
Systematically eliminate where a digit can go by checking rows and columns — the essential first step in every puzzle.
What is Scanning Technique?
Scanning is the most fundamental Sudoku technique and the starting point for every solve session. It involves looking at where a digit already appears on the board and eliminating the rows and columns where that digit cannot appear again. By crossing out (eliminating) every row and column that already contains the target digit, you reduce the candidate cells within any given 3×3 box. When only one cell in a box remains uncrossed, the digit goes there. This visual cross-hatching process is fast, requires no pencil marks, and can often solve a significant portion of an easy puzzle in seconds. Scanning teaches solvers to think in rows, columns, and boxes simultaneously — a tripartite view of the grid that underpins every other Sudoku technique. Even expert solvers return to scanning after each digit placement, treating it as an automatic reflex rather than a deliberate technique. The mental model of "crossing out" rows and columns with an imaginary grid of lines is a powerful visual shortcut that bypasses complex arithmetic and works directly on the board's spatial structure.
When to Use Scanning Technique
Use scanning at the very start of every puzzle and after every single digit placement throughout the solve. It is especially powerful for digits that already appear 6 or more times on the board, as they have very few remaining placements, making forced cells easy to find by elimination. A good scanning workflow is to cycle through digits 1–9 at the start of the puzzle: for each digit, check all nine boxes and see if any box has only one possible cell. If yes, place the digit. After each placement, immediately re-scan all affected regions. As the board fills up and scanning yields fewer direct placements, gradually escalate to Hidden Singles, Naked Pairs, and so on. The transition from pure scanning to candidate-based techniques is a natural progression that most solvers experience organically.
How to Apply Scanning Technique — Step by Step
- 1
Pick a high-frequency digit
Choose a digit that already appears frequently on the board — ideally 5 or more times. The more times a digit appears, the more rows and columns are already "crossed out" for it, leaving fewer possible positions and making forced placements easier to spot.
- 2
Mark all existing positions
Locate every cell on the board that already contains your chosen digit. Mentally (or physically) note the rows and columns of each occurrence. You are building a picture of all the "forbidden zones" for this digit across the entire grid.
- 3
Apply cross-hatching to each empty box
For each of the nine 3×3 boxes that does not yet contain your digit, draw imaginary lines through all the rows and columns that already contain it. Any cell within the box that lies in a crossed-out row or column is eliminated as a candidate. Only cells in uncrossed rows AND uncrossed columns remain valid.
- 4
Identify forced placements
If only one cell in a box survives after cross-hatching (i.e., all other cells were eliminated by row or column constraints), that cell must contain the digit. Place it immediately and re-scan all units that share the row, column, and box of the newly placed digit for further forced placements.
💡 Pro Tip
Use "cross-hatching" — draw imaginary lines through rows and columns containing the digit. Where those lines intersect a box, only the uncrossed cells remain as candidates. For a powerful mental shortcut: after identifying which rows are forbidden (contain the digit), check how many rows in a band (group of three horizontal boxes) are forbidden. If two out of three rows in a band are crossed out, the digit is confined to the third row in all three boxes of that band — this is called "band scanning" and can reveal hidden singles across multiple boxes simultaneously.
Practice Scanning Technique Now
Put this technique to the test on a live puzzle. The Practice Mode lets you work through real examples with candidate marking.