Scanning

Scanning is the most fundamental Sudoku technique. It's the first thing you should do when approaching any puzzle.

What is Scanning?

Scanning means systematically looking at rows, columns, and boxes to find where a specific number can be placed.

How to Scan

Pick a Number

Start with a number that appears frequently in the givens. If you see five 7s already placed, scanning for the remaining 7s is easier than scanning for a number that appears only twice.

Scan Rows

For your chosen number:

  1. Look at each row
  2. Is the number already present? Skip that row.
  3. If not, which cells in the row could hold it?
  4. Check each candidate cell's column and box
  5. If only one cell works, place the number

Scan Columns

Same process, but vertically:

  1. Look at each column
  2. Find columns missing your number
  3. Check which cells are valid
  4. Place when only one option remains

Scan Boxes

For each 3×3 box:

  1. Is the number present? Skip if yes.
  2. Which cells are empty?
  3. Check row and column constraints for each
  4. Place when you find the answer

Cross-Hatching

A powerful scanning variation:

  1. Pick a number
  2. Look at a box missing that number
  3. Draw imaginary lines through the box from all instances of that number in crossing rows and columns
  4. The remaining cells are candidates
  5. Often only one cell survives

Scanning Order

Some solvers go 1-9 in order. Others:

  • Start with the most common number on the grid
  • Scan numbers that complete a row/column/box
  • Focus on crowded areas first

Experiment to find your rhythm.

When Scanning Isn't Enough

Scanning alone solves Easy puzzles and starts Medium ones. When scanning produces no results:

  • It's time for candidates/notes
  • Look for hidden singles
  • Apply intermediate techniques

Don't force scans—if nothing's obvious, move to other techniques.

Practice Drill

Load an Easy puzzle and try this:

  1. Scan for all 1s, place what you can
  2. Scan for all 2s, place what you can
  3. Continue through 9
  4. Repeat until solved

Time yourself. With practice, scanning becomes automatic.