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Scanning in Sudoku

Learn how to scan a Sudoku grid systematically to find missing digits, Singles, and useful candidate restrictions.

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Scanning is the habit of inspecting a Sudoku grid with a specific target instead of staring at all 81 cells at once.

You can scan:

  • a nearly complete row, column, or box;
  • one digit across several houses;
  • one promising empty cell;
  • the houses changed by your last move;
  • one candidate after an elimination.

Scanning is usually a search method, not the final deduction. It helps you discover deductions such as a Full House, Naked Single, Hidden Single, or later a candidate restriction.

Sudoku visualScanning is a family of search modes
Row
5
3
7
6
1
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Box
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One digit
5
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Peers
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Why scanning works

Every unsolved cell belongs to three houses:

  • one row;
  • one column;
  • one 3×3 box.

A digit can be placed only if it is legal in all three.

At the same time, each placed copy of a digit restricts where that same digit can appear elsewhere in related rows, columns, and boxes.

Scanning works because you repeatedly look at those overlapping constraints from a limited viewpoint. Instead of asking the impossible question “what can I solve anywhere?”, you ask a smaller question such as:

What is missing from this row?

or:

Where can digit 6 go in this box?

or:

What candidates remain for this cell?

The logic is basic. The skill is choosing the next useful viewpoint.

Scan method 1: nearly complete houses

Start with rows, columns, or boxes that contain many solved digits.

Suppose a row contains:

1 2 3 4 5 6 7 . 9

Only digit 8 is missing, so the empty cell must be 8.

That is the simplest unit-first placement: a Full House / last missing digit.

With two or three empty cells, the same scan is still useful. List the missing digits, then use the crossing columns and boxes to decide which cells can accept them.

Scan method 2: one digit at a time

Choose one digit — for example 5 — and locate every solved 5 in the grid.

Now ask where 5 is still missing.

In each target box or line:

  1. mark mentally which positions are blocked by existing 5s;
  2. ignore cells that are already solved;
  3. look for a house where only one legal position for 5 remains.

If one position remains, you have found a Hidden Single for 5.

Digit-first scanning is powerful because Sudoku requires every digit to appear once in every row, column, and box. You are following one repeated constraint through the whole grid instead of considering all nine digits simultaneously.

Scan method 3: one box, one digit

Choose a box and one digit that the box still needs.

Use solved copies of that digit in crossing rows and columns to rule out positions inside the box.

If only one cell survives, the digit is forced there.

This focused box-and-digit method is commonly taught as crosshatching.

Techniques · BeginnerCrosshatching in SudokuLearn how crosshatching works in Sudoku by using solved digits across rows and columns to restrict positions inside a box.

Crosshatching is therefore a specialised form of scanning. The search method is crosshatching; the resulting proof is normally a Hidden Single.

Scan method 4: promising cells

Scanning can also be cell-first.

Choose an empty cell whose row, column, and box already contain many different digits. Then ask which digits are absent from all of its peers.

If eight digits are blocked and only one legal digit remains, the cell is a Naked Single.

This viewpoint is particularly useful when:

  • the cell sits in a dense part of the puzzle;
  • you are solving on paper without full candidates;
  • a recent placement changed several nearby possibilities.

Cell-first scanning and digit-first scanning complement each other. One asks what can this cell contain?; the other asks where can this digit go?

Scan method 5: re-scan what just changed

After every placement or candidate elimination, three houses have changed immediately:

  • the cell's row;
  • its column;
  • its box.

Those houses should normally be your first re-scan targets.

A new solved digit can:

  • complete a house;
  • reduce another cell to one candidate;
  • leave one position for the same digit in a neighbouring house;
  • create a Locked Candidates restriction;
  • start a cascade of Singles.

This local re-scan is one of the easiest ways to avoid missing simple progress after a harder step.

A practical scanning order

For a beginner or an easy puzzle, use a repeatable order rather than random visual searching.

  1. Check houses with one empty cell.
  2. Check other nearly complete boxes, rows, and columns.
  3. Scan one digit through the boxes.
  4. Repeat for other well-placed digits.
  5. Inspect promising cells for Naked Singles.
  6. Re-scan the houses affected by every placement.
  7. If placements stop, add or update candidates and continue with elimination techniques.

You do not have to follow this order forever. It is a training structure for building reliable visual habits.

Experienced solvers move between these views more fluidly because they recognise which part of the grid currently contains the most information.

What is crosshatching?

Crosshatching is a specific digit-first scanning method.

You focus on one digit and use existing copies in rows and columns to block positions inside a box. If one position remains, that position is a Hidden Single for the target digit.

The dedicated Crosshatching Guide explains bands, stacks, blockers, and the point where the same observation stops giving a direct placement and begins to support Locked Candidates.

Scanning vs candidates

Scanning and candidate notation solve different problems.

Scanning helps you search the visible structure efficiently.

Candidates make unresolved possibilities explicit when the visible structure alone is no longer enough.

Early in a puzzle, filling every candidate immediately can create more visual noise than value. Many placements are easier to see from solved digits and nearly complete houses.

Later, candidates become essential because techniques such as subsets, Fish, Wings, Coloring, and Chains depend on exact possibility information.

A useful progression is:

scan → place Singles → add/update candidates when needed → eliminate → scan again

not:

write every candidate once → stop looking at the large structure of the grid.

Scanning after candidate eliminations

Scanning remains useful after beginner solving ends.

Suppose an intermediate technique removes candidate 7 from two cells. That elimination may leave:

  • one cell with only {7} — a Naked Single;
  • one remaining position for 7 in a column — a Hidden Single;
  • 7 confined to one row inside a box — a Locked Candidates pattern.

The elimination itself may have been advanced, but the next useful observation can be simple.

This is why a strong solving loop always returns to scanning after progress.

Scanning on paper vs digital Sudoku

The underlying logic is identical, but the interface changes what is easy to notice.

On paper

Solved digits dominate the visual field, so nearly complete houses, digit scans, and crosshatching can be especially natural. Full candidate notation may be delayed until the grid becomes less transparent.

With automatic candidates

Naked Singles become visually obvious because one-candidate cells stand out. Hidden Singles can sometimes be harder to notice because the grid contains many small candidate marks.

Candidate filters can restore a digit-first view by showing one number at a time.

The interface changes recognition, not the proof.

Common scanning mistakes

Scanning without a target

Move with a purpose: one house, one digit, one cell, or one recently changed area. Unstructured staring creates fatigue without guaranteeing coverage.

Ignoring one house type

A cell that looks legal from its row may be blocked by its column or box. Always remember that the three constraints overlap.

Not re-scanning after a placement

Every solved digit changes three houses and may unlock several simple moves immediately.

Looking only for placements

As the puzzle becomes harder, a scan may reveal a useful restriction rather than a solved cell. A digit confined to two cells on one line inside a box is not placeable yet, but the information may support Locked Candidates.

Moving too quickly through digits

If you choose digit 7, actually inspect the relevant houses for 7 before jumping to 8. A systematic pass is more reliable than a visual skim.

Treating a failed scan as proof that no move exists

Scanning is only one search method. If one viewpoint yields nothing, change viewpoint: cell-first, digit-first, house-first, or candidate-based.

FAQ

Is scanning a formal Sudoku technique?

The term is often used broadly for systematic visual searching rather than one single deduction rule. In VeyraPlay Guides, scanning is a foundation search skill that helps you discover valid deductions.

Can scanning solve a whole Sudoku?

Many Easy puzzles can be solved largely through repeated scanning, Full Houses, and Singles. Harder puzzles eventually require explicit candidate eliminations and more advanced techniques.

What should I scan first?

Start with houses that are nearly complete. Then try one digit across several boxes, followed by promising cells and the houses changed by your last move.

Do I need pencil marks for scanning?

No. Basic scanning works directly from solved digits. Candidates become more useful when the puzzle stops yielding visible placements or when you need to prove eliminations.

Is scanning the same as crosshatching?

No. Crosshatching is one focused form of scanning: one digit is traced through rows and columns to restrict its positions inside a box.

What to learn next

Learn Crosshatching for a focused digit-scanning method, then study Naked Singles and Hidden Singles so you can distinguish the two fundamental forced-placement proofs your scans reveal.

Techniques · BeginnerCrosshatching in SudokuLearn how crosshatching works in Sudoku by using solved digits across rows and columns to restrict positions inside a box.Techniques · BeginnerNaked SingleLearn what a Naked Single is, why one remaining candidate forces a placement, and how to find Naked Singles with or without pencil marks.Techniques · BeginnerHidden SingleLearn what a Hidden Single is, how it differs from a Naked Single, and how to find a digit with only one legal position in a row, column or box.Strategies · BeginnerBeginner Sudoku StrategiesLearn beginner Sudoku strategies using last missing digits, scanning, Naked Singles, Hidden Singles, simple candidate notes and a repeatable solving order.
Keep exploring

Related Sudoku Guides

Fundamentals · BeginnerHow to Solve SudokuLearn a reliable Sudoku solving process: scan the grid, place forced digits, manage candidates and use techniques only when needed.Fundamentals · BeginnerHow to Play SudokuLearn how to play Sudoku from your first scan to your first placements, with a simple beginner workflow for rows, columns and boxes.Techniques · BeginnerFull House / Last DigitLearn Full House, also called the Last Digit technique in Sudoku: how to find one missing digit in a row, column, or box and place it with certainty.Techniques · IntermediateLocked CandidatesLearn Locked Candidates in Sudoku, including Pointing and Claiming, and use box-line intersections to make logical candidate eliminations.Techniques · IntermediatePointing PairsLearn Pointing Pairs and Pointing Candidates: when a digit is locked to one line inside a box and can be removed elsewhere on that line.Techniques · IntermediateBox-Line ReductionLearn Box-Line Reduction, also called Claiming, and use a row or column that is confined to one box to eliminate candidates inside it.