Sudoku help

Sudoku strategy guide

A practical guide to solving Sudoku boards logically, using notes well and avoiding random guesses.

Start with certainty

The safest Sudoku method is to begin with numbers you can prove. Look at each row, column and 3x3 box and ask where a digit can legally fit. If a digit has only one possible square, place it. If there are several possibilities, make a note and keep moving.

Early progress often comes from repeated scanning. Pick one number that already appears often on the board and search every box for missing copies. Then repeat with the next common number.

Use notes as evidence

Notes are not clutter when they are used carefully. They are a record of what is still possible. When you remove a candidate, do it because another placed number or box restriction proves that it cannot remain.

If notes become messy, pause and re-check one section of the grid instead of guessing. A clean row or box can often unlock the next step.

Techniques by difficulty

Easy

Use singles, row scans and box scans. Most values can be found without maintaining a full candidate list.

Medium

Look for locked candidates and pairs. If a number must sit in one row within a box, it can often be removed from the rest of that row.

Hard and above

Expect slower progress. Track candidates, compare overlapping boxes and be willing to revisit earlier assumptions when a new value is placed.

When to use a hint

A hint is most useful when it teaches you what you missed. After revealing a hint, do not simply continue. Ask why that number belongs there. Check its row, column and box until the logic becomes clear.

Common solving traps

  • Placing a number because it “looks right” instead of being proven.
  • Ignoring a box because the row already feels busy.
  • Forgetting that one new number can change several candidate lists.
  • Keeping outdated notes after placing a value.

Worked solving routine

1. Scan rows

Choose a digit, then inspect rows that already contain it. If two nearby rows contain the digit and the third row in the same band does not, the missing position may be restricted to a single box.

2. Scan boxes

For each 3x3 box, list the missing digits. A row or column may block most candidate squares, leaving only one legal location.

3. Re-check after placements

Every placed number removes possibilities from one row, one column and one box. Re-check recently affected areas before moving elsewhere.

Candidate pairs

If two cells in a row can only contain the same two digits, those digits are locked into that pair. Other cells in the row cannot use them. The same idea works in columns and boxes. Pair logic is one of the first techniques that moves a player from easy boards toward medium and hard boards.

The key is not to overuse pairs. Only treat something as a pair when both cells truly have the same two options and no other options remain.

Keeping the grid readable

Too many notes can hide the actual solve. Add candidates when the puzzle slows down, then clean them whenever a confirmed number is placed. If a candidate was eliminated by a new value, remove it everywhere affected.

A readable grid makes it easier to spot singles, pairs and locked candidates without scanning the same uncertainty repeatedly.