How to solve sudoku

1. Recognize the three units

In classic sudoku, each cell belongs to a row, a column and a 3 × 3 box. Place digits 1–9 exactly once in each unit. Printed givens stay unchanged. There is no requirement to add rows or put digits in numerical order.

2. Start with an obvious missing digit

Look for a unit with few blanks. A row containing 1, 2, 3, 4, 5, 6, 7 and 9 is missing 8. Before entering it, inspect that cell’s column and box. It must satisfy all three rules. If there is a conflict, review an earlier entry rather than changing a given.

3. Write candidate notes

For a cell with no immediate answer, rule out digits already present in its row, column or box. If only 2 and 7 remain, note both in small writing. Once another cell in that column is confirmed as 7, eliminate it and leave 2. Remove candidates that become impossible after each step.

4. Look for a single possible position

You can also study one digit within a unit. If a box has only one allowed position for 5, that cell must be 5 even if it contains other notes. Distinguish this from a cell having one candidate: one method examines positions for a digit, while the other examines digits for a cell.

5. When stuck, review one relationship

Return to boxes affected by a new digit and inspect their rows and columns. Do not give a provisional guess the same appearance as a confirmed entry. When using a hint, check which rule supports it before continuing. On paper, pencil and readable notes make corrections easier without hiding clues.

6. Check and try another variant

At the end, review every row, column and box as well as the original givens. A 4 × 4 grid uses 1–4 and 2 × 2 boxes; a 6 × 6 grid uses 1–6 and the indicated boxes. Killer adds cages with sums and no repeated digits inside them; read those rules before treating it as classic sudoku.