The Rules of Sudoku

A standard sudoku puzzle is a 9×9 grid divided into nine 3×3 boxes. Some cells already have numbers filled in (the “givens”). Your goal is to fill in every empty cell so that:

  • Every row (horizontal line) contains the digits 1–9 exactly once
  • Every column (vertical line) contains the digits 1–9 exactly once
  • Every 3×3 box contains the digits 1–9 exactly once

Basic Solving Techniques

  1. 1

    Scan for naked singles (one possibility)

    For each empty cell, look at its row, column and box. Cross out any digits already used in those three areas. If only one digit remains possible for a cell, write it in. This is the simplest technique and alone solves many easy puzzles. Go through every empty cell systematically.

  2. 2

    Scan for hidden singles

    For each digit 1–9, look at each row, column and box. If a digit can only go in one cell within that row, column or box — even if that cell has multiple other possibilities — place it there. Example: if the digit 7 can only go in one cell in a particular row, write 7 there regardless of what other digits might also be possible for that cell.

  3. 3

    Write candidate numbers in empty cells

    For medium and hard puzzles, pencil all possible candidates (small numbers) in the corners of each empty cell. Update these as you fill in cells. Seeing all candidates makes advanced techniques easier to apply.

  4. 4

    Naked pairs and triples (intermediate)

    If two cells in the same row, column or box share exactly the same two candidate numbers (e.g. both show only 3 and 7), those two digits must go in those two cells. Remove those digits from all other cells in the same row, column and box. This elimination often unlocks further naked singles.

Tips for Beginners

  • Start with easy or beginner puzzles — they require only naked and hidden singles to solve
  • Work in pencil so mistakes can be erased
  • Never guess — if you do not have enough information to place a number logically, use a different technique first
  • Focus on one digit at a time when scanning — where can the 5 go in this box?
  • Take breaks when stuck — fresh eyes often immediately spot what was missed
Free sudoku resourceswebsudoku.com offers free puzzles at four difficulty levels. The New York Times Games app and ABC Sudoku (Australia) offer daily puzzles. Sudoku.com (app) is highly rated for mobile play. Most newspapers still include a daily sudoku puzzle.

Frequently Asked Questions

Sudoku exercises logical reasoning, pattern recognition and working memory. Regular puzzle solving is associated with maintained cognitive function as people age, though research on whether it directly prevents cognitive decline is mixed. It is mentally engaging and a beneficial cognitive activity — particularly because it is an active problem-solving exercise rather than passive entertainment. The benefits are greatest when the puzzles are challenging (slightly above your current skill level) rather than too easy.
Go back to basics: re-scan every row, column and box for naked singles you may have missed. Check your candidate lists are up to date. Look for hidden singles (a digit that can only go in one cell in a group). Try looking at specific digits — where can the 4 go in each box? If still stuck on a hard puzzle, advanced techniques like X-Wing, Swordfish and XY-Wing exist — search for tutorials on these. As a last resort, use an online solver to reveal a single next step (not the full solution).