Sudoku is one of the most popular logic puzzles in the world. The rules are simple to understand, but the puzzles can range from easy to extremely challenging. Here's everything you need to know to start playing.
Sudoku is played on a 9x9 grid, divided into nine 3x3 boxes (also called "regions" or "blocks"). The goal is to fill every empty cell with a number from 1 to 9, following three simple rules:
1. Each row must contain the numbers 1 through 9, with no repeats. Look across any horizontal line — every digit from 1 to 9 must appear exactly once.
2. Each column must contain the numbers 1 through 9, with no repeats. Look down any vertical line — every digit from 1 to 9 must appear exactly once.
3. Each 3x3 box must contain the numbers 1 through 9, with no repeats. Within each of the nine bold-outlined boxes, every digit from 1 to 9 must appear exactly once.
That's it! No math is required — Sudoku is purely a logic puzzle. You never need to add, subtract, or perform any calculations (unless you're playing Killer Sudoku).
When you start a Sudoku puzzle, some cells are already filled in. These are called givens or clues. The number of givens determines the puzzle's difficulty:
Easy puzzles typically have 36-45 givens. Many cells can be solved immediately.
Medium puzzles have 30-35 givens. Requires some scanning and elimination.
Hard puzzles have 25-29 givens. Multiple techniques needed.
Expert puzzles have 22-26 givens. Advanced strategies required.
Master/Extreme puzzles have 20-24 givens. The most challenging puzzles for experienced players.
Step 1: Scan for Singles
Look at each row, column, and box. If a number can only go in one cell, fill it in. This technique is called "Naked Single" — the most fundamental solving method.
Step 2: Use Pencil Marks (Notes)
For cells where multiple numbers are possible, write small candidate numbers (pencil marks) in the cell. This helps you track possibilities and spot patterns.
Step 3: Eliminate Candidates
When you place a number, remove that number from the pencil marks of all cells in the same row, column, and box. This often reveals new singles.
Step 4: Look for Hidden Singles
Even if a cell has multiple candidates, if a number can only go in one place within a row, column, or box, it must go there. This is called a "Hidden Single."
Naked Pairs/Triples
If two cells in the same row, column, or box contain only the same two candidates, those numbers can be eliminated from other cells in that group. The same logic extends to triples.
Pointing Pairs
If a candidate number in a box is restricted to a single row or column, that number can be eliminated from other cells in that row or column outside the box.
X-Wing
If a candidate appears in exactly two cells in each of two different rows, and those cells share the same two columns, the candidate can be eliminated from other cells in those columns.
Swordfish
An extension of X-Wing involving three rows and three columns. This advanced technique can unlock cells that simpler methods cannot.
Killer Sudoku combines classic Sudoku with an extra constraint. In addition to the standard rules:
Cages: The grid is divided into groups of cells called "cages," outlined with dotted lines. Each cage has a small number in its corner showing the target sum.
Sum Rule: The numbers in each cage must add up to the target sum.
No Repeats in Cages: No number can repeat within a cage, even if the cage spans multiple rows, columns, or boxes.
Killer Sudoku often starts with no givens at all — you must use the cage sums and Sudoku constraints together to solve the puzzle.
Start easy. Build confidence with Easy puzzles before moving up in difficulty.
Use notes. Pencil marks are essential for anything above Easy difficulty. Don't try to keep everything in your head.
Focus on one area. Rather than jumping around the grid, focus on rows, columns, or boxes that are nearly complete.
Don't guess. Sudoku should always be solvable through logic alone. If you feel stuck, look for a technique you haven't tried yet.
Take breaks. Sometimes stepping away for a few minutes helps you see patterns you missed.