Welcome to the ultimate Sudoku guide! Whether you're a complete beginner or looking to refine your solving skills, this comprehensive guide will teach you everything you need to know about Sudoku—from basic rules to advanced techniques used by championship solvers.
Apply these techniques in our free online Sudoku with daily challenges and 4 difficulty levels!
Play Free Sudoku →Sudoku is a logic-based number placement puzzle that has captivated millions worldwide. Despite using numbers 1-9, it requires zero mathematical ability—it's purely about logic, pattern recognition, and systematic thinking. The puzzle's beauty lies in its elegant simplicity combined with surprising depth.
The name "Sudoku" comes from Japanese: 数独 (sūdoku), meaning "single number." Each puzzle presents a 9×9 grid partially filled with numbers, and your task is to complete it following three simple rules.
That's it! These three rules govern every Sudoku puzzle, from beginner to extreme difficulty. The challenge comes in applying logical deduction to discover where each number must go.
You could play Sudoku with letters A-I, symbols, or colors—numbers are just convenient! You never add, subtract, multiply, or divide. Success depends on logical thinking and pattern recognition, not arithmetic.
These fundamental techniques will help you solve Easy and many Medium-difficulty puzzles. Master these before moving to advanced methods!
The most basic and essential technique. Scan each row, column, and box systematically to find where a specific number must go.
How to do it:
When only one number can possibly go in a cell, fill it in! This happens when all other numbers are eliminated by the row, column, and box constraints.
Example: If a cell's row contains 1,2,3,4,5,6,7,8, and its column contains 9, that cell must be 9!
When a number can only go in one place within a row, column, or box, it must go there—even if other numbers could also go there.
Example: If 7 can go in three cells of a row, but examining the boxes shows it can only fit in one of them, place it there.
For harder puzzles, mark small "pencil marks" (candidates) in cells showing which numbers could possibly go there. As you fill in numbers, eliminate them from other cells' candidates.
Our online Sudoku has built-in notes mode for this! Click the pencil icon to toggle notes mode and mark candidates.
Once you've mastered the basics, these techniques will help you tackle Hard-difficulty puzzles.
If two cells in a row, column, or box can only contain the same two numbers (e.g., both can only be {3, 7}), then those two numbers can be eliminated from all other cells in that group.
Why it works: Those two cells "claim" those two numbers, so no other cell in the group can use them.
If two numbers can only appear in two specific cells within a row, column, or box (even if those cells have other candidates), all other candidates can be eliminated from those two cells.
Example: If 4 and 9 can only appear in cells A and B of a box, remove all other pencil marks from cells A and B.
If a candidate number in a box is restricted to one row or column, it can be eliminated from the same row/column in other boxes.
If a candidate in a row or column is restricted to one box, it can be eliminated from other positions in that box.
These powerful techniques are needed for Xtreme-difficulty puzzles and competitive Sudoku.
An advanced pattern-matching technique. If a candidate appears in exactly two positions in two different rows, and these positions align in the same two columns, that candidate can be eliminated from other positions in those two columns.
Extension of X-Wing across three rows and three columns. Rare but powerful when it appears!
A three-cell pattern that creates logical eliminations through a chain of candidates.
Advanced techniques involving marking cells with "colors" to track logical implications. Used in the hardest puzzles.
For: Complete newcomers to Sudoku
Features: Master Sudo guides you step-by-step, showing possible numbers and explaining why
Techniques needed: Just follow the guidance and learn!
Typical time: 15-30 minutes while learning
For: Building confidence and practicing basics
Clues: 28-32 given numbers
Techniques needed: Scanning, single candidates, single positions
Typical time: 5-15 minutes
For: Experienced solvers seeking a challenge
Clues: 22-26 given numbers
Techniques needed: All basics plus naked pairs, hidden pairs, pointing pairs
Typical time: 15-40 minutes
For: Expert solvers and puzzle masters
Clues: 17-21 given numbers
Techniques needed: Advanced patterns (X-Wing, Swordfish, chains, coloring)
Typical time: 30-90+ minutes
No! Every proper Sudoku puzzle has a unique solution that can be found through pure logic. If you find yourself guessing, there's a logical technique you haven't discovered yet. Our puzzles are all logically solvable without guessing.
Practice consistently, starting with easier difficulties and gradually progressing. Learn basic techniques thoroughly before moving to advanced ones. Use notes mode to track candidates. When you validate and find errors, understand why—this is how you learn!
Absolutely, especially for Hard and Xtreme puzzles! Professional Sudoku solvers always use pencil marks. It's not cheating—it's an essential technique that lets you track possibilities and apply advanced solving methods. Our online game has built-in notes mode!
It varies greatly! Easy: 5-15 minutes, Hard: 15-40 minutes, Xtreme: 30-90+ minutes. Don't worry about speed initially—focus on understanding the logic. Speed naturally improves with practice.
Scan the entire grid first to get familiar with it. Look for rows, columns, and boxes with many given numbers. Use scanning technique for frequently-appearing numbers. Fill in obvious singles before adding pencil marks everywhere.
Yes! Our Sudoku is fully optimized for mobile with large, easy-to-tap cells, intuitive touch controls, and a streamlined interface. Your progress auto-saves, so you can play on phone, tablet, or computer seamlessly.
Put these techniques into practice! Play free online Sudoku with daily challenges, 4 difficulty levels, notes mode, hints, and more.
Play Sudoku Now →Last Updated: January 2025 | Try Solitaire Home | Play Sudoku