These are the most frequently asked questions about Sudoku. Whether you’re a complete beginner or an experienced solver looking for specifics, you’ll find clear answers here.
The Basics
What is Sudoku?
Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. The goal is to fill every row, column, and box with the digits 1 through 9 without repeating any number. No math, guessing, or special knowledge is required — just logical deduction.
Learn more: How to Play Sudoku | Sudoku Rules
Does Sudoku require math?
No! Sudoku uses numbers as symbols, but requires zero arithmetic. It’s purely a logic and pattern recognition puzzle. The digits 1–9 could be replaced with letters, colors, or shapes and the game would work identically.
Learn more: Is Sudoku Math?
How do I start playing Sudoku?
Start with an Easy puzzle — you can solve it using just the process of elimination. If you want a guided introduction, read our How to Play Sudoku guide or follow the Start Here learning path.
What are pencil marks (candidate notation)?
Pencil marks are small numbers written in cells to track which digits are still possible. They’re essential for any technique beyond basic scanning.
Learn more: Pencil Marks Explained
Rules & Structure
What is the 45 rule in Sudoku?
The digits 1 through 9 add up to 45, so every complete row, column, or 3×3 box must sum to 45. This is most useful in Killer Sudoku where cage sums are given as clues.
Learn more: What Is the 45 Rule in Sudoku?
How many solutions does a valid Sudoku puzzle have?
Exactly one. A properly constructed Sudoku puzzle has a single unique solution reachable through logic alone. If a puzzle has multiple solutions, it is not considered a valid Sudoku.
What is the minimum number of clues in a Sudoku puzzle?
17 clues. This was proven mathematically in 2012 by Gary McGuire and his team at University College Dublin. No valid 9×9 Sudoku puzzle with 16 or fewer clues can have a unique solution.
Learn more: How Many Sudoku Puzzles Exist?
How many valid Sudoku grids exist?
Approximately 6.67 × 10²¹ (6.67 sextillion) valid completed grids. When accounting for symmetry equivalences (rotations, reflections, relabeling), there are about 5.47 billion essentially different grids.
Learn more: How Many Sudoku Puzzles Exist? | Sudoku and Mathematics
Difficulty
How is Sudoku difficulty determined?
Difficulty is determined by the number of starting clues and, more importantly, the complexity of solving techniques required:
| Difficulty | Clues | Key Techniques | Guide |
|---|---|---|---|
| Easy | 36–45 | Naked singles, hidden singles | How to Solve Easy |
| Medium | 30–35 | + Naked pairs, hidden pairs, pointing pairs | How to Solve Medium |
| Hard | 26–30 | + Triples, box-line reduction | How to Solve Hard |
| Expert | 22–26 | + X-Wing, Swordfish, XY-Wing | How to Solve Expert |
| Evil | 17–22 | + Chains, coloring, ALS | How to Solve Evil |
Learn more: Sudoku Difficulty Explained
Can every Sudoku puzzle be solved without guessing?
Yes. Every valid Sudoku puzzle can be solved through pure logical deduction. If you feel the need to guess, it means there is a technique you haven’t learned yet or a deduction you’ve missed. Our technique progression guide shows exactly which techniques to learn for each difficulty.
What difficulty should I start with?
If you’re brand new, start with Easy. Once you can solve Easy puzzles consistently in under 10 minutes, move to Medium. Follow our Start Here learning path for a structured progression.
Techniques & Strategy
What are the best techniques for beginners?
Two techniques are all you need for Easy puzzles:
- Naked Single — A cell with only one possible candidate
- Hidden Single — A candidate that appears only once in a row, column, or box
Learn more: Beginner Sudoku Strategies
What are advanced Sudoku techniques?
Advanced techniques handle patterns that span the entire grid:
- Fish patterns: X-Wing, Swordfish, Jellyfish
- Wing techniques: XY-Wing, XYZ-Wing, W-Wing
- Single-digit patterns: Skyscraper, Two-String Kite, Empty Rectangle
- Chain logic: Coloring, Alternating Inference Chains
- Uniqueness: Unique Rectangle, BUG
- Set logic: Almost Locked Sets
Learn more: Advanced Sudoku Strategies | Complete Technique Progression
What order should I learn techniques in?
Follow our Complete Technique Progression guide, which maps out 8 stages from beginner to expert. The short version:
Singles → Pairs → Triples → Fish → Wings → Single-digit patterns → Uniqueness → Chains/ALS
How do I improve my solve time?
Read our 12 Speed-Solving Tips. Key advice: master scanning before pencil marks, learn techniques in order, avoid backtracking, and practice at a specific difficulty until it feels easy.
Learn more: How Long Does It Take to Solve Sudoku?
About SudokuPulse
Is SudokuPulse free?
Yes, completely free. All puzzles, technique guides, strategy articles, and tools are available at no cost with no registration required.
How many puzzles does SudokuPulse have?
Unlimited. We generate puzzles algorithmically across six difficulty levels (Easy, Medium, Hard, Expert, Evil, Mini) plus a fresh Daily puzzle every day.
Who created SudokuPulse?
James Hoss, a competitive Sudoku solver and puzzle designer with over 15 years of experience. Every technique guide and strategy article is written and reviewed by James.
Learn more: About SudokuPulse
How can I contact you?
Visit our Contact page to get in touch. We welcome puzzle feedback, content suggestions, and bug reports.