The basics
Sudoku frequently asked questions
Short, clear answers, each linked to a longer guide.
Sudoku is a 9 by 9 logic puzzle where every row, column and 3 by 3 box must contain 1 to 9 once. It never needs guessing or math, and every proper puzzle has exactly one solution. Below are answers to the questions people ask most.
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Frequently asked questions
What is Sudoku?
A logic puzzle on a 9 by 9 grid. You fill in digits so every row, column and 3 by 3 box has 1 to 9 exactly once. See how to play.
Do you need math for Sudoku?
No. The digits are just symbols. Sudoku needs logic only.
Should I guess in Sudoku?
No. A proper puzzle can always be solved by logic. If you feel you must guess, look for a pattern from the techniques list.
What is the best strategy for Sudoku?
Scan for singles first, add notes when they run out, then use pairs, pointing pairs and so on. See how to solve Sudoku.
What are pencil marks?
Small candidate digits you write in an empty cell. See notes and pencil marks.
How many Sudoku puzzles are there?
There are 6,670,903,752,021,072,936,960 valid completed grids, and many more puzzles can be made from each. About 5.47 billion grids are essentially different once you count rotations, reflections and relabeling.
What is the minimum number of clues in a Sudoku?
17. No valid puzzle with 16 clues exists. See why a Sudoku has one solution.
What does Sudoku mean?
It is short for a Japanese phrase that means roughly that the digits must remain single. See the history of Sudoku.
What is the hardest Sudoku level?
It depends on the site. Here, Extreme needs XY-Wing or coloring. See difficulty levels.
What is Killer Sudoku?
A variant with cages that have a target sum instead of many given digits. See Killer Sudoku.
Is Sudoku good for your brain?
It is a pleasant mental workout. Research on long-term benefits is mixed. See what science says.
Can I print Sudoku puzzles?
Yes, for free. Use printable Sudoku.
Is there a free Sudoku solver?
Yes. The solver gives the answer or walks through each step.