Last updated: March 2026
What Is Sudoku?
Sudoku is the world's most popular number puzzle, played by over 55 million people who search for it online every month. Originally popularized in Japan in the 1980s, it now appears in newspapers across 80+ countries and has become a staple of puzzle culture worldwide.
The rules are elegantly simple: fill every row, column, and 3x3 box with the numbers 1 through 9, with no repeats in any row, column, or box. Despite the simplicity of the rules, Sudoku puzzles range from relaxing warm-ups to deeply challenging logic exercises that require advanced techniques and careful reasoning.
Our version gives you unlimited puzzles at four difficulty levels, a built-in solver, pencil marks for tracking candidates, a timer, and a Daily Challenge that is the same puzzle for every player each day. Every puzzle is generated fresh and guaranteed to have exactly one valid solution.
How to Play Sudoku
Select a cell by clicking or tapping it, then type a number 1-9 on your keyboard or use the mobile number pad. Bold dark numbers are the given clues and cannot be changed. Your numbers appear in blue.
Use notes mode (press N or tap the pencil icon) to add small candidate numbers to a cell. This is essential for harder puzzles where you need to track which numbers are still possible in each cell. Notes are automatically cleaned up when you place a number.
The Hint button reveals one correct number (up to 3 per game). The Check button highlights any incorrect numbers in red for 3 seconds. If you get completely stuck, the Solve button fills in the entire solution with an animated cascade.
Arrow keys navigate between cells on desktop. The timer starts when you place your first number and pauses when you switch tabs. On completion, you get a summary of your time, hints used, and mistakes, with an option to share your results.
Frequently Asked Questions
How many Sudoku puzzles can I play?
Unlimited. Each time you start a new game, a unique puzzle is generated on the fly using a backtracking algorithm with randomization. You also get a Daily Challenge that is the same puzzle for all players each day.
What are the difficulty levels?
There are four levels. Easy gives you 46-51 numbers to start with, Medium gives 36-45, Hard gives 29-35, and Expert gives just 23-28. Higher difficulty means more empty cells and requires more advanced solving techniques.
How does notes mode work?
Press N on your keyboard or tap the pencil icon to toggle notes mode. In notes mode, tapping a number adds a small pencil mark to the cell showing possible candidates. When you place a final number, pencil marks for that number are automatically removed from the same row, column, and 3x3 box.
Can the solver handle any valid Sudoku puzzle?
Yes. The built-in solver uses a backtracking algorithm that can solve any valid Sudoku puzzle. If you get stuck, hit Solve to see the answer fill in with an animated cascade. You can also use up to 3 hints per game to reveal individual cells.
How does the Daily Challenge work?
The Daily Challenge uses the current date as a seed for the random number generator, so every player gets the same puzzle each day. Difficulty rotates through the week: Monday is Easy, Tuesday is Medium, Wednesday is Hard, Thursday is Expert, Friday is Hard, Saturday is Medium, and Sunday is Easy.