Sudoku generator
Choose easy, medium or hard and print a fresh sudoku with its solution. Every puzzle is checked to have exactly one solution. The seed number lets you recreate the same puzzle later, so you can keep the key.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Pick a level and press New puzzle, or type a seed.
- Print the page. The solution is on a separate page when the key option is ticked.
- Untick the key if you only want to hand out the puzzle.
How it works
A solver fills the cell with the fewest candidates first and backtracks on a dead end. With a seeded random digit order it produces a random complete grid. Digits are then removed in a random order until the target number of givens is reached, and each removal is kept only if the puzzle still has exactly one solution.
Example
Medium keeps 32 digits, so 49 cells are empty. The solution page shows the given digits in black and the filled-in ones in blue on screen.
Tips
- Solve in pencil first; the hard level rarely has an obvious next move at the start.
- Write down the seed with the level to print the solution again later.
Frequently asked questions
How do you know there is only one solution?
The generator builds a complete valid grid with randomised backtracking, then removes digits one at a time. After each removal a solver counts solutions up to two; the digit stays removed only if exactly one solution remains.
What do the levels mean?
Levels are set by how many digits are given: 40 for easy, 32 for medium and 26 for hard. This is a rule of thumb. Fewer givens usually make a puzzle harder, but difficulty also depends on which digits remain, and it is not rated by solving technique.
Can the same puzzle be made again?
Yes. The generator is seeded, so the same level and seed always give the same puzzle and solution, on any device.
How few digits can a sudoku have?
Mathematicians proved in 2012 that no 9 by 9 sudoku with a single solution can have fewer than 17 given digits. This tool stops at 26, which keeps puzzles solvable by logic for most people.