How to use the sudoku generator
- Choose a difficulty: easy, medium, hard or expert. Set how many puzzles you want, from 1 to 12.
- Pick a title and how many puzzles go on each page. One per page gives large digits, four per page saves paper.
- Keep answer key pages on if you want solutions. They print after all the puzzles, with the original givens in bold.
- Use the puzzle set code to get the same puzzles again, or press Make new puzzles for a fresh set.
- Press Download PDF or Print. Solve in pencil, and check against the key when you finish.
How each puzzle is made
The generator first builds a complete, valid sudoku grid by filling it in with a randomized search. It then removes digits one at a time, in random order. After every removal a solver counts the solutions of the grid that remains, stopping as soon as it finds a second one. If the count is anything other than exactly one, the digit goes back. What is left is a puzzle with a single solution that you can reach by logic, with no need to guess.
The solver is a backtracking search that always works on the empty cell with the fewest possible digits, using bit masks for rows, columns and boxes. It is tested against a well-known published puzzle and its published solution, and the tests also check that damaged puzzles are reported as having no solution or more than one. Every puzzle you print has passed the single-solution check, and the answer key is that solution.
What the difficulty levels mean
Difficulty here is based on two measurable things. The first is how many digits are given: the targets are 40 for easy, 33 for medium, 28 for hard and 24 for expert, and the page tells you the real range for your set. The second is whether naked and hidden singles alone can finish the puzzle. A naked single is a cell that has only one digit left that fits; a hidden single is a digit that fits in only one cell of a row, column or box. Easy and medium puzzles are always solvable with those two techniques. Hard and expert puzzles are not, so they call for pairs, pointing digits or other patterns.
This is a simple scale, not an official rating, and different solvers find different puzzles hard. Fewer givens often means harder, but not always. Expert puzzles are the most demanding of the ones the generator can find quickly; it does not search for the very fewest givens, which is 17 according to published research, and such puzzles are rare and very hard to find.
Pencil marks, strategy and fairness
There are about 6.67 x 10^21 distinct completed sudoku grids, so you will not run out of puzzles. Solve by scanning for singles first, then write small candidate digits in each empty cell. Never guess: a valid puzzle from this tool always has a logical path. If you get stuck, compare your grid with the answer key to find the first wrong digit rather than starting over.
Puzzle generation runs entirely in your browser. Larger sets of expert puzzles take a few seconds because every removal needs a solver check.
Assumptions and limits
- Every puzzle is checked to have exactly one solution, but difficulty is only an estimate based on givens and single-digit logic. Another solver may rate a puzzle differently.
- The standard 9 by 9 sudoku is the only size offered. Variants such as killer, jigsaw or 16 by 16 grids are not built in.
- The same set code, difficulty, count and symmetry option always give the same puzzles. Changing any of them gives different puzzles.
- Up to 12 puzzles are made at a time to keep the page responsive, especially for expert difficulty.
- The page does not check your answers. Use the answer key pages, or print the puzzle and solve it on paper.
Frequently asked questions
How do I know each sudoku has only one solution?
After every digit is removed, a solver counts how many solutions remain and stops at two. A digit is only removed if exactly one solution is left, so every printed puzzle has a single answer.
What is the minimum number of givens in a sudoku?
Published research found that no valid sudoku has fewer than 17 given digits. The expert level here aims for 24, because puzzles near 17 are very rare and hard to generate.
What makes hard puzzles harder than easy ones?
They have fewer givens and cannot be finished using only naked and hidden singles. You will need extra techniques such as pairs, pointing digits or box-line reductions.
Can I get the same puzzles again later?
Yes. Keep the puzzle set code and enter it later with the same difficulty, count and symmetry choice. The set is rebuilt exactly, and answer keys match.
Are the puzzles free to print and share?
Yes. They are generated on your device for you to use, and the tool places no restriction on printing them for classes, newsletters or personal use.
What is the answer key page?
It shows each completed grid. The original givens are bold and the digits you must find are regular weight, so you can check a single cell without seeing everything. For number games, see the bingo card generator.