Does every puzzle really have only one solution?

Yes, and that is the point of the tool. A clue is only ever removed if the puzzle still has exactly one completion afterwards; if a second appears the clue goes straight back. Generators that skip this check routinely produce puzzles with several valid answers, which is why a solver sometimes finds their answer disagreeing with the printed key.

How hard is a 23-clue puzzle?

Hard for most people, but clue count is a rough proxy. Difficulty really depends on which solving techniques the grid demands, and two puzzles with identical clue counts can feel very different. Use the labels as a guide rather than a grading.

Why does the 6x6 grid have boxes that are not square?

Because six cannot be made from two equal factors. A 6x6 uses boxes two rows deep and three columns wide, which still gives six cells per box and preserves the rule exactly. It is a standard variant, not an approximation.

Can it make a sudoku that needs no guessing?

It does not promise that. Guaranteeing a puzzle is solvable by pure logic requires a solver that knows the human techniques and checks the puzzle yields to them, which is a much bigger piece of work than uniqueness. In practice the gentle and moderate settings almost always solve without guessing.

Can I use these puzzles in a school newsletter?

Yes. The puzzles are generated fresh in your browser and are not taken from any published collection, so there is nothing to license. Printing, photocopying and sharing them is fine.

Sudoku Generator

Makes a sudoku and guarantees the thing that actually matters: exactly one solution. Most quick sudoku generators fill a grid, delete some cells and print it, which produces puzzles with several valid answers — and the solver, reaching a point where two digits both work, assumes they have made a mistake. Every puzzle here is checked for uniqueness before a single clue is removed.

Grid size

Difficulty

Symmetric clues, like a newspaper puzzle

Seed (optional)

A word makes the puzzle reproducible, so you can reprint the identical grid later.

Your sudoku will appear here.

What this generator does

Produces a sudoku grid with some cells filled in, together with the single completion that fits them. The generator builds a complete valid grid first, then removes clues one at a time, re-checking after every removal that only one solution still exists and putting the clue back if a second has appeared. That check is the whole product: it is what separates a sudoku from a grid with some numbers missing, and it costs a full solve per attempted removal, which is why the solver underneath is written for speed.

How to use this tool

Choose a grid size — 9x9 is the real game, 4x4 and 6x6 are for younger children learning the rule.
Choose a difficulty. Fewer clues means harder.
Leave symmetry on for a puzzle that looks like a newspaper one, or turn it off for the hardest grids.
Press Make sudoku, then print it or reveal the answer key.

Understanding the controls

Grid size: 9x9 is standard sudoku, with 3x3 boxes. 6x6 uses boxes two rows by three columns and 4x4 uses 2x2 — both are genuine sudoku with the same rule, small enough for a child to finish and to see why the rule works.
Difficulty: Sets how few clues to aim for. Gentle leaves around 38 clues on a 9x9 and tough aims for 23. The tool never gives you fewer clues than the setting asks for, so a puzzle is never harder than its label; where it cannot remove enough it says so instead of pretending.
Symmetry: Removes clues in diagonally opposite pairs, which is the pattern newspaper sudoku uses. It is purely about how the grid looks and not about how it solves — but because it takes cells two at a time, a symmetric puzzle bottoms out at more clues than an asymmetric one. Turn it off for the hardest grids.
Seed: Makes the puzzle reproducible. The same seed, size, difficulty and symmetry setting always give exactly the same grid, so a class set can be reprinted next year or a missing sheet replaced.

Worked examples

A 9x9 moderate: Around 32 clues in a 180-degree symmetric pattern — a normal newspaper puzzle, solvable without guessing.
A 4x4 for a five-year-old: Digits 1 to 4, boxes of two by two, around 8 of the 16 cells given. The rule is the same and the grid is finishable in a couple of minutes.
The same seed twice: Seed “year-5-autumn” with 9x9 moderate gives an identical grid every time, so a lost worksheet can be reprinted exactly.
Symmetry turned off: The clue pattern looks irregular and the grid usually holds two to four fewer clues, because cells no longer have to be removed in pairs.

Common use cases

A printable starter or early-finisher task for a maths lesson
Teaching the sudoku rule to Key Stage 1 with a 4x4 grid
A puzzle page for a school or club newsletter
Practice at a difficulty that a book does not happen to contain
A quiet activity that needs no screen once it is printed

How this generator works

A complete grid is filled first by backtracking, trying the digits in a shuffled order so each run produces a different grid. Clues are then removed one at a time in a random order, and after each removal the puzzle is solved again — not to find the answer, but to count how many answers exist. The count stops as soon as it reaches two, because uniqueness only ever needs to distinguish one from more than one, and that bound is what makes a solve-per-removal affordable. If a second solution has appeared, the clue goes straight back and the next cell is tried. The solver always branches on the cell with the fewest remaining candidates, which on a sparse grid is the difference between milliseconds and seconds: a plain left-to-right scan spends its time exploring subtrees that were already doomed.

Randomness and fairness

Only one thing is random: the order the digits are tried when the complete grid is filled, and the order the cells are tried when clues are removed. Everything else follows deterministically — which clues can go and which must stay is decided entirely by whether the puzzle still has one solution. By default that ordering comes from the browser's cryptographic random number generator. Entering a seed switches it to a reproducible, non-cryptographic generator, which is deliberate and is the only way to reprint an identical grid; it is not cryptographically secure and nothing on this page needs it to be.

For how randomness is produced across the whole site, see how Generate Random works.

Assumptions this tool makes

You want a puzzle to print or copy out rather than one to solve on screen.
Standard sudoku rules apply: each digit once per row, once per column and once per box.

Limitations and good to know

Difficulty is set by how many clues remain, not by which solving techniques the puzzle needs. Two grids with the same clue count can feel quite different, and a 23-clue puzzle occasionally solves more easily than a 27-clue one.
It does not guarantee a puzzle is solvable by logic alone without guessing — verifying that needs a technique-aware solver, which is a different and much larger piece of work.
It will not reliably reach the 17-clue minimum that a 9x9 sudoku can theoretically have. Finding those grids is a search problem in its own right rather than something a carve arrives at.
The 4x4 and 6x6 grids are for learning the rule. Even at the toughest setting they take a couple of minutes, because there are simply not many cells to reason about.
There is no interactive grid to type into — the output is a printable puzzle and its answer key, which is what a worksheet needs.

Common mistakes

Assuming a puzzle with a lot of blanks is a hard puzzle: Clue count and difficulty are related but not the same thing. What makes a sudoku hard is which techniques it needs, and this tool sets clue count only — so treat the labels as a guide rather than a grading.
Printing without the answer key and having no way to check: Reveal the key and print it on a separate sheet, or note the seed. The same seed and settings reproduce the identical puzzle, so the answer can always be recovered.
Giving a 4x4 to an older child as a challenge: A 4x4 has sixteen cells and cannot be hard. It is for teaching the rule. Move to 6x6 as a bridge and 9x9 as soon as the rule has landed.
Turning symmetry off expecting a different kind of puzzle: Symmetry changes how the grid looks, not how it solves. Its only real effect is that an asymmetric grid can go a few clues lower, which makes it slightly harder.

Practical tips

Generate the whole class set with seeds you write down — one seed per table means every table gets a different puzzle and you can reprint any of them.
For a first sudoku lesson, print a 4x4 alongside a 6x6 so the class can see that the box shape changes and the rule does not.
If you want a genuinely hard grid, choose tough and turn symmetry off; that combination reliably gives the fewest clues this tool can reach.

Troubleshooting

The puzzle has more clues than the difficulty suggests: The grid it started from would not let any more go without a second solution appearing. The tool says so under the puzzle. Generate again for a different grid, or turn symmetry off.
I want the same puzzle again and did not note the seed: Without the seed it cannot be recovered — the grid was one of astronomically many. Copy the puzzle text before you leave the page; it includes the answer key.

Privacy and your data

Nothing is entered and nothing is stored. The puzzle is generated in your browser, no grid is uploaded or written into the page address, and analytics records only that the tool was used along with the size and difficulty chosen — never a grid or a seed.

Frequently asked questions

Does every puzzle really have only one solution?: Yes, and that is the point of the tool. A clue is only ever removed if the puzzle still has exactly one completion afterwards; if a second appears the clue goes straight back. Generators that skip this check routinely produce puzzles with several valid answers, which is why a solver sometimes finds their answer disagreeing with the printed key.
How hard is a 23-clue puzzle?: Hard for most people, but clue count is a rough proxy. Difficulty really depends on which solving techniques the grid demands, and two puzzles with identical clue counts can feel very different. Use the labels as a guide rather than a grading.
Why does the 6x6 grid have boxes that are not square?: Because six cannot be made from two equal factors. A 6x6 uses boxes two rows deep and three columns wide, which still gives six cells per box and preserves the rule exactly. It is a standard variant, not an approximation.
Can it make a sudoku that needs no guessing?: It does not promise that. Guaranteeing a puzzle is solvable by pure logic requires a solver that knows the human techniques and checks the puzzle yields to them, which is a much bigger piece of work than uniqueness. In practice the gentle and moderate settings almost always solve without guessing.
Can I use these puzzles in a school newsletter?: Yes. The puzzles are generated fresh in your browser and are not taken from any published collection, so there is nothing to license. Printing, photocopying and sharing them is fine.