Sudoku Generator

Generate sudoku puzzles whose single solution is proved by a solver, play them in the browser with pencil marks, or print up to 20 with answer keys.

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🔒 Private by design: everything runs locally in your browser and never uploaded to any server.

Written and maintained by Last updated

The property that makes a sudoku a sudoku

A grid of digits with some cells blanked is not necessarily a sudoku. It becomes one only if exactly one arrangement completes it. A puzzle with two solutions is not a harder puzzle, it is a broken one, because at some point the only honest move is to guess and the deduction that makes the game worthwhile stops being possible.

Plenty of generators skip this. They remove a fixed number of clues and hand the result over, on the reasonable assumption that a grid with 30 clues is usually unique. Usually is doing a lot of work in that sentence.

Here every removal is followed by a solver that counts solutions and stops at the second one it finds. A clue whose removal would allow a second solution is restored immediately and the digger moves on. The cost is that generating a puzzle runs a full solve for each of up to 81 candidate removals, which on a modern device still takes a small number of milliseconds.

Playing and printing

  1. Pick Play online or Print as PDF at the top.
  2. In play, choose a difficulty and press the new puzzle button. The header shows the level and the exact clue count of the grid you got.
  3. Click a cell and type a digit, or use the on screen pad. Turn on Pencil marks to record candidates instead of answers.
  4. Turn on Highlight mistakes if you want wrong digits flagged as you go, or leave it off for a stricter game.
  5. In print, choose a difficulty, a puzzle count from 1 to 20, one, two or four puzzles per page and a paper size, then download.

Difficulty is a clue count, stated plainly

The four levels are 45, 36, 30 and 24 clues. That is the entire definition, and it is worth being blunt about it because sudoku difficulty labels elsewhere often imply a grading system that is not there.

Real difficulty grading works by running a solver that only uses human techniques, then recording the hardest technique required. A puzzle needing nothing beyond scanning is easy, one needing a chain of implications is hard. This generator does not do that, so it cannot promise that an expert grid will demand anything in particular.

What it can promise is that fewer clues means less to work with, which in aggregate produces the experience the labels suggest. If you solve a run of expert puzzles you will find them consistently slower than the medium ones. You will also occasionally find one that collapses in a few minutes, and that is the honest limit of a clue count as a difficulty measure.

What the play mode adds over paper

Pencil marks are the main reason to solve on screen rather than on paper. A cell with no answer yet can hold any subset of the digits one to nine as candidates, and the same digit pad writes them, so you can build up a full candidate picture without a pencil sharp enough to fit nine numbers in a box.

The peer highlight is the second. Selecting a cell dims nothing but visibly marks every other cell in its row, its column and its three by three box, which is the exact set you scan when testing a candidate. On paper you do that with your eyes and make mistakes.

The mistake highlight is optional on purpose. Leaving it off gives you the paper experience where an error propagates until something contradicts, which is part of the game for some people and infuriating for others.

The printed sheet

The PDF is generated as a real document with vector grid lines, not an image of the screen, so it stays sharp whatever you print it on. Puzzle pages come first and the answer keys follow, one key page for each puzzle page, and the page total is shown before you commit to the download.

Four puzzles per page is genuinely compact and suits a commute or a waiting room. One per page is the right choice if anyone is going to write in it with a pen rather than a fine pencil.

Other things to print or play

For a different kind of grid puzzle on the same PDF pipeline, Maze Generator and Word Search Maker both produce printable sheets with solutions. Bingo Card Generator covers a group activity rather than a solo one, and Math Worksheet Generator is the classroom equivalent.

If you want a timed brain exercise rather than a puzzle, Memory Test runs in the browser. The rest of the printable tools are on the pdf hub.

See it in action

Screenshot of the Sudoku Generator tool with a puzzle generator offering play-online or print-as-PDF, with difficulty set by how many clues are given - easy at 45, medium at 36, hard at 30 and expert at 24
Sudoku Generator mid-process: a puzzle generator offering play-online or print-as-PDF, with difficulty set by how many clues are given - easy at 45, medium at 36, hard at 30 and expert at 24.
Diagram: where the work happens on a SysFenix page that has no file input at all: the tool arrives as ordinary JavaScript inside the page, works the answer out on your own device and renders it in place, so the upload, queue and server-side record a typical online tool needs never happen
Where the work happens on a SysFenix page that has no file input at all: the tool arrives as ordinary JavaScript inside the page, works the answer out on your own device and renders it in place, so the upload, queue and server-side record a typical online tool needs never happen.

Frequently asked questions

Is every puzzle guaranteed to have exactly one solution?

Yes, and it is proved rather than assumed. The generator starts from a complete valid grid and removes clues one at a time. After every single removal it runs a solver that counts solutions, stopping as soon as it finds a second one. If the count is anything other than exactly one, that clue is put straight back before the next attempt. The puzzle you receive is the last state that passed that check, so uniqueness is a property that was tested, not one inferred from how many clues remain.

What does difficulty actually control?

The number of clues left on the board and nothing else. Easy aims for 45, medium 36, hard 30, expert 24. It does not grade puzzles by the solving techniques a human would need, so it will never tell you that a puzzle requires a particular pattern of reasoning. Fewer clues correlates with harder in practice, which is why the levels work, but the correlation is not a guarantee and an occasional expert grid will fall apart quickly.

Why does an expert puzzle sometimes show 25 clues instead of 24?

Because the uniqueness check gets the final word. Digging stops when the target is reached or when no remaining clue can be removed without creating a second solution, whichever comes first, so the achieved count is always at or above the target rather than exactly on it. In practice easy, medium and hard reach their targets consistently, and expert is the level that regularly stops one clue short.

Can I get the same puzzle again later?

No. Every random choice comes from the platform's cryptographic random source, and the interface has no seed field, so a puzzle cannot be reproduced once you leave it. Restart returns the current puzzle to its starting clues, which covers the usual reason for wanting one back, but there is no way to share a specific grid with someone else by reference.

What does the play mode give me beyond a printed grid?

A timer with pause and resume, a digit pad so it works on a phone, pencil marks for candidate digits within a cell, and an optional mistake highlight that marks any entered digit which disagrees with the solution. Selecting a cell also highlights its row, its column and its box, which is the visual aid that makes scanning for a candidate practical on a screen. There is also a reveal solution button for when you are done arguing with it.

How does the printable sheet work?

You choose a difficulty, how many puzzles from 1 to 20, whether to fit one, two or four puzzles per page, and A4 or Letter paper. The tool then builds a PDF whose puzzle pages are followed by the same number of answer key pages, and the interface states that page total before you download so there are no surprises. The layout is generated as a real PDF rather than a screenshot, so the grid lines stay crisp at any print size.

Does anything get uploaded, and does it need an internet connection while I play?

Nothing is uploaded. Puzzle generation, the solver, the play interface and the PDF are all produced inside your browser tab, so no puzzle, no solve time and no progress is ever sent anywhere. Once the page has loaded there is nothing further to fetch, which is why it keeps working on a plane or a train with no signal.

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