Sudoku Generator

The Sudoku generator makes puzzles with exactly one solution, in the size and difficulty you pick

In Sudoku you fill the empty cells with digits so that every row, every column and every box (the blocks outlined in bold) contains each digit exactly once. This tool makes 9×9 puzzles in 6 difficulty levels and 4×4 and 6×6 puzzles for children in 3 levels. Every puzzle it makes has exactly one solution and can be solved by logic alone, with no guessing.

You can solve a puzzle right on the screen. Notes, fill notes, undo, a timer and a mistake count are built in, and when you get stuck, a hint marks the next move on the grid and explains why. If you prefer paper, print 1, 2, 4 or 6 puzzles per sheet with answer pages attached, or send the same puzzle to someone with a link that contains the puzzle ID.

When it helps to make your own Sudoku

Puzzles in books and newspapers come in a fixed difficulty and quantity, and they never tell you why a digit belongs where it does. Making your own lets you fit the puzzle to the person and the situation.

• Worksheets for children: for a child learning the rules, print 4×4 puzzles, then 6×6 once they are comfortable, six to a sheet. The answers come on separate pages, so checking is easy.
• Classroom or club contests: print several copies of the same puzzle ID and everyone solves the same problem, so times can be compared.
• Practicing solving techniques: from Hard upward, a technique such as Pointing, a pair or an X-Wing is needed at least once. Press Hint when you are stuck and you can see where on the grid that technique applies.
• One puzzle a day: your best time and solved count for each difficulty stay in this browser, so you can see how much faster you get at the same level.

How to use it

1. Under 'New puzzle' on the right (below the grid on a phone), choose the grid size and difficulty. If nothing is in progress, a new puzzle appears at once. If you are in the middle of a puzzle, only the setting changes until you press the 'Make new' button.
2. Click a cell on the grid, then click a digit in the row below. On a keyboard, move with the arrow keys and type the digit. The clock starts when you enter the first digit.
3. To write candidates, turn on 'Notes' or hold Shift while typing a digit. 'Fill notes' writes every digit that can go in each empty cell in one go. When you place a digit, the same note in its row, column and box is erased automatically.
4. If you are stuck, press 'Hint'. The first press shows the name of the technique for the next move and the reason, and the grid highlights the supporting cells and the cell to fill. Press once more to place that digit.
5. To solve on paper, choose how many puzzles per sheet and how many sheets under 'Print and save', then print. The first puzzle is the one currently on the screen, and the rest are newly made in the same size and difficulty.

The ring around each digit shows how many of that digit are already on the grid, and the small number at its top right is how many remain. When you finish a row, column or box correctly, a wave runs through those cells, and when the whole grid is complete, it spreads across the grid from the last cell.

Difficulty is set by the technique needed, not by the number of blanks

Many generators grade difficulty by the number of blanks. But with the same 28 clues, one puzzle may be finished by filling cells one at a time, while another can only be solved by spotting an X-Wing. This tool solves each puzzle the way a person would, in order, and sets the level by the hardest technique that was truly needed to finish. The table below comes from making 200 puzzles (IDs 1 to 200) for each 9×9 level with this tool on October 23, 2026.

From Medium to Expert, the average number of clues is almost the same, between 27.3 and 28.0. That means the number of clues alone cannot tell you the difficulty. What does change is how rare a fitting puzzle is: in this measurement, getting one puzzle took an average of 7.1 attempts at Hard, 18.9 at Very hard and 119.4 at Expert. An Expert puzzle can take a few seconds to make depending on the device, and the status line shows the attempt count meanwhile.

For Very easy and Easy, clues are removed until 44 or 36 remain; from Medium upward, clues keep being removed for as long as the puzzle can still be solved with that level's techniques. Clues are removed in pairs that stay in the same place when the grid is rotated 180 degrees around its center, so the clue layout is symmetric, like in newspaper Sudoku.

How we check that there is only one solution

If you remove too many clues, the grid ends up with more than one solution. Such a grid is not a proper Sudoku, because at the end you would have to pick either of two digits. Each time this tool removes a pair of clues from a completed grid, it tries to solve what remains from start to finish using only the techniques of that difficulty. Every technique makes only certain deductions, of the form 'this cell can only be this digit' or 'this digit cannot go here', so a grid that gets solved all the way has exactly one solution. If it cannot be solved, the removed clues are put back.

A 9×9 Sudoku with one solution needs at least 17 clues. McGuire, Tugemann and Civario searched exhaustively by computer for 16-clue puzzles and found none, which is how they showed that the minimum is 17 (2014). If a puzzle with only 11 or 14 clues is advertised as having one solution, that claim is not correct. The fewest clues in this tool's measurement was 22, at Expert level.

Solving techniques: the names used in hints

Hints and the 'Techniques needed to finish' bars use the names below. The names and definitions follow the technique documentation of the Sudoku solver HoDoKu. A 'unit' means a row, column or box, and a 'candidate' is a digit that can still go in an empty cell.

Singles: techniques that fix one digit

• Full House: when only one empty cell is left in a unit, the missing digit goes there.
• Hidden Single: if a digit has only one cell where it can go within a unit, that cell is the digit's place. Looking at boxes first finds these fastest.
• Naked Single: if a cell's row, column and box already contain 8 digits, the one digit left is the only one that fits. It is harder to spot than a Hidden Single because you have to look at one cell at a time.

Locked candidates: Pointing and Claiming

Pointing is the case where, inside one box, all the candidates for a digit lie in one row (or column). That digit must go somewhere in that row inside that box, so you remove it from the cells of the same row outside the box. Claiming is the reverse: in one row (or column), all the candidates for a digit lie inside one box, so you remove that digit from the other cells of that box. From this level on, you do not place a digit right away; you remove candidates to create the next single.

Naked and Hidden Pairs and Triples

If two cells in a unit have exactly the same two candidates (a Naked Pair), those two cells share the two digits between them, so you remove both digits from the other cells of the unit. With three cells and three digits it is a Triple, and with four cells and four digits a Quad. Conversely, if two digits can go in exactly the same two cells of a unit (a Hidden Pair), you remove every other candidate from those two cells.

X-Wing and Swordfish

If a digit has candidates in just two cells in each of two rows, and those cells lie in the same two columns, you have an X-Wing. The digit will use those two columns across the two rows, so you remove it from the other cells of the two columns. Looking at columns instead of rows works the same way. Extend it to three rows and three columns and you have a Swordfish.

XY-Wing and XYZ-Wing

An XY-Wing consists of a pivot cell with two candidates (X and Y) and two pincer cells (X·Z and Y·Z) that share a unit with the pivot. Whether the pivot is X or Y, one of the two pincers becomes Z, so a cell that sees both pincers can have Z removed. In an XYZ-Wing the pivot has three candidates, X, Y and Z, and you remove Z from a cell that sees all three cells.

See it on the grid When you get stuck on a Very hard or Expert puzzle, press Hint: cells with candidates to remove are hatched, the supporting cells are shown darker, and the cell to fill is outlined. If you use Fill notes first, the note digits that will be removed appear struck through.

How 4×4, 6×6 and 9×9 differ

The 4×4 and 6×6 sizes only make puzzles that singles can solve, and the clue count separates Easy, Medium and Hard. A 4×4 grid has only four digits and four boxes to keep track of, which suits a child learning the rules, while 6×6 has wide rectangular boxes and trains you to tell rows and boxes apart. The three 9×9 values are the clue counts for Very easy, Easy and Medium, three of its six levels.

Printing and saving

Printing uses your browser's print dialog. Change the destination printer to 'Save as PDF' there to get a PDF file. The paper is US Letter, and the puzzles are placed inside 12 mm margins. Each puzzle has its ID printed below it; add ?sudoku= and the ID to the end of this tool's address (for example ?sudoku=9-3-1cn) to reopen the same puzzle. The page footer shows this address format too. Answer pages are collected after the puzzle pages, with up to 6 answers per sheet.

Saving as PNG turns the current puzzle into an image 1,800 pixels wide. If you save while 'Show solution' is on, you get an image of the solution. 'Link' copies an address that contains the puzzle ID. The ID has the form 'grid size-difficulty-seed' (for example 9-3-1cn), and the same ID makes the same puzzle again on any device.

Common mistakes

• Entering a guess and continuing: one wrong digit wrongly erases candidates elsewhere, and a contradiction shows up later. If you are not sure, leave it as a note, and if you have already entered it, use undo to go back to that point.
• Filling in notes and never tidying them: this tool erases the same note in the same row, column and box when you place a digit, but candidates that Pointing or a pair would remove have to be erased by you. The hatched cells in a hint are exactly those.
• Judging difficulty by the clue count: as the table above shows, a puzzle with about 28 clues can be Medium or Expert.
• Turning Show solution on and off: you return to your puzzle, but a puzzle whose solution you saw is not recorded as a best time even when you finish it. A puzzle where you used hints counts toward your solved total but is left out of the best time.
• Looking for your records on another device: puzzles in progress and records are stored only in this browser. To continue on another device, open the same puzzle with a link and start from the beginning.

Limits

• The techniques used to grade difficulty are the 15 listed above. Puzzles that would need harder techniques such as chains, Unique Rectangles or coloring are not made, so even Expert stays within what these 15 can solve.
• Difficulty is set by technique, so it can differ from how hard a puzzle feels to a person. Even among Expert puzzles, using an XYZ-Wing once feels different from using it several times.
• Only 4×4, 6×6 and 9×9 grids are offered. Variants such as 16×16, Killer Sudoku and Diagonal Sudoku are not made.
• You cannot type in a puzzle copied from elsewhere. You can only solve and get hints for the puzzles this tool makes.
• Puzzles in progress, records and settings live only in this browser's storage and are not sent to a server. They disappear if you clear site data or close a private window.

Good to know about Sudoku

The name 'Sudoku' (数独) was given by the Japanese puzzle company Nikoli. Nikoli introduced the puzzle, which had appeared in an American puzzle magazine under the name 'Number Place', to Japanese readers in 1984, and explains that its first name, 数字は独身に限る ('the digits must be single'), was shortened to 数独. In Japan 'Sudoku' is a registered trademark of Nikoli, so other companies use the name 'Number Place'.

There are 6,670,903,752,021,072,936,960 completed 9×9 grids that follow the rules (Felgenhauer and Jarvis, 2005). If you count grids that become identical by relabeling digits, rotating or reflecting the grid, or swapping rows, 5,472,730,538 remain (Russell and Jarvis). This tool builds a new random completed grid every time and picks the clues on it, so unless you use the same ID, the same puzzle will practically never come up again.

Sources

As of The abstract and bibliographic details of the minimum-clue paper, the grid counts, the origin of Nikoli's name and the HoDoKu technique definitions were checked against the originals on October 23, 2026. The difficulty and grid-size tables are values measured the same day by making 200 puzzles per level with this tool, and the printed grid sizes are calculated for US Letter paper after subtracting 12 mm margins and the header and footer.

• 01Gary McGuire, Bastian Tugemann, Gilles Civario — There Is No 16-Clue Sudoku: Solving the Sudoku Minimum Number of Clues Problem via Hitting Set EnumerationExperimental Mathematics 23(2), 2014, pp. 190-217, DOI 10.1080/10586458.2013.870056. Shows by exhaustive computer search that no 16-clue puzzle exists, so the minimum is 17 clues
• 02Bertram Felgenhauer, Frazer Jarvis — Enumerating possible Sudoku grids (2005, archived PDF)The calculation and method behind the count of 6,670,903,752,021,072,936,960 completed 9×9 grids
• 03Ed Russell, Frazer Jarvis — There are 5472730538 essentially different Sudoku gridsThe number of grids left when relabeling, rotation, reflection and row swaps are counted as the same
• 04Nikoli — 数独 (Sudoku) introduction and rulesThe origin of the name, introduced in 1984 as 数字は独身に限る and shortened to 数独, plus the English page's notes on 'Number Place' and the trademark
• 05HoDoKu — Solving TechniquesDefinitions of Full House, Hidden Single, Naked Single, Locked Candidates, Naked/Hidden Subsets, X-Wing, Swordfish, XY-Wing and XYZ-Wing

Frequently asked questions

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