Sudoku Puzzles
Nine rows, nine columns, nine boxes, and the digits 1 to 9 once in each. Every grid here is generated fresh, proved to have exactly one answer, and proved solvable without a single guess — then explained square by square when you want to know why.
Also known as number place, the name it was first published under in 1979.
Related Practice
The Three Rules, And Nothing Else
A sudoku gives you a nine by nine grid carrying some digits already. Fill in the rest so that every row holds 1 to 9 once each, every column holds 1 to 9 once each, and every three by three box holds 1 to 9 once each. That is the whole specification. There is no arithmetic, nothing is concealed, and no fact from outside the grid is ever needed.
It follows that the digits are not really numbers. They are nine distinct symbols, never added or compared, and the puzzle would be identical printed in nine colours. People who say they are “bad at maths” are turning down a puzzle that contains none.
The Only Two Moves You Need
Every square on this site can be settled by one of two deductions. Knowing which one you are looking for is most of the skill, because they ask you to look at the grid in opposite directions.
| Move | What you look at | What settles it |
|---|---|---|
| Naked single | One square | Cross off every digit already in its row, column and box. One digit survives. |
| Hidden single | One digit | Walk a row, column or box asking where that digit can still go. One square remains. |
Most people find the naked single first because it needs no plan — you simply stare at a square until it gives up. The hidden single is the one worth practising: it requires you to hold a digit in mind and sweep the grid, and it is what breaks a puzzle open when every square still has two or three candidates. When you ask for a hint here, the working names which of the two it used and which row, column or box did the forcing.
Clue Count Is A Poor Measure Of Difficulty
Sudoku is almost always sold by the number of printed digits, and that number is a weak signal on its own. Measuring our own generator across many grids turned up three at the same 24 clues that needed 1, 9 and 22 hidden singles respectively. The first is a gentle puzzle and the third is a hard one, and nothing on the surface distinguishes them.
So every family here declares two things: how many digits it prints, and the minimum share of the solve that must be hidden singles. A grid that fails either is discarded rather than shipped under a name it has not earned.
| Family | Clues printed | Hidden singles | Level |
|---|---|---|---|
| Learning the grid | about 34–37 | none required | Easy |
| Steady grids | about 31–32 | up to a twentieth | Easy |
| Classic symmetry | about 29–30 | a tenth or more | Medium |
| Scanning practice | about 28–30 | a quarter or more | Medium |
| Fewer givens | about 26–28 | a fifth or more | Medium |
| Digit hunting | about 25–26 | a third or more | Hard |
| Lean grids | about 24–25 | a quarter or more | Hard |
| Sparsest here | about 24 | a third or more | Hard |
Seventeen, And Why These Grids Print More
The fewest clues a sudoku can carry and still have exactly one answer is seventeen. That is not folklore: it was settled in 2012 by Gary McGuire, Bastian Tugemann and Gilles Civario, who proved by exhaustive computer search that no sixteen clue sudoku exists.
The leanest grids here print about 24, and the gap is worth explaining rather than hiding. A seventeen clue grid is uniquely solvable, but not by elimination and scanning — it needs techniques like pointing pairs and box–line reduction, and past those, chains that amount to controlled trial and error. This site promises every grid can be talked through move by move, so it only ships grids that genuinely can be. Twenty four is where that promise runs out, not where the puzzle does.
Why Symmetry Costs A Few Clues
A printed sudoku almost always has a grid pattern with rotational symmetry: every given digit has a partner the same distance the other side of the centre square. It is purely visual — symmetry gives a solver no help at all — but it is what makes a grid look designed rather than scattered, and its absence is noticeable.
It has a measurable price. Because clues have to be removed in mirrored pairs, a symmetric grid cannot be stripped as far: ours settle around 28 clues where a freely stripped grid reaches 24. The gentler families here are symmetric for the look of them, and the leanest are not, because at that end the extra difficulty is worth more than the tidiness.
How The One Answer Is Proved
Two separate checks run before any grid reaches you, and they answer different questions.
The first is a solver restricted to the two moves above. If it cannot finish unaided, the grid is thrown away — that is what makes “no guessing” a checked property rather than a slogan. The second is an exhaustive search that counts solutions and stops at two; a grid with a second answer is discarded, because a puzzle with two answers marks a correct solver wrong.
The test suite then refuses to take the generator's word for any of it. It rebuilds the nine boxes from arithmetic, re-checks the finished grid against the three rules directly, and counts the solutions again with a plain backtracking search written in the test file that shares no code with the site. Across 480 grids the two routes agreed every time, and the recorded walkthrough accounted for every empty square in all of them.
Sudoku Puzzles Frequently Asked Questions
How do you solve a sudoku puzzle?
Two moves account for every square on this site. A naked single is a square with one candidate left once you cross off what its row, column and box already hold. A hidden single is a digit with one square left in a row, column or box — you find it by scanning for the digit rather than eliminating for the square. Work boxes first, then rows and columns. Every grid here is proved solvable by those two moves alone, so if you are stuck there is always a real next move and the hint will name it.
What are the rules of sudoku?
Three rules, and no others. Every row holds the digits 1 to 9 once each. Every column holds 1 to 9 once each. Every three-by-three box holds 1 to 9 once each. There is no arithmetic, nothing is hidden, and no outside knowledge is needed — everything you require is printed on the grid.
Is sudoku maths?
No. The digits are just nine distinct symbols and nothing is ever added, multiplied or compared. Replace them with nine colours or nine letters and the puzzle is identical. What sudoku actually trains is elimination and scanning, which is reasoning rather than arithmetic. Kakuro and killer sudoku are the grids on this site that genuinely involve sums.
What is the fewest clues a sudoku can have?
Seventeen. That no sudoku with sixteen clues exists was settled in 2012 by Gary McGuire, Bastian Tugemann and Gilles Civario, who searched the space exhaustively by computer. Grids that lean are far harder than anything solvable by elimination and scanning alone, which is why the leanest here print about 24.
Do I need to guess at any point?
Never, and that is checked rather than claimed. Before a grid is shown to you it is solved twice: once by a solver limited to the two moves a person actually uses, and once by an exhaustive search that confirms there is exactly one answer. A grid that fails either test is thrown away and never reaches the page.
What makes one sudoku harder than another?
Not the number of clues on its own. Grids with the same clue count can differ enormously: our own measurements found three 24-clue grids needing 1, 9 and 22 hidden singles respectively. The share of hidden singles is the better signal, because scanning for where a digit belongs is the move most people find harder, so every family here declares one.
Is it free, and do I need an account?
Free with nothing held back, and no account. There is no sign-up, no puzzle limit and nothing to install. Every grid is generated in your browser at the moment you load the page, so nothing you do is sent anywhere.