What are the rules of Sudoku?

Fill the nine-by-nine grid so that every row, every column and every three-by-three box contains the digits 1 to 9 exactly once. There is no arithmetic involved — the digits are just nine distinct symbols.

Does Sudoku require guessing?

No. Every puzzle here has a unique solution reachable by logic alone. If you feel the need to guess, there is a deduction you have missed — most often a single-position scan asking where a digit can go in a box.

What is the naked pair technique?

If two squares in the same row, column or box have exactly the same two candidates, those two digits must occupy those two squares in some order. You can then eliminate both digits from every other square in that unit.

How many clues does each difficulty give?

Roughly 35, 28 or 23 depending on the level. The count is approximate, because the generator aims for a target while keeping the puzzle valid and uniquely solvable.

What is the symmetry option for?

It makes the starting clues rotationally symmetric about the centre — the traditional aesthetic from Japanese puzzle magazines. Hard puzzles cannot always be both symmetric and valid, so the option can be switched off.

What is the minimum number of clues a Sudoku can have?

Seventeen. This was proven in 2012 by an exhaustive computer search led by Gary McGuire — no valid puzzle with a unique solution exists with sixteen or fewer.

Sudoku

Sudoku contains no luck, no hidden information and nothing to guess. Every square has a determinable answer, which means being stuck is always a fact about you rather than about the puzzle.

Click a square and type or tap a number. Use pencil marks to note candidates; hold Shift to force a small mark.

Players	1
Deck	9×9 grid, digits 1–9
Play time	5–25 minutes depending on difficulty
Difficulty	Three levels — roughly 35, 28 or 23 starting clues

How to play Sudoku

The grid is nine by nine, divided into nine three-by-three boxes. Some squares are filled in at the start — those are the clues. Your job is to fill the rest so that three conditions hold simultaneously:

Every row contains the digits 1 to 9, each exactly once.
Every column contains the digits 1 to 9, each exactly once.
Every three-by-three box contains the digits 1 to 9, each exactly once.

That is the entire rule set. There is no arithmetic — the digits are simply nine distinct symbols, and the puzzle would work identically with nine colours or nine letters.

Controls

Click a square and enter a number, either from the on-screen keypad or your keyboard. With Multinumber on you can place several candidate numbers in one square as pencil marks, shown small. To force a single small mark — a number you suspect but have not confirmed — hold Shift while pressing the digit.

Difficulty and symmetry

Three levels give roughly 35, 28 or 23 clues. Fewer clues means more work, though the count is approximate: the generator gets as close to the target as it can while keeping the puzzle valid. You can also choose whether the clues are rotationally symmetric — the traditional aesthetic, rotated 180 degrees about the centre. Easy and medium puzzles are almost always symmetric; hard ones sometimes cannot be, so the option can be turned off.

Solving techniques

Sudoku is a ladder of techniques, and most puzzles fall to the first two or three.

Scanning: the single candidate

Take an empty square and eliminate every digit already present in its row, its column and its box. If exactly one digit survives, it goes there. On easy puzzles this alone will solve the grid, and it is the technique to exhaust before trying anything cleverer.

Scanning the other way: the single position

Instead of asking "what can go in this square", ask "where can this digit go in this box". Often a digit is excluded from eight of a box's nine squares by the rows and columns crossing it, which places it immediately. Beginners rarely think this way round, and it is frequently faster.

Pencil marks, used sparingly

Filling every square with every candidate is exhausting and mostly unnecessary. Mark candidates only in the region you are actively working on — a single box, or a row you suspect is nearly determined. Pencil marks are a working tool, not a bookkeeping obligation.

Naked pairs

If two squares in the same row, column or box both have exactly the same two candidates — say 4 and 7 — then those two digits occupy those two squares in some order, and 4 and 7 can be eliminated from every other square in that unit. This is the first genuinely non-obvious technique, and it cracks most medium puzzles.

Never guess

Every puzzle here has a unique solution reachable by deduction. If you find yourself about to guess, there is an inference available that you have not found — usually a single-position scan you skipped. Guessing works occasionally and teaches you nothing, and a wrong guess propagates errors that are very hard to unpick.

Variants

Killer Sudoku — cages of squares with a target sum, adding arithmetic to the logic.
Jigsaw (Squiggly) Sudoku — the nine boxes are irregular shapes rather than 3×3 blocks.
Samurai Sudoku — five overlapping grids solved together.
Hyper Sudoku — four extra shaded regions that must also contain 1 to 9.
Sizes other than nine — 4×4 grids for children, 16×16 for people with too much time.

Where Sudoku came from

The mathematical basis is older than the puzzle. Leonhard Euler studied Latin squares — grids where each symbol appears once per row and column — in the eighteenth century, though without the box constraint that makes Sudoku work.

The modern puzzle was published in 1979 by Howard Garns, a retired architect from Indiana, in Dell Pencil Puzzles and Word Games under the name Number Place. It attracted no particular attention. In 1984 the Japanese publisher Nikoli picked it up, named it sūji wa dokushin ni kagiru — "the digits must be single" — and shortened that to Sudoku. It became a staple of Japanese puzzle magazines, and Nikoli added the convention of symmetric clues.

Its global explosion has a specific cause. Wayne Gould, a retired New Zealand judge, spent six years writing a program to generate puzzles and then persuaded The Times of London to run one in November 2004. Within months it was in newspapers worldwide and 2005 became, by common agreement, the year of Sudoku. In 2012 mathematicians led by Gary McGuire proved by exhaustive computation that no valid Sudoku puzzle can have fewer than 17 clues.

Questions people ask about Sudoku

What are the rules of Sudoku?: Fill the nine-by-nine grid so that every row, every column and every three-by-three box contains the digits 1 to 9 exactly once. There is no arithmetic involved — the digits are just nine distinct symbols.
Does Sudoku require guessing?: No. Every puzzle here has a unique solution reachable by logic alone. If you feel the need to guess, there is a deduction you have missed — most often a single-position scan asking where a digit can go in a box.
What is the naked pair technique?: If two squares in the same row, column or box have exactly the same two candidates, those two digits must occupy those two squares in some order. You can then eliminate both digits from every other square in that unit.
How many clues does each difficulty give?: Roughly 35, 28 or 23 depending on the level. The count is approximate, because the generator aims for a target while keeping the puzzle valid and uniquely solvable.
What is the symmetry option for?: It makes the starting clues rotationally symmetric about the centre — the traditional aesthetic from Japanese puzzle magazines. Hard puzzles cannot always be both symmetric and valid, so the option can be switched off.
What is the minimum number of clues a Sudoku can have?: Seventeen. This was proven in 2012 by an exhaustive computer search led by Gary McGuire — no valid puzzle with a unique solution exists with sixteen or fewer.