What is Sudoku? A Plain-English Guide for Beginners

Sudoku is not a maths puzzle. That is the first thing people get wrong about it, and the reason a lot of people decide they don't like sudoku before they've actually tried it. The digits could be apples and oranges, or letters of the alphabet, or any nine symbols you like. The puzzle would be exactly the same.

What sudoku actually is: a logic puzzle on a 9×9 grid. You're given a few starting digits, and your job is to fill in the rest so that every row, every column and every 3×3 box contains the digits 1 through 9 exactly once. No addition. No multiplication. No arithmetic at all.

The grid, the clues, the goal

A sudoku grid is nine cells across by nine cells down, eighty-one cells in total. Inside, thicker lines mark out nine 3×3 boxes. When a puzzle is printed in a newspaper or shown on a screen, some of those cells already have digits in them. Those are the clues, sometimes called the givens.

Everything else is empty. You fill the empties one digit at a time until the grid is complete, never breaking any of the three rules. A well-made puzzle has exactly one valid finish, so there is always a right answer waiting at the end.

The three rules in one breath

Here is the entire rulebook:

• Each row contains the digits 1 to 9, each once.
• Each column contains the digits 1 to 9, each once.
• Each 3×3 box contains the digits 1 to 9, each once.

That is it. There is no fourth rule hiding anywhere. If your puzzle breaks one of those rules, you've gone wrong. If it satisfies all three, you've finished. For a longer walkthrough with examples of what counts as legal, see the rules explained or the formal rules page.

Why digits, if it's not maths?

The choice of 1 through 9 is a historical accident, plus a practical convenience. Sudoku descends from a family of mathematical objects called Latin squares, which the Swiss mathematician Leonhard Euler wrote about in the 1780s. Those used digits or letters interchangeably. When the modern puzzle was published in 1979, the inventor used digits because they're easy to print, easy to read at a glance, and every reader could already tell nine of them apart.

You could absolutely play sudoku with the symbols A through I, or with nine different colours, or with nine little animal icons. Some children's books for early readers do exactly that. The logic is the same.

"Sudoku is a puzzle about constraint. You can do it without numeracy, and you can solve a hard one without ever doing a sum."

Where the difficulty comes from

A puzzle's difficulty is not about how many clues it gives you. It's about what kind of thinking you need to do to finish it. Easy puzzles can be solved by simple scanning. Hard puzzles need patterns that take a while to learn.

If you're brand new, the difficulty labels are mostly a promise about what the puzzle won't ask of you. An easy sudoku won't make you pencil-mark candidates. A medium one might. A hard one almost certainly will. Beyond that, you learn techniques as the puzzles demand them. There is no curriculum to memorise upfront.

How a solve actually feels

A satisfying sudoku solve has a rhythm. You scan, you spot a placement, you make it, that placement opens up another, and so on. On easy and medium puzzles, the cascade does most of the work. You rarely stop to think for more than a few seconds.

On harder puzzles, you'll hit a wall. The easy placements run out. Now you have to look more carefully, sometimes by writing tiny candidate digits in each empty cell. From there, patterns start to appear: a digit that can only go in one place in a box, two cells that together must hold a specific pair, and so on. Most of these patterns have names, and we cover them in the strategies section.

Five things sudoku is not

A few quick myths to set aside, because they keep getting in the way:

• Not maths. No arithmetic, ever. The digits are labels.
• Not a guessing game. A well-made puzzle is fully deductive.
• Not about memory. Everything you need is on the board in front of you.
• Not always Japanese. The name is, but the puzzle itself was invented in Indianapolis in 1979.
• Not just for older people. The audience is everyone from 7 to 97, despite the cliché.

The shape of a typical session

First minute

Look for cells where only one digit fits

Minutes 2 to 5

Place the easy ones, watch the cascade

Around minute 6

Hit a slower patch. Switch to scanning by digit

Last stretch

Pencil-mark the remaining empties if it's harder than easy

Final cells

Place, place, place, done

Why people keep coming back

Sudoku has been a fixture for two decades now, and a fringe pastime for forty years before that. The reasons it persists are simple. It fits in five minutes or it fits in an hour. It doesn't need words, so it travels. It rewards practice. It has a clear win condition. And when you're done, you're done, no notifications, no streaks nagging at you, just a filled grid and a quiet feeling that your brain worked for a few minutes.

If you want to try one, an easy puzzle takes about five minutes once you've read the rules. A medium one is fifteen to twenty. If you'd rather see the whole catalogue of variants and printables first, the variants page and the printables page are both worth a look. Or, if you'd rather walk through the rules first, read this next.