Sudoku — Free Online Puzzle Game

How to Play Sudoku

1. 1Fill the 9×9 grid so every row, column, and 3×3 box contains the digits 1 to 9 exactly once.
2. 2Click a cell (or use the arrow keys) to select it, then type or tap a number 1–9.
3. 3Use Notes (press N) to pencil in candidates, and Hint if you get stuck.
4. 4Conflicting numbers turn red. Finish the grid with no conflicts to win.

The One Rule Behind Every Solve

Sudoku has a single rule — each digit 1–9 appears once per row, once per column, and once per 3×3 box — and everything else follows from it. There is never any need to guess: a properly made puzzle can always be solved by pure logic, which is exactly why this generator guarantees a unique solution for every board. If you ever feel forced to guess, there is a deduction you haven't spotted yet, not a flaw in the puzzle.

The highlighting is there to help you think. Selecting a cell shades its row, column, and box so you can instantly see which numbers are already committed, and picking a number lights up every other cell holding that digit. Combined with pencil notes, this turns the grid into a working space where the logic becomes visible rather than something you hold entirely in your head.

A Brief History of Sudoku

Although it feels timeless, Sudoku is a modern puzzle. Its direct ancestor, “Number Place,” was designed by the American architect Howard Garns and first published in 1979. It found its true home in Japan in the mid-1980s, where the publisher Nikoli named it sudoku — short for a phrase meaning “the digits must be single.” The global craze arrived in 2004, when a New Zealander named Wayne Gould persuaded The Times of London to run computer-generated puzzles; within a year Sudoku columns had spread to newspapers worldwide. What makes it universal is that it uses no language and no arithmetic — the digits 1–9 are just nine distinct symbols, so a Sudoku puzzle reads the same in every country.

The Rules in Detail

A Sudoku grid is 9×9, divided into nine 3×3 boxes. You must fill every cell so that each of the three kinds of group — every row, every column, and every 3×3 box — contains the digits 1 through 9 exactly once. That is the entire rule set. The starting numbers, called givens or clues, are fixed and cannot be changed. A well-formed puzzle has exactly one solution reachable by logic alone, which is the property this generator guarantees for every board. There is no arithmetic involved at any point — despite the numbers, Sudoku is pure constraint logic, not maths.

Difficulty Levels Explained

Difficulty in Sudoku is not really about how many clues you start with — it is about which solving techniques the puzzle forces you to use. This tool sets four levels by clue count as a practical proxy: Easy (around 45 givens) can usually be solved by simple scanning; Medium (about 36) needs hidden singles and a little note- taking; Hard (about 30) requires candidate elimination and pairs; and Expert (around 26) demands chains of advanced deductions and disciplined pencil marks. Fewer clues generally mean the logic runs deeper before each cell can be proven — but every level here is still solvable without a single guess.

Solving Techniques — Beginner to Expert

Sudoku is a ladder of techniques. You do not need the advanced ones for easy puzzles, but each new method unlocks the next tier of difficulty. Here they are, roughly in order of power.

Naked singles (the starting point)

A naked single is a cell where eight of the nine digits already appear in its row, column, or box — so only one number can go there. Scanning for naked singles, and filling them in, is usually enough to finish an easy puzzle on its own.

Hidden singles (the workhorse)

A hidden single is subtler: within a single row, column, or box, a particular digit can only legally fit in one cell, even though that cell has other candidates too. Hidden singles are the most common breakthrough on medium puzzles — train your eye to ask, “where can the 7 go in this box?” rather than only “what goes in this cell?”

Naked and hidden pairs

When two cells in the same group share exactly the same two candidates, those two digits are locked to those cells — you can remove them from every other cell in that group. A hidden pair is the mirror image: two digits that can only appear in the same two cells, letting you strip away their other candidates. Pairs are where pencil notes stop being optional.

Pointing pairs and box-line reduction

If, within a box, a digit's only candidates all lie in one row or column, then that digit must come from the box — so it can be eliminated from the rest of that row or column outside the box. The reverse (box-line reduction) works too. These interactions between a box and a line crack most hard puzzles.

X-Wing and beyond (expert)

The X-Wing is the gateway to expert solving. When a digit's candidates in two different rows fall in exactly the same two columns, they form a rectangle; the digit must occupy one diagonal pair, which lets you eliminate it from those columns elsewhere. Further up the ladder sit techniques like the Swordfish, XY-Wing, and colouring — but reaching for these is only necessary on the hardest expert boards.

Why Sudoku Is Good for Your Brain

Sudoku is one of the most popular pastimes in the world for a reason beyond fun. It exercises working memory, pattern recognition, and logical reasoning, and it demands sustained, distraction-free focus — a rare thing in a world of notifications. Because a proper puzzle is always solvable by deduction, it rewards patience and careful thinking rather than luck, which makes finishing one genuinely satisfying. Playing a few minutes a day is a light, screen-based way to keep those mental muscles active. For the full ladder of methods with worked examples, read our complete Sudoku solving guide.