Sudoku - Free Online Number Puzzle

About the Sudoku

Sudoku is the world's most popular logic puzzle: a 9×9 grid split into nine 3×3 boxes, partly filled with clues. Your task is to complete it so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. There is no arithmetic and no guessing required — a well-formed Sudoku always has exactly one solution reachable by pure logic. Ours generates a brand-new, uniquely solvable board every time, with Easy, Medium, Hard and Expert difficulties, pencil-mark notes and an optional hint when you are truly stuck.

How to play

1. Pick a difficulty. A fresh grid appears with some cells already filled in.
2. Tap an empty cell, then tap a number (or type 1–9) to place it.
3. Turn on Notes to pencil in candidates; use Erase to clear a cell.
4. Fill every cell correctly — each row, column and 3×3 box needs all of 1 to 9.

What it measures

This test focuses on your working memory, logical reasoning and sustained concentration. An easy Sudoku takes most people 5–10 minutes; a hard one can run 20–30 minutes or more. Speed matters less than a clean, no-mistake solve — that is the real mark of solid technique.

Why play it

• A pure logic workout — no maths, just deduction and pattern-spotting
• Holding candidate numbers in your head exercises working memory
• Endlessly replayable: every puzzle is newly generated and has one solution

Did you know?

• Despite the Japanese name, the modern puzzle was designed by American Howard Garns in 1979 as "Number Place" before Japan's Nikoli popularised it in the 1980s.
• "Sudoku" is short for a Japanese phrase meaning "the digits must be single" — the numbers are just symbols, so the puzzle is not really about maths at all.
• A valid Sudoku has exactly one solution, and it has been proven that at least 17 clues are needed for a puzzle to be uniquely solvable.
• There are 6,670,903,752,021,072,936,960 distinct completed Sudoku grids — but only about 5.5 billion once you account for symmetry.

The 17-clue theorem

A Sudoku needs at least 17 given clues to have a unique solution. That had been suspected for years from the fact that nobody could construct a valid 16-clue puzzle, but it was only settled in 2012, when Gary McGuire, Bastian Tugemann and Gilles Civario proved it exhaustively — roughly 800 core-years of computation, reduced to a week on a supercomputer at University College Dublin.

The total count of valid completed grids is also known exactly: 6,670,903,752,021,072,936,960. Once you factor out symmetries — rotations, reflections, digit relabelling — that falls to 5,472,730,538 essentially different grids.

Not actually Japanese, and not actually about numbers

The modern puzzle was designed by Howard Garns, a retired American architect, and published by Dell in 1979 as Number Place. Nikoli picked it up in Japan in 1984 and gave it the name Sudoku; it only became a global phenomenon after Wayne Gould's computer-generated puzzles reached The Times in 2004.

The digits are arbitrary labels — you could play with nine colours or nine symbols and nothing about the logic would change. No arithmetic is ever required, which is why Sudoku is a constraint-satisfaction puzzle rather than a maths one.