Sudoku

How Sudoku Works

Sudoku is a logic puzzle on a 9x9 grid divided into nine 3x3 boxes. The single rule: fill every empty cell so that each row, each column and each 3x3 box contains the digits 1 through 9 exactly once. No arithmetic is involved — the digits are labels, and nine letters or nine colours would play the same puzzle. A properly made Sudoku has exactly one solution.

The board starts with some digits already placed. Easy puzzles give around 44; Expert puzzles give around 24. Fewer givens usually means a longer chain of reasoning before the next digit falls, but where they sit matters as much as how many there are. Select a cell, type a digit (or tap the number pad on mobile), and the cell fills in. Pressing the same digit again removes it. Notes mode lets you jot small candidate digits into a cell as a reminder of what is still possible there — they are bookkeeping, not part of the answer. Auto Notes fills every empty cell with its remaining candidates in one press. You get three Hints per puzzle, each revealing the correct digit for the selected cell.

A good starting move is to pick the digit that already appears the most, then scan its rows, columns and boxes to find where the last copies must go. When that runs dry, look for the row, column or box closest to full — fewer blanks mean fewer digits to sort out. These two habits carry most of Easy and Medium. Beyond that, named techniques like Naked Pairs, Pointing Pairs and X-Wings exist for harder grids; they all boil down to asking the same rule a more targeted question. If a wrong digit goes in and Auto-Check is on (the default), the cell is flagged immediately — three mistakes end the game, though the Continue option lets you resume the same grid with the counter reset. Your puzzle, statistics and best times are saved in this browser automatically.