Sudoku

How to play Sudoku

A Sudoku grid has 81 cells split into nine 3×3 boxes. Some cells start with a number, and the goal is to fill the rest so every row, every column and every box holds the digits 1 to 9 once. Select a cell, then pick a number on the pad or type the number on a keyboard. Arrow keys move the selection, N switches notes on and off, Backspace clears a cell and Ctrl+Z undoes the last move.

Difficulty levels

Easy puzzles start with about 40 given numbers, medium with 33, hard with 28 and expert with 24 or fewer. Fewer clues mean longer chains of reasoning before the next safe number shows up. Every puzzle comes from a generator with a backtracking solver, and the solver confirms each puzzle has exactly one solution, so logic alone always reaches the end without guessing.

Notes, hints and checking

Notes mode writes small candidate numbers into a cell, and placing a number removes the same candidate from the row, column and box. Red numbers break a rule, such as a duplicate in a row. Check marks every filled cell with a wrong number, and Hint fills the selected cell, or a random empty one, with the right answer. The game saves after every move in this browser, so a reload continues the same puzzle.