A Sudoku board is a 9x9 grid split into nine 3x3 boxes, and there is only one rule to follow. Each digit from 1 to 9 appears exactly once in every row, exactly once in every column, and exactly once in every 3x3 box. A puzzle starts with some cells already filled, and every remaining cell can be worked out from that single rule alone.

No. The digits are simply nine symbols that have to stay distinct, so replacing them with nine letters or nine colours would leave the puzzle unchanged. You never add, subtract or multiply anything. Solving a Sudoku means ruling out the possibilities that would break the rule until only one option is left.

Start where the board is already crowded. A row, column or box with only two or three blanks left offers far fewer possibilities than an empty one. Then take a single digit and check which rows, columns and boxes already contain it, because those lines rule the digit out everywhere they cross. When a box has only one cell left that the digit can legally occupy, you have found a placement. This method is called scanning, and on an easy puzzle it fills in a large part of the board on its own.

Two techniques cover most easy and medium puzzles. A naked single is a blank cell where every digit except one has been ruled out, so that digit has to go there. A hidden single is a digit that still fits in only one cell of a particular row, column or box, even when that cell looks like it has several options. The two work from opposite ends, one starting from the cell and the other from the digit, and alternating between them clears a board surprisingly fast. On this page, turn on Notes to write candidates into a cell as small digits; they clear themselves when a digit that rules them out is placed nearby.

Pick a difficulty in the lobby and start. Tap a cell, then tap a digit on the keypad or type it; the selected cell's row, column and box are shaded and every copy of the same digit is highlighted so you can scan. Tap the same digit again or press Erase to clear a cell. Notes switches the keypad to pencil marks. Undo steps back one change. With mistake marking on, a wrong digit turns red at once; with it off, you only find out when the board is full. Hint lights up the area to look at and names the technique, and fills the digit only when you press it a second time. Progress is saved on this device, so the lobby offers to continue an unfinished game.

Each puzzle is generated in your browser when you start: a random complete grid is created, then digits are removed in symmetric pairs while a solver checks that exactly one solution remains. The level is then decided by solving the puzzle with human techniques. Easy can be finished with naked singles alone, Normal needs hidden singles, Hard needs locked candidates or naked pairs, and Expert contains at least one step that none of those four can make. The generator keeps trying until it gets the level you asked for, so the label describes the puzzle you actually get rather than a blank count. Everything runs locally, with nothing to install and no account to create.