How to play Sudoku

Sudoku is played on a 9 × 9 grid, split into nine 3 × 3 boxes. Some cells already hold a digit when the puzzle starts. These are the givens, and they can’t be changed.

Your job is to fill every empty cell with a digit from 1 to 9 so that:

every row holds each digit from 1 to 9 exactly once,
every column holds each digit from 1 to 9 exactly once,
every 3 × 3 box holds each digit from 1 to 9 exactly once.

Every puzzle here has exactly one solution, and you can reach it by reasoning alone. You never need to guess.

Controls

Tap a cell to select it, then tap a digit on the number pad. On a computer you can also use the keyboard: the arrow keys move around the board, 1 to 9 place a digit, and Backspace erases it.

Notes switches notes mode on and off. In notes mode a digit is written small inside the cell, as a reminder of what could go there. On a keyboard, Shift plus a digit writes a note without switching modes.
Fill notes writes every possible digit into every empty cell in one go.
Undo takes back your last change, as far back as you want.
Hint shows you the next step and why it works, one step at a time. You have three hints per puzzle.

A digit that repeats in its row, column or box turns both cells red. Wrong digits are shown in red too; you can switch that off in Settings if you would rather find them yourself. There is no mistake limit.

Your puzzle is saved as you play, so you can close the page and carry on later. Your best time for each level is kept on this device.

Five techniques to start with

Scan a box for one digit. Pick a digit, say 4, and a box that has no 4 yet. Look at the 4s in the rows and columns that cross that box. Each one rules out a whole line of the box. If only one empty cell is left, the 4 goes there.
Find the last digit for a cell. Look at the digits already in an empty cell’s row, column and box. If eight different digits are there, the ninth is the answer.
Finish nearly full lines first. A row with seven digits has two gaps and two missing digits. Check the columns of the two gaps. If one missing digit already appears in the column of one gap, it must go in the other gap.
Keep notes on harder puzzles. When scanning stops working, write the possible digits into each cell. Every digit you place disappears from the notes of the cells that share its row, column or box, and new answers often appear.
Look for pairs. If two cells in the same row can only hold 3 and 8, then 3 and 8 belong to those two cells, even before you know which is which. You can cross 3 and 8 off the notes of every other cell in that row. The same works in a column or a box.

Which level to choose

Easy puzzles need only the first two techniques. Medium adds pairs, and Hard needs triples and the way a box and a line limit each other. Expert puzzles call for patterns such as the X-Wing, where a digit can only sit in the same two columns of two different rows. New to Sudoku? Start with Easy and move up when it feels quick.