How to Play Sudoku

Sudoku is a number puzzle you solve with logic. You never need to do any math.

The Rules

The grid has 9 rows, 9 columns, and 9 boxes of 3 by 3 squares.
Fill every empty square with a number from 1 to 9.
Each row, each column, and each box must contain every number from 1 to 9 exactly once.
The dark numbers are given at the start and cannot be changed.

Every puzzle on this site has exactly one correct answer. We prove this with a computer before a puzzle is published, so you never need to guess.

Enter a number: tap a square, then tap a number on the number pad. Tap the same number again to remove it.
Notes: turn Notes on to pencil in small numbers you are considering. When you place a number, matching notes nearby are cleared for you.
Erase: clears the selected square.
Undo: takes back your last move. You can press it again and again.
Show mistakes right away: when this is on, a wrong number turns red as soon as you enter it. Turn it off for more of a challenge.
Check: shows any wrong numbers in red and tells you how many squares are left.
Reveal square fills in the selected square. Reveal puzzle shows the whole answer, but the puzzle will not count toward your streak.
Timer: counts how long you have played. It pauses when you leave the page.

Arrow keys move between squares. Number keys 1 to 9 enter a number. Backspace or Delete erases. Press N to switch Notes on or off, and U to undo.

Easy puzzles can be solved by looking for the one place a number can go, or the one number that fits a square.
Medium puzzles also need pairs: two squares in a row, column or box that must hold the same two numbers.
Hard puzzles need more advanced patterns, such as triples and the X-wing. They are still solvable by logic alone.

Pick a number, such as 1, and look at each box. If only one square in a box can hold that number, write it in.
Look for rows, columns or boxes that are almost full. The missing numbers are easy to find.
When you get stuck, use Notes to list the possible numbers in a few squares. A square with only one possible number is ready to fill.