How to play Sudoku
The grid has 81 cells in nine rows, nine columns and nine 3×3 boxes. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each box holds every digit exactly once. The dark digits are the givens and can't change. Every puzzle here has exactly one solution, so each digit can be worked out by logic: there is never a point where you have to guess.
- Pick a level (Easy, Medium, Hard or Expert) or the Daily puzzle.
- Tap a cell, then a number. On a keyboard, move with the arrow keys and type 1 to 9. The row, column and box of the cell light up, and so does every copy of the same digit, which makes gaps easy to spot.
- Pencil in candidates. Turn on Notes (or press N, or hold Shift while typing a digit) to jot small digits in a cell. When you place a digit, it disappears from the notes of every cell it rules out. Fill notes writes every candidate into every empty cell in one go.
- Undo freely. Undo and Redo go back as far as you like, and your game, notes and time are saved on this device if you close the tab.
The timer starts with your first tap and pauses by itself when you switch tabs or scroll the board out of view. Pause hides the board, so a break doesn't give you free thinking time.
How the levels are graded
A puzzle with fewer givens isn't always harder. What makes a Sudoku hard is the kind of reasoning it demands, so every puzzle is solved here the way a person would, always trying the simplest technique first, and gets the level of the hardest step it needed:
- Easy: hidden singles only. Look at a box, row or column and find the one cell where a digit still fits. Easy puzzles keep 35 to 40 givens.
- Medium: also naked singles, cells where only one digit is left once you check their row, column and box.
- Hard: at least one pointing pair, box-line reduction, naked pair or hidden pair. Notes help from here on.
- Expert: at least one naked or hidden triple, X-Wing, XY-Wing or Swordfish.
Solving techniques in brief
Hidden single. In a row, column or box, a digit has only one cell left where it can go. Naked single. A cell has only one digit left that its row, column and box don't already hold. Every puzzle starts with these, and Easy and Medium puzzles need nothing else.
Harder levels add patterns that rule out candidates until a single appears again. Hard Sudoku explains the pointing pair, box-line reduction and pairs, with a routine for finding them in your notes; Expert Sudoku covers the XY-Wing, X-Wing, triples and Swordfish.
Hints, the mistake checker and the daily puzzle
A hint shows the next digit you can find by logic and explains why, step by step, highlighting the cells involved: “Only place for 7 in row 5”, or on harder puzzles a pointing pair first and then the single it leads to. If there is a wrong digit on the board, the hint points to it before anything else. The mistake checker marks wrong digits in red as you place them and counts them; with it off, only two copies of a digit in one unit are flagged.
The daily puzzle is built from the date on your device, so everyone who plays on the same day gets the same grid. It climbs from Easy on Monday to Expert on Saturday. Solve it on consecutive days to grow your streak, and when you finish, share your time: the link opens the same puzzle for your friend, with your time to beat.
Did you know?
There are 6,670,903,752,021,072,936,960 ways to fill a Sudoku grid (OEIS A107739), and a puzzle needs at least 17 givens to have only one solution: in 2012 McGuire, Tugemann and Civario searched every grid by computer and found no valid 16-clue puzzle (arXiv:1201.0749).
Want a break between puzzles? Roll some 3D dice, test your reaction time, or print graph paper for your own grid puzzles. The online timer works for a timed Sudoku race at a table, too.