How to play
The grid has 81 cells in 9 rows, 9 columns and 9 boxes of 3 by 3 cells. Some cells are filled in at the start. These clues are shown in bold and cannot be changed. Your job is to fill every empty cell so that each row, each column and each box contains the digits 1 to 9 exactly once. No arithmetic is needed. The digits are just nine different symbols.
Tap a cell, then tap a digit on the number pad, or type it on a keyboard. If the same digit already sits in the cell's row, column or box, both cells turn red. A red pair does not tell you which of the two digits is wrong, only that they cannot both stay. When the last cell is filled and every digit is right, the timer stops and the game shows your time.
The four levels
Each level is defined by two things the generator controls: how many clues the puzzle starts with, and whether the two simplest solving steps are enough to finish it. Those two steps are the naked single, a cell where only one digit is still possible, and the hidden single, a digit that has only one possible cell left in a row, column or box.
| Level | Clues at the start | Can singles finish it? |
|---|---|---|
| Easy | 38 to 42 | Yes |
| Medium | 32 to 35 | Yes |
| Hard | As few as possible, at most 30 | Yes |
| Expert | Minimal, at most 30 | No, at least one harder step is needed |
Medium starts with fewer clues than easy, so there is less to work from at the start. Hard keeps removing clues until one more removal would make singles fail, so every step is still a single, but the singles are harder to see. In our test run of 200 puzzles per level, hard puzzles had 22 to 28 clues. Expert puzzles stop working with singles at some point. Then notes help: write the possible digits into the cells, look for pairs of cells that share the same two candidates, and remove those digits from the rest of the unit.
How the puzzles are made
The page makes a new puzzle on your device each time you press New puzzle. First it fills three boxes on the diagonal with random digits and lets the solver from our Sudoku solver complete the grid. Then it goes through the 81 cells in random order and tries to remove each clue:
- For easy, medium and hard, a clue stays out only if naked and hidden singles can still finish the puzzle. A puzzle that singles can finish has exactly one solution, because every step is forced.
- For expert, a clue stays out only if an exact solver still finds one solution and no second one. When no clue can be removed any more, the puzzle is kept only if singles alone get stuck on it. If they do not, the page starts over with a new grid.
Easy and medium stop removing clues at a random count in their range, so not every puzzle of a level looks the same. Making a puzzle takes a fraction of a second on a computer. Expert puzzles can take a moment longer on an older phone, because the solver runs after every removed clue.
Notes, hints and checking
- Notes: switch on Notes to write small candidate digits into a cell. Fill notes writes all candidates at once, which saves time on expert. Placing a digit removes it from the notes of the cell's row, column and box.
- Highlights: the selected cell's row, column and box are shaded, and cells with the same digit are marked, so you can see at a glance where a digit is still missing. The small number under each key on the pad says how many of that digit are left to place.
- Hint: fills one correct digit. A wrong digit of yours is corrected first. Otherwise the hint picks a cell you could find with a single and explains the reason. On expert, where no single may be left, it fills the cell with the fewest possible digits.
- Check: crosses out wrong digits without showing the right ones. Undo takes back any move, including a hint or a restart.
A worked example
Say the top-left box is missing a 7, and it has three empty cells. One of them is in row 2, which already has a 7 further right. Another is in column 3, which has a 7 further down. The third cell sees no 7 at all. That cell must be the 7, even if other digits would also fit there on their own. This is a hidden single, and it is the step the hint button most often points to. Now look at any empty cell and count the different digits in its row, column and box together. If eight different digits are there, the ninth is the answer: a naked single.
Saving and limits
The game is saved in your browser's local storage after every move, so a reload continues where you stopped. Nothing is uploaded. If local storage is blocked, for example in some private windows, the game still works but starts over on reload. Copy puzzle copies the clues as text, and Open in the solver loads the puzzle into the Sudoku solver, where you can type your own puzzles from a newspaper or book.
How to play sudoku online
- 1Pick a level and press New puzzle. The puzzle appears in a moment and the timer starts.
- 2Tap a cell and a digit on the pad, or use the arrow keys and type. Switch on Notes to write candidates.
- 3Red cells show a clash. Use Undo to step back, Check to find wrong digits, or Hint when you are stuck.
- 4Fill the last cell to finish. Your time is shown and kept as a best time if you used no hint.
Related guides
- How to Solve Sudoku: Techniques From Singles to X-Wing
- Letter Frequency in English and the Best Starting Words
- How Crossword Clues Work: Abbreviations, Tense and Tricks
Frequently asked questions
Does every puzzle have exactly one solution?
Yes. Each puzzle is made on your device from a full random grid. Clues are taken out one by one, and a clue only stays out if the puzzle still has a single solution. For easy, medium and hard this is guaranteed because the puzzle can be finished with naked and hidden singles alone. For expert, an exact solver counts the solutions after every removed clue.
What makes expert harder than hard?
Hard puzzles have few clues, but you can always find the next digit with a naked or a hidden single. Expert puzzles reach a point where neither step works, so you need notes and a harder technique, or a careful guess. Expert puzzles are also minimal: take away any one clue and the puzzle would have more than one solution.
How do pencil notes work?
Press Notes or the N key, then tap digits to write small candidates in the selected cell. Tap a digit again to remove it. When you place a digit, the same digit is removed from the notes in its row, column and box, unless you turn that option off. Fill notes writes every digit that does not clash yet into all empty cells.
Is my game saved?
Yes, in this browser only. The puzzle, your digits, your notes, the time and the hints used are written to local storage after every move, so a reload or a closed tab continues where you stopped. Nothing is sent to a server. A private window or clearing your browser data removes the saved game.
What do Hint, Check and Reveal do?
Hint fills one cell with the correct digit. If one of your digits is wrong, it corrects that one first; otherwise it picks a cell you could find with a single and says why. Check crosses out every wrong digit without showing the right one. Reveal fills in the whole solution and ends the game.
Can I play with a keyboard?
Yes. Click the grid or press Tab to reach it, move with the arrow keys, type 1 to 9 to place a digit and Backspace, Delete or 0 to clear a cell. N switches notes on and off, and Ctrl+Z or Cmd+Z undoes the last move. Each cell has a label with its row, column and content for screen readers.
When does the timer stop?
The timer runs while the puzzle is open and the tab is visible. It pauses when you switch to another tab or app and goes on when you come back. It stops for good when the grid is solved or the solution is revealed. Best times are kept per level, and only for puzzles solved without a hint.