About Sudoku
Sudoku is a 9x9 grid split into nine 3x3 boxes. Some cells start with a number, and your job is to fill the rest so that every row, every column and every box contains the digits 1 to 9 exactly once. There is no arithmetic involved. The numbers are just nine different symbols, and the whole puzzle is about placement and elimination.
We built our version around one promise: every puzzle has exactly one solution. Each grid is generated fresh when you start a game. Our generator fills a random complete grid, then removes clues in symmetric pairs. After every removal it checks the puzzle again, and if a removal would allow a second solution, that clue goes back. You never have to guess between two equally valid answers, and every number can be found by reasoning.
We also grade each puzzle by the techniques it actually needs, not only by how many clues it shows:
| Level | Clues | What it takes to solve |
|---|---|---|
| Easy | about 38 | Naked and hidden singles only |
| Medium | 27–33 | Still singles only, but fewer clues and longer searches |
| Hard | usually 24–32 | Needs at least one pair, triple, pointing pair or box/line claim |
| Expert | usually 24–32 | Needs an X-Wing, Swordfish or XY-Wing |
Our grader runs the same logic a person would use. A Hard puzzle cannot be finished with singles alone, so if you stall there, that is expected. It is time to look for pairs. When we generated hundreds of puzzles in testing, almost every Expert grid came out solvable with those three patterns. A rare one needs a longer chain.
How to play
- Select a cell by tapping or clicking it. Its row, column and box get a soft highlight, and every cell holding the same number is highlighted more strongly.
- Enter a number with the number pad or the 1–9 keys. Tapping the same number again clears the cell. The small figure under each pad button shows how many of that digit are still missing. When all nine are placed, the button fades out.
- Take notes. Press the Notes button (or N) to switch into pencil mode. Numbers you enter then appear as small candidates in the corner grid of the cell. Press it again to go back to normal entry.
- Auto-remove notes. When you place a digit, that digit is automatically removed from the notes in the same row, column and box. You can turn this off in the menu if you prefer to manage every mark yourself.
- Mistakes. With “Show mistakes” on (the default), a digit that doesn’t match the solution turns red and the Mistakes counter goes up. Turn it off in the menu for a stricter, paper-style game where nothing tells you you’re wrong until the grid won’t close.
- Undo reverses your last entry, erase or note change, including any notes that were auto-removed.
- Hints. You get three per puzzle. A hint first fixes a wrong entry if you have one. Otherwise it fills the selected cell, or picks the open cell with the fewest possible candidates, which is usually the next logical move.
When the last correct digit goes in, the board ripples and your time is compared against your best for that difficulty. The timer stops whenever the game is paused or the tab is hidden. Your puzzle, notes and time are saved automatically, so you can close the page and pick up where you left off.
Strategy tips
Scan for singles before you write notes
On Easy and Medium, you can often finish without a single pencil mark. Pick a digit, say 7, and look at each box that doesn’t have one. The 7s already placed in nearby rows and columns block cells in that box. If only one cell is left, that’s your 7. The number pad counts help here. Digits with only one or two copies left are the fastest to finish.
Use the same-number highlight as a scanning tool
Select any cell containing a 4 and every 4 on the board lights up, so the rows and columns they block are easy to see. In testing, this cut our own solve times more than anything else.
Note in bulk, then prune
On Hard and Expert, switch to notes mode and fill in candidates for one box or one row at a time. With auto-remove on, every digit you place later cleans up the notes around it. That keeps the grid honest without extra work.
Look for pairs when singles dry up
If two cells in the same row, column or box share the same two candidates and nothing else, those two digits are locked into those cells. Remove them from every other cell in that unit. Hard puzzles are built so that you need this kind of step at least once.
Check boxes against lines
If all the candidates for a digit inside one box sit in a single row, that digit must go in that row within the box. You can delete it from the rest of the row. The reverse works too: if a row’s only spots for a digit are inside one box, clear that digit from the rest of the box.
Save your hints for real dead ends
A hint on a random empty cell tells you one number. A hint when you’re truly stuck usually unlocks a whole chain of singles.
We cover pairs, pointing pairs and X-Wings step by step in our guide, Sudoku techniques from beginner to expert.
FAQ
Do the puzzles repeat?
No. Every puzzle is generated when you press a difficulty, from a random complete grid and a random removal order. The same grid showing up twice is extremely unlikely.
Can a puzzle ever need guessing?
Easy, Medium and Hard puzzles are checked by our logic solver before you see them, so none of them need guessing. Expert puzzles are chosen to need an X-Wing, Swordfish or XY-Wing. On rare occasions the generator falls back to one that needs a longer chain, but it still has a single solution.
Does using hints or making mistakes affect my best time?
Your best time is simply the fastest solve for each difficulty. The win screen tells you how many hints you used and how many mistakes you made, so you can hold yourself to a stricter standard.
Where is my progress stored?
In your browser’s local storage on this device. Clearing site data resets your saved puzzle, best times and solve counts.
