Sudoku — Generated Fresh, With One Solution Guaranteed
Sudoku has one rule: every row, every column and every three-by-three box holds the digits 1 to 9 once each. Everything else about the game — the scanning, the pencil marks, the moment a box suddenly resolves — follows from that single constraint.
Every puzzle on this page is generated the moment you ask for it rather than pulled from a stored library, so "new puzzle" never hands you one you have already solved. Generation starts from a complete valid grid and removes numbers one at a time, keeping a removal only if the puzzle still has exactly one solution. That check is the important one. A puzzle with two solutions is not a hard sudoku, it is a broken one: you can fill every square legally and still be told you are wrong. Nothing here can do that.
The three settings differ in how many numbers you start with — around 40 on easy, 32 on medium, and as few as 25 on hard. Those counts translate into genuinely different work. Every easy grid here can be finished with the two simplest techniques, single-candidate cells and single-position digits, in whatever order you spot them. Medium grids nearly always can — about one in sixteen needs more. Hard grids are the split: roughly half of them need something beyond singles, such as a pair of cells that between them own two digits, or a digit whose only homes in a box all sit in one row.
Numbers go in by tapping a square and then a number, or with the keyboard — arrow keys to move, 1 to 9 to place, backspace to clear. The number pad greys out a digit once all nine are placed, which is the count every paper solver keeps in their head anyway. A duplicate in any row, column or box is always shown in red, because that is just the rule being visible rather than a hint. What is optional is "flag wrong digits", which compares what you have typed against the solution; it is off by default here, since being told a digit is wrong the moment you place it removes most of the deduction. Turn it on if you would rather catch a mistake than unwind fifteen minutes of work built on it.
The timer runs from your first entry and stops when the grid is complete and clean. "Reveal a cell" fills the square you have selected, or the first empty one if you have not selected any, and the count of reveals you used is shown at the end next to your time — a solve with help is still worth having, it is just worth knowing about.
How to play Sudoku
- Pick Easy, Medium or Hard. Switching difficulty deals a new grid.
- Tap an empty square, then tap a number from the pad below the grid.
- Or use the keyboard: arrow keys to move, 1–9 to place, backspace to clear.
- Duplicates in a row, column or box turn red. Tick "flag wrong digits" to also catch numbers that clash with nothing yet but cannot be right.
- Stuck? "Reveal a cell" fills one square from the solution and counts it.
- Fill the grid with no clashes and the puzzle is solved — your time is on the card at the end.