One rule across three kinds of group
Fill the empty squares with 1 through 9. Each row, each column and each outlined 3×3 box must contain every digit once. A number that fits a row can still conflict with its column or box. Start by checking all three groups that touch the square, not just the nearest visible numbers.
Enter numbers without fighting the page
Choose Start, select a square and use the number pad beneath the grid. A computer keyboard also accepts 1–9; arrow keys move the selection and 0 clears it. Fixed starting clues cannot be replaced. On a phone, tap the square before its number so your input goes to the intended place.
Easy, medium or hard?
The three difficulty controls generate a new puzzle, so finish your current attempt before changing level. This implementation checks that a generated puzzle has one solution. Difficulty labels are practical presets rather than a promise of a particular advanced technique. Easy is the most useful starting point when you are learning to scan rows and boxes.
Try singles before guessing
Look for a square with only one possible digit, or a digit that can fit in only one square of a row, column or box. Filling that square changes the neighbouring possibilities. Repeat the scan after each certain placement. A fast guess can leave a plausible-looking but inconsistent board; a slower deduction is easier to check.
Timer, best times and the version on this page
This is the MIT-licensed Web Games Sudoku by mashukui. It generates repeatable new attempts without an account and records best times locally when storage is available. It does not promise cloud-saved unfinished grids. Use fullscreen for a larger board and the catalogue heart to keep the page handy; try Minesweeper for another deduction puzzle afterwards.
