About Sudoku
Sudoku is a number placement puzzle on a 9 x 9 grid divided into nine 3 x 3 boxes. Some numbers are given, and you fill in the rest so that each row, column and box contains the digits 1 to 9 exactly once. Every puzzle here is generated fresh and checked to have a single solution.
How to play
- Each row, each column and each of the nine 3 x 3 boxes must contain every digit from 1 to 9 once.
- Given numbers are shown in bold and cannot be changed. You fill in the empty cells.
- There are three levels. Easy aims for 38 given numbers, Medium for 30 and Hard for 25. The generator never removes a number if doing so would allow a second solution, so a puzzle can end up with a few more clues than the target.
- Your entries are checked against the solution as you type. A wrong number turns red and adds one to the Mistakes counter, but it stays on the board until you change or erase it.
- The puzzle is solved when every cell matches the solution. The timer runs from the moment the puzzle appears.
- Score is 1000 (Easy), 2000 (Medium) or 3000 (Hard), minus 2 points per second and 50 points per mistake, with a minimum of 10.
Controls
- Mouse or touch: click a cell to select it, then press a number on the pad below the grid. The erase button clears the cell.
- Keyboard: once a cell is selected, type 1 to 9 to fill it, and Backspace, Delete or 0 to erase. The arrow keys move the selection.
- Choosing a level from the menu starts a new puzzle at that level.
Tips and strategy
- Look for hidden singles first: pick a digit and a box, and check where that digit could go. If only one cell in the box is possible, it goes there.
- Use the highlights. Selecting a cell shades its row and column, and every cell holding the same number lights up, which makes it quick to see where a digit is still missing.
- Look for naked singles: a cell whose row, column and box together already contain eight different digits can only hold the ninth.
- Since wrong numbers count against your score even if you fix them, only enter a digit when you are sure. Work out uncertain cells on paper or in your head first.
- On Hard, look for pairs: if two cells in a row can only hold the same two digits, those digits can be ruled out from the rest of that row.
Background
The modern puzzle was created by American architect Howard Garns and first published in 1979 by Dell Magazines under the name Number Place. In the 1980s the Japanese puzzle publisher Nikoli introduced it in Japan and gave it the name Sudoku, a shortening of a phrase meaning that each digit must appear only once. It became an international craze in 2004 and 2005 after British newspapers began printing it daily.
Questions
- Does every puzzle have exactly one solution?
- Yes. The generator checks the puzzle after removing each clue and puts the clue back if more than one solution becomes possible.
- Can I make pencil marks?
- No. Each cell holds one number at a time, so keep candidate lists in your head or on paper.
- Does correcting a mistake remove it from the count?
- No. Every wrong digit you enter adds to the Mistakes counter, even if you erase it straight away, and each one costs 50 points.