Sudoku

About Sudoku

Sudoku is a number placement puzzle on a 9 by 9 grid split into nine 3 by 3 boxes. Some squares start filled in. Your job is to fill the rest so that every row, every column and every box holds each digit from 1 to 9 exactly once. There is no arithmetic in it. The digits could just as well be nine colours; it is pure logic.

Every Sudoku on this page is made fresh in your browser the moment you ask for one. The program first builds a complete, valid grid at random. Then it removes digits one at a time, and after each removal it runs a solver to check that the puzzle still has only one answer. If taking a digit out would allow a second solution, the digit goes back. That is why every puzzle here can be solved by reasoning alone, with no guessing required. Easy leaves 40 clues, Medium 32 and Hard 26.

How to play Sudoku

1. Pick a level and press New puzzle. The clock starts with your first entry.
2. Select a square by clicking or tapping it, or move to it with the arrow keys.
3. Type a digit from 1 to 9, or tap it on the number pad. Enter the same digit again, or press Erase, 0 or Backspace, to clear it.
4. Turn on Pencil (or press N) to jot small candidate digits in a square instead. Placing a real digit removes that digit from the pencil marks in its row, column and box.
5. A digit that clashes with another in its row, column or box turns red. Undo (or U) steps back one entry.
6. Fill the whole grid with no clashes to finish. Your fastest time on each level is kept in this browser.

Techniques that solve most puzzles

Start with scanning. Pick a digit, say 5, and look at each box. If the 5s already placed in nearby rows and columns rule out every square in a box but one, that square must be a 5. This is called a hidden single, and it cracks most Easy puzzles on its own. The reverse also works: if a square can only take one digit because the other eight already appear in its row, column or box, write it in. That is a naked single.

Medium and Hard puzzles need pencil marks. Note every candidate in a box or row, then look for pairs. If two squares in the same row can only be 3 or 7, those two digits are spoken for, and you can strike 3 and 7 from every other square in that row. Another useful pattern is pointing: if all the candidates for a digit inside a box sit on one row, that digit cannot appear anywhere else on that row outside the box. Work slowly, and use the red clash marks as a check, not a guide.