How Sudoku works in a minute | isthebestsudoku.com

The rule

The board has 81 squares: 9 rows, 9 columns and, marked off by heavier lines, 9 boxes of 3×3 squares. Some squares already hold a digit. These are the givens, and they never change.

You fill in every other square so that each row, each column and each box holds each digit from 1 to 9 exactly once. So no digit may appear twice in any row, any column or any box.

There is no arithmetic. The digits are just nine different symbols; you never add them up.

A well-made puzzle has exactly one solution, and every puzzle on this site does: before a puzzle goes up, a solver confirms it.

Move 1: only one digit fits this square

Pick an empty square. Cross off every digit already used in its row, in its column and in its box. If only one digit is left, it goes in.

One row of a Sudoku: 5, 3, an empty square marked with a question mark, then 6, 7, 8, 9, 1 and 2.
5	3	?	6	7	8	9	1	2

This row already uses 1, 2, 3, 5, 6, 7, 8 and 9. Only 4 is left, so the square marked ? is 4.

In a real puzzle, the used digits are usually spread across the square’s row, column and box together. You cross them all off the same way.

Move 2: this digit fits only one square

Pick a box, a row or a column that still needs a digit, say 7. Look at each empty square in it. If a 7 already sits in that square’s row or column, the 7 cannot go there. When only one square is left, the 7 goes in it.

The top three rows of a Sudoku. Row 1 has a 7 in its seventh square, in the top-right box. Row 2 has a 7 in its fifth square, in the top-middle box. Row 3 begins 4, an empty square marked with a question mark, then 6. In the top-left box, the six squares in rows 1 and 2 are crossed out: a 7 cannot go there.
ruled out	ruled out	ruled out		7
ruled out	ruled out	ruled out	7
4	?	6

The top-left box still needs a 7. Rows 1 and 2 already have one, so a 7 cannot go in the crossed-out squares. In row 3 of the box only the square marked ? is empty, so the 7 goes there.

Notes

When a square could still hold more than one digit, you can pencil in small notes: the digits still possible there. As you place more digits, you cross notes off. Harder puzzles need longer chains of reasoning over those notes.

Playing on this site

Tap a square, then a number on the pad. Once the page has loaded, the phone keyboard never opens.
Tap Notes to switch the pad to notes, and tap it again to switch back.
Erase clears the chosen square. Undo takes back your last change, as many times as you like, notes included.
Tapping a number, or a square that holds one, rings every square holding that number, so a repeated digit is easy to spot. The chosen square’s row, column and box are shaded too. Given digits are always bold, and yours are not.
On a keyboard, type 1 to 9 to place a number, press Backspace or Delete to erase, and use the arrow keys to move between squares.
Check my grid asks our server to check your grid against the rule. It marks every square in a clash with a ! and names each clash in words, or tells you the grid is complete. It never shows an answer, and there is no limit on checks or mistakes.
There is no timer and no score.

Choose a puzzle