How to play Sudoku

Play Sudoku

The rules

A Sudoku grid is a 9×9 square split into nine 3×3 boxes. Some cells already hold digits: these are the givens. Fill every empty cell so that:

• every row contains the digits 1 to 9, each exactly once;
• every column contains the digits 1 to 9, each exactly once;
• every 3×3 box contains the digits 1 to 9, each exactly once.

Every puzzle has exactly one solution, and logic techniques alone are enough to reach it. You never have to guess.

How to play here

• Select a cell: click it, or use the arrow keys.
• Enter a digit: keys 1–9, or the number pad below the grid.
• Notes (small candidates): press N or the Notes button. In notes mode, a digit you type is recorded as a small candidate instead of an answer.
• Erase: Delete, Backspace or 0.
• Undo and redo: Ctrl+Z and Ctrl+Y (on a Mac: ⌘Z and ⌘⇧Z).
• Hint: each press of Hint tells you more: first where to look, then which technique works there, and finally the digit itself. If there is a mistake on the board, the hint shows you where.

Your progress is saved automatically in your browser. You can close the page and come back later.

Difficulty levels

The level isn’t set by the number of givens. It’s set by the hardest technique the puzzle requires:

There is a daily puzzle at each of the four levels. Easy puzzles have at least 34 givens, and at least one row, column or box with five or more, so they are easy to start.

Techniques

Naked single

A cell has only one possible digit, because every other digit already appears in its row, column or box. This is the most basic technique.

Hidden single

Within a row, column or box, a digit fits in only one cell, even if that cell has other candidates. Scan digit by digit: “where can the 7 go in this box?”

Pointing pair

If, inside a box, a digit can only go in cells on the same row, it must be on that row within the box. So you can remove it from every other cell of that row outside the box. The same works for columns.

Box-line reduction

The reverse of a pointing pair: if in a row a digit can only go in cells inside one box, remove it from the other cells of that box.

Naked pairs and triples

If two cells in the same unit hold exactly the same two candidates (say 3 and 8), those two digits are taken by them. Remove 3 and 8 from every other cell in the unit. A triple works the same way with three cells and three digits.

Hidden pairs and triples

If two digits in a unit can only go in the same two cells, those cells are reserved for them. Remove every other candidate from those cells.

X-Wing

If a digit can appear in two rows only in the same two columns, it appears once in each of those columns within those rows. So you can remove it from every other cell of the two columns.

XY-Wing

Three cells with two candidates each: a pivot {X,Y} and wings {X,Z} and {Y,Z} that see the pivot. Either way one wing is Z, so any cell that sees both wings can’t be Z.

Swordfish

An X-Wing extended to three rows and three columns.

Tips

1. Start by scanning digit by digit: take a digit that appears often and look for where it is missing.
2. Write notes only when scanning gets stuck. Early notes are more confusing than helpful.
3. When you place a digit, check its row, column and box right away: it often opens the next step.
4. In hard puzzles, look for cells with only two candidates. They are the building blocks of pairs and wings.