How to Play Sudoku: Rules and a First Solving Pass

The one rule

A standard sudoku is a 9x9 grid divided into nine 3x3 boxes. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each box contains every digit exactly once.

That is the whole game. You never add, subtract or guess a total. You only decide which digit belongs in which cell.

Words you will see

A few terms come up in every sudoku guide, so it helps to know them first.

• Cell: one square in the grid.
• Row: a horizontal line of nine cells. Column: a vertical line of nine cells.
• Box: one of the nine 3x3 blocks, outlined with heavier lines.
• Givens or clues: the digits already printed when you start.
• Candidates: the digits that could still go in an empty cell.

A first solving pass

Start with the digit that appears most often among the givens, say 5. Find each box that does not yet have a 5. Look at which rows and columns already hold a 5, and mentally cross those lines out. Often only one cell in the box is left, and that is where the 5 goes.

Then move to the next digit and repeat. After you have worked through 1 to 9, go around again, because every new digit you place gives you new information.

• Pick a digit and scan the grid for it.
• Check one box at a time for a spot where that digit can go.
• Write the digit only when it is the single possibility.
• Move on to the next digit and repeat until a full pass adds nothing.

When you get stuck

If a full pass places nothing, pick a cell with only two or three candidates and note them lightly in pencil. The strategies guide on this site explains how pencil marks work. Do not guess a digit and keep going, because one wrong digit can spoil the whole grid.

Checking your answer

When the grid is full, check each row, column and box and make sure it holds 1 to 9 with no repeats. Every puzzle generated on this site has exactly one solution, so a grid that passes every check is the answer.

If you printed the puzzle with an answer key, compare your grid to it. If you play on screen, the check button flags mistakes, and you can use a hint when you want a nudge instead of the full answer.