How to Play Sudoku
Sudoku has exactly three rules, and you can learn all of them in about five minutes. No math, no language — just logic. Here's everything you need before starting your first puzzle.
The Goal
A Sudoku grid is a 9×9 board divided into nine 3×3 boxes (the thicker lines). Some cells start with a digit from 1 to 9 — these are the given numbers, or clues. Your job is to fill every empty cell so that each digit 1 through 9 appears exactly once in every row, every column, and every 3×3 box.
That's the whole game. The three rules below are simply that goal stated one unit at a time.
Rule 1: Every Row Contains 1–9
Each horizontal row of nine cells must contain the digits 1 through 9 — exactly once each. No digit can repeat within a row, and no digit can be missing.
In the example, one row already has eight digits: 2, 7, 1, 5, 6, 8, 4, and 3. The only digit missing from 1–9 is 9, so the single empty cell at the end of the row has to be 9. This kind of deduction — "eight down, one to go" — is the most common move in the entire game.
Whenever a row holds eight of the nine digits, the ninth digit belongs in the one empty cell. No guessing needed.
Rule 2: Every Column Contains 1–9
The same rule applies vertically: each of the nine columns must contain the digits 1 through 9, exactly once.
In the example, the highlighted column already holds 5, 3, 9, 1, 8, 6, 4, and 2 — every digit except 7. So the empty cell at the bottom of the column must be 7. Notice how rows and columns work together: a digit you place rules out cells in its row and its column at the same time.
A single placement affects two lines at once — its row and its column. That's why every new digit you write in opens up deductions elsewhere.
Rule 3: Every 3×3 Box Contains 1–9
The third rule is what makes Sudoku a Sudoku: the grid is divided into nine 3×3 boxes (marked by the thicker lines), and each box must also contain the digits 1 through 9, exactly once.
In the example, the highlighted box near the top already holds 6, 1, 9, 4, 3, 2, 8, and 7 — every digit except 5. The one empty cell in the box must therefore be 5. Boxes are often the fastest way to make progress early in a puzzle, because nine cells packed close together create plenty of overlap between rows and columns.
Every cell belongs to three units at once: a row, a column, and a box. A digit is legal only if it breaks none of the three rules.
Good to Know Before You Start
Do I need any math to play Sudoku?
Can I change the given numbers?
Do I ever have to guess?
What makes a puzzle easy or hard?
Does every puzzle have exactly one solution?
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Solving Techniques
Now that you know the rules, learn the techniques that make solving fast and fun.