- Sudoku is a 9×9 grid — 81 cells total, divided into nine 3×3 boxes
- Every row, column, and 3×3 box must contain the digits 1–9, each exactly once
- No math needed — the numbers are just symbols, like labels
- Always start with the rows, columns, or boxes that already have the most numbers filled in
- Use pencil marks to track your options when multiple numbers could fit a cell
What Is Sudoku?
Sudoku is a number-placement puzzle. Your job is to fill a grid with the digits 1 through 9, following one simple rule. That's really all there is to it — but figuring out where each number goes is where the fun begins.
You don't add, subtract, or multiply anything. The numbers 1–9 are just symbols. You could use stars, letters, or colors instead and the puzzle would work exactly the same way. It's about logic, not arithmetic.
The Three Groups You Need to Know
Every cell in a Sudoku grid belongs to three groups at the same time:
| Group | How Many in the Grid? | What It Looks Like |
|---|---|---|
| Row | 9 rows total | A horizontal line of 9 cells across the grid |
| Column | 9 columns total | A vertical line of 9 cells top to bottom |
| Box | 9 boxes total | A 3×3 block of cells (nine of these tile the grid) |
Every cell is part of exactly one row, one column, and one box. That's why solving Sudoku means checking three different groups every time you place a number.
The Golden Rule of Sudoku
No digit can appear twice in the same row, column, or 3×3 box.
This single rule is the engine behind every puzzle. If a 5 already appears in row 3, you can't place another 5 anywhere else in row 3. Same goes for its column and its box.
In practice, that means: when you look at an empty cell, you can eliminate any number that already appears in its row, column, or box. The numbers left over are your candidates. Sometimes there's only one left — and that's your answer.
Step-by-Step: Your First Move
- Scan the grid — look for rows, columns, or boxes with 7 or 8 numbers already filled in.
- Find the missing digit — if a row already has 8 numbers and only 6 is missing, write in the 6.
- Move to the next easiest spot — keep scanning until you can't find any more obvious answers.
- Write pencil marks — for cells where multiple numbers are still possible, list the candidates lightly in the corner of the cell.
- Use elimination — as you fill in more numbers, some pencil marks become impossible and can be erased.
Finding Easy Cells First
Not all empty cells are equally hard. A cell in a box that already has 8 numbers filled in is almost certain — you just need to figure out which one number is missing. Here's how to rank your targets:
Always attack the easiest cells first. Each number you place makes other cells easier, like a chain reaction.
When You Get Stuck: Use Pencil Marks
If no cell has an obvious answer, it's time to write candidate notes — small numbers in the corners of each empty cell that show which digits are still possible there.
For example, if a cell's row, column, and box together already contain the digits 1, 2, 4, 5, 6, 7, 8, and 9 — only 3 is missing. That cell must be 3. Fill it in.
Once your pencil marks are in place, look for:
- A cell with only one candidate — fill it in immediately (this is called a Naked Single).
- A digit that can only go in one cell within a row, column, or box — that's where it belongs (this is called a Hidden Single).
Your First Puzzle Starts Right Here
You now know everything a beginner needs: the grid, the rule, the three groups, and how to find easy cells first. The only thing left is to open a puzzle and try it.
Start on Easy difficulty. Easy puzzles always have at least one cell where only one number is possible — so you'll never be completely stuck. As you fill in numbers, others become obvious, and the grid slowly comes to life.
One puzzle in, and you'll understand Sudoku better than any explanation can teach you. When you're ready for a fresh challenge every day, try the Daily Challenge.