- Sudoku has exactly one rule: no digit 1–9 can repeat in any row, column, or 3×3 box
- You never need to add, subtract, or guess — every move follows from pure logic
- Always start by scanning rows, columns, and boxes that are almost full
- Pencil marks (candidate notes) are the most important tool you're not using yet
- Easy puzzles are designed so there's always at least one obvious move — you'll never be truly stuck
What Is Sudoku?
Sudoku is a logic puzzle played on a 9×9 grid. Your goal is simple: fill every empty cell with a digit from 1 to 9. The catch is that each digit can only appear once in every row, once in every column, and once in every 3×3 box.
That's the entire rulebook. No math, no language, no prior knowledge required. Every puzzle has exactly one correct solution, and every step toward it follows logically from the rule above.
If you've been putting off trying Sudoku because it looks complicated, this guide will show you it's much simpler than it appears — and far more satisfying once you start.
Understanding the Grid
Before your first move, it helps to see the grid for what it is: three overlapping systems, all sharing the same 81 cells.
| Unit | Count | Size | Must Contain |
|---|---|---|---|
| Rows | 9 | 9 cells wide | Digits 1–9, each once |
| Columns | 9 | 9 cells tall | Digits 1–9, each once |
| Boxes | 9 | 3×3 cells | Digits 1–9, each once |
Every cell belongs to exactly one row, one column, and one box. That's why placing a single number can eliminate candidates across three different groups at once — and why Sudoku creates such satisfying chain reactions.
The Four Moves Every Beginner Should Know
You don't need advanced strategies to solve Easy and Medium puzzles. These four techniques will carry you through most of them:
1. Last Remaining Number
If a row, column, or box already has 8 of its 9 digits filled in, the missing digit is the only one that can go in the empty cell. No thinking required — just fill it in.
2. Scanning (Cross-Hatching)
Pick a digit — say, 7. Look at each 3×3 box and ask: can a 7 go here? Check which rows and columns already have a 7, and those lines rule out cells in the box. If only one cell in the box can hold a 7, place it.
3. Naked Single
Look at an empty cell. Cross off every digit that already appears in its row, column, or box. If only one digit is left, that's your answer. This is the core move in Naked Singles — one of the most important techniques in Sudoku.
4. Hidden Single
Look at all the empty cells in a row, column, or box. For each digit 1–9, ask: how many cells in this group could hold this digit? If only one cell is possible for a particular digit, place it there — even if that cell has multiple candidates. This is a Hidden Single.
Three Mistakes Every Beginner Makes
Knowing what not to do is as valuable as knowing what to do:
Never place a number unless you can prove it's correct. A wrong guess creates a cascade of errors that is almost impossible to untangle. If you're stuck, use pencil marks and look harder — don't guess.
Beginners often try to hold candidates in their head. That works for Easy puzzles, but breaks down fast on Medium and above. Using pencil marks (small candidate digits in each cell) is not cheating — it's the correct technique.
New players often jump to the emptiest part of the grid. Instead, find the rows, columns, or boxes with the most digits already filled in — those are where your easiest wins are hiding.
Which Difficulty Should You Start With?
Every difficulty level requires a different set of tools. Here's what each one actually demands:
| Difficulty | Techniques Required | Right For You If… |
|---|---|---|
| Easy | Last Remaining, Naked Singles | You've never solved a Sudoku before |
| Medium | + Hidden Singles, scanning | Easy feels too fast to finish |
| Hard | + Naked/Hidden Pairs, candidates | You're comfortable with pencil marks |
| Expert | + X-Wing, Swordfish, advanced chains | Hard no longer challenges you |
| Master / Extreme | All advanced techniques | You want a serious mental workout |
Start one level easier than you think you need. Finishing a puzzle cleanly teaches you more than getting halfway through a harder one and giving up.
How to Use Pencil Marks
Once you've placed every number that's obvious at first glance, pencil marks take over. Here's how to use them:
- For each empty cell, write small candidate digits in the corners — only the digits that aren't yet ruled out by the cell's row, column, or box.
- When you place a number, erase that digit from the candidates of every cell in the same row, column, and box.
- When a cell has only one candidate left, that's your answer — fill it in immediately.
- When only one cell in a row, column, or box has a particular candidate, place it there.
On Sudoku.by, tap the pencil icon to enter notes mode — you can fill in candidates without committing to a final answer. Read the full guide on how to play for a walkthrough of every feature.
Start Your First Puzzle Right Now
The single best thing you can do after reading this guide is open a puzzle. Reading about Sudoku and solving Sudoku are completely different experiences — the rules click into place in a way that no explanation fully captures.
Start with an Easy puzzle. Once you've finished one, try a Medium. When you're ready for a guaranteed daily challenge, the Daily Challenge resets every midnight with a fresh puzzle at a difficulty you choose.
For a deeper look at individual solving techniques, our techniques library covers every strategy from Last Free Cell through Swordfish — with worked examples at each step.