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Sudoku looks like a maths puzzle, but it isn’t. There are no sums, no equations and no guessing. It is a game of pure logic, and once you know the one rule and a couple of simple moves you can solve your first puzzle today. This guide takes about ten minutes to read and walks you through everything, with real puzzle examples.
What is Sudoku?
The numbers are only symbols: you could play the same puzzle with nine letters or nine colours. What matters is that each one appears once per row, column and box. A good Sudoku has exactly one solution, and you can always reach it by reasoning, step by step.
Rows, columns and boxes
The board has 81 cells, grouped in three ways. Sudoku players call each group a unit:
- 9 rows, running left to right.
- 9 columns, running top to bottom.
- 9 boxes, the 3×3 squares outlined with thicker lines.
Every cell sits in three units at once. That is the whole trick of Sudoku: when you place a number, it affects its row, its column and its box.
The one rule
There is only one rule, and it applies to all 27 units:
| Unit | How many | Cells in each | Must contain |
|---|---|---|---|
| Row | 9 | 9 | 1–9, each exactly once |
| Column | 9 | 9 | 1–9, each exactly once |
| 3×3 box | 9 | 9 | 1–9, each exactly once |
Because a unit has nine cells and needs nine different numbers, a number can never repeat in a row, column or box. Turned around, that gives you your main solving tool: if a number already appears in a row, it can’t go anywhere else in that row.
Solve your first puzzle, step by step
Almost every move in an easy Sudoku is one of two kinds. Learn these and you can finish easy puzzles on your own.
Move 1: find the only place for a number (hidden single)
Pick a box and a number that box still needs. Look along the rows and columns that pass through the box. Each time you see that number in one of them, cross off the cells of the box in that line, since the number can’t go there. If only one cell is left, that is where the number goes. This is called scanning or cross-hatching, and the move is a hidden single.
The same idea works for a row or a column: if a row needs a 2 and only one of its empty cells can take a 2, it goes there.
Move 2: find the only number for a cell (naked single)
Now flip it around. Choose an empty cell and list the numbers already used in its row, column and box. If eight different numbers are taken, the ninth is the answer. This is a naked single, sometimes called the “last possible number”.
Pencil marks (notes)
In harder puzzles you’ll reach a point where no single number jumps out. That’s when players write pencil marks, also called notes or candidates: small numbers in a cell showing every digit that could still go there.
Two habits make notes useful instead of messy:
- Only note what’s possible. Check the row, column and box before writing a candidate.
- Clean up as you go. When you place a number, erase it from the notes of every cell in the same row, column and box. (Sudoku.my does this for you automatically.)
Once you’re comfortable with notes, you’re ready for the techniques in our guide to 10 Sudoku strategies, from locked candidates to the X-Wing.
Difficulty levels explained
Difficulty isn’t about how many numbers are missing. It’s about which techniques you need. Here is how levels work on Sudoku.my, where every puzzle is graded by the logic it requires:
| Level | Starting clues | Techniques needed | Typical first-time solve |
|---|---|---|---|
| Easy | 38–42 | Hidden and naked singles | 10–20 min |
| Medium | 30–34 | Singles, plus locked candidates | 15–30 min |
| Hard | 25–29 | Adds naked and hidden pairs/triples | 25–45 min |
| Expert | about 22–26 | Adds X-Wing, XY-Wing and Swordfish | 40 min or more |
Solve times are rough guides for beginners. They drop quickly with practice: many regular players finish easy puzzles in under five minutes.
Beginner mistakes to avoid
| Mistake | Why it hurts | Do this instead |
|---|---|---|
| Guessing when stuck | One wrong guess spreads errors that are hard to find later. | Switch technique: scan a different number, or write pencil marks. |
| Checking only rows | You miss most moves, which come from boxes and columns. | Scan box by box first. Boxes give the most hidden singles. |
| Writing every candidate at the start | It’s slow, and easy puzzles don’t need it. | Use notes only once singles run out. |
| Forgetting to update notes | Old candidates lead to wrong conclusions. | Erase a number from its row, column and box right after placing it. |
| Starting on a hard level | Frustration, before the basic moves are automatic. | Do five to ten easy puzzles, then move up one level at a time. |
7 tips to get better fast
- Start with the most common number. If six 4s are already placed, the last three are easy to find.
- Work box by box. Boxes are where beginners find the most moves.
- Look for nearly-full units. A row with seven numbers has only two gaps and two missing numbers.
- Use both directions. Ask “where can this number go?” and “what can this cell be?”.
- Don’t rush the last few cells. Most mistakes happen when the end is in sight.
- Learn one new technique at a time. Practise it on several puzzles until you spot it without looking.
- Play a little every day. A daily puzzle builds pattern recognition faster than a long weekly session.
Teaching a child? Start with a smaller 4×4 or 6×6 grid first: our guide to Sudoku for kids has puzzles and answers to try together.
Sudoku glossary
| Term | Meaning |
|---|---|
| Clue / given | A number printed at the start. It can’t be changed. |
| Unit | A row, column or box. There are 27 in total. |
| Peer | Any cell that shares a unit with another cell. Each cell has 20 peers. |
| Candidate | A number that could still go in a cell, often written as a pencil mark. |
| Hidden single | The only cell in a unit where a number can go. |
| Naked single | A cell with only one possible number left. |
| Scanning | Looking along rows and columns to rule out cells for a number. |
A very short history
The puzzle we now call Sudoku first appeared in 1979 in the American Dell Pencil Puzzles and Word Games magazine under the name Number Place, and is credited to puzzle designer Howard Garns. In the 1980s the Japanese publisher Nikoli popularised it in Japan and gave it the name Sudoku, short for a phrase meaning “the digits must be single”. It became a worldwide craze in 2004 and 2005, after The Times in London started printing it daily. Read the full story in our history of Sudoku.
There are about 6.67 × 1021 possible completed grids, so you will never run out of puzzles. And in 2012, mathematicians proved that a Sudoku needs at least 17 clues to have a single solution.
Frequently asked questions
Do you need to be good at maths to play Sudoku?
No. Sudoku uses numbers only as symbols. You never add or multiply anything; you could play the same puzzle with nine letters or colours. It is pure logic.
Does every Sudoku have only one solution?
A well-made Sudoku has exactly one solution. Every puzzle on Sudoku.my is checked to have one solution that can be reached by logic alone, without guessing.
What is the minimum number of clues in a Sudoku?
Seventeen. In 2012 mathematicians proved by computer search that no standard 9×9 Sudoku with 16 or fewer clues has a unique solution.
How long does it take to solve a Sudoku?
Beginners often need 10–20 minutes for an easy puzzle. With practice, easy puzzles take 3–5 minutes, while expert puzzles can take 20 minutes or more.
Is it OK to guess in Sudoku?
You can, but you shouldn’t need to. A good puzzle can always be solved by logic, and guessing often leads to errors that are hard to undo. Use pencil marks instead.