How to Play Sudoku: A Beginner's Guide With a Worked Example

If you want to know how to play sudoku, here is the short version: fill the 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. That's the whole rulebook. There is no arithmetic, no trivia, and no guessing required — a properly made sudoku can always be finished by pure logic.

The rules take thirty seconds to learn. Getting comfortable takes an afternoon. This guide covers both: the rules, the three techniques that will carry you through 90% of easy and medium puzzles, and a real puzzle solved step by step with actual grid positions so you can follow along instead of just reading about it.

The rules of sudoku, in full

A standard sudoku is a 9×9 grid divided into nine 3×3 boxes. Some cells already contain digits — those are the givens, or clues. Your job is to fill the rest.

Three constraints, and that's it:

1. Each row of 9 cells contains the digits 1–9, each exactly once.
2. Each column of 9 cells contains the digits 1–9, each exactly once.
3. Each 3×3 box contains the digits 1–9, each exactly once.

A few things people expect to be rules but aren't:

• The diagonals don't matter (unless you're playing a variant that says so).
• The digits are just symbols. You could play with nine colors or nine animals. Nothing is added or compared.
• You never need to guess. If a puzzle is well made, it has exactly one solution and every step can be justified by logic. If you feel forced to guess, you've missed something — or the puzzle is broken.

That last point is worth taking seriously. Good generators, including the one on this site, build a complete valid grid first and then remove clues only while the puzzle still has exactly one solution. That's what makes "solvable by logic alone" a promise rather than a hope.

How to read grid positions

Throughout this guide I'll use RxCy notation: R3C5 means row 3, column 5. Rows are numbered 1–9 from the top, columns 1–9 from the left. Boxes are numbered 1–9 left to right, top to bottom, so box 1 is the top-left 3×3 and box 8 is the bottom-middle.

You don't need this notation to play. It just makes it possible to talk about a specific square without waving at a screen.

The puzzle we'll solve

Here's the grid. Dots are empty cells.

``` C1 C2 C3 C4 C5 C6 C7 C8 C9 R1 | 5 3 . | . 7 . | . . . | R2 | 6 . . | 1 9 5 | . . . | R3 | . 9 8 | . . . | . 6 . |

|-----------+-----------+----------|

R4 | 8 . . | . 6 . | . . 3 | R5 | 4 . . | 8 . 3 | . . 1 | R6 | 7 . . | . 2 . | . . 6 |

|-----------+-----------+----------|

R7 | . 6 . | . . . | 2 8 . | R8 | . . . | 4 1 9 | . . 5 | R9 | . . . | . 8 . | . 7 9 | ```

Grab a pencil if you like. Every move below is verified — none of it is hand-waving.

How to play sudoku: your first three techniques

Almost every easy and medium puzzle falls to three ideas. Learn these and you're genuinely playing, not just filling in obvious blanks.

Technique 1: Crosshatching (scanning for a digit)

Crosshatching means picking one digit, picking one box, and using the rows and columns that pass through that box to rule out cells until only one is left.

Let's hunt for the 8 in box 2 — that's the top-middle box, rows 1–3 and columns 4–6.

Box 2 currently looks like this:

`` . 7 . 1 9 5 . . . ``

Five empty cells: R1C4, R1C6, R3C4, R3C5, R3C6. Where can the 8 go?

• Row 3 already has an 8, at R3C3. So no cell in row 3 can take another one. That kills R3C4, R3C5, and R3C6.
• Column 4 already has an 8, at R5C4. That kills R1C4.

One cell survives: R1C6 = 8. Write it in.

That is crosshatching in its purest form. Notice what made it work — I wasn't looking at a cell and asking "what goes here?" I was looking at a digit and asking "where can it go?" That flip in perspective is the single biggest jump most beginners make.

Technique 2: The hidden single

A hidden single is a digit that has only one legal home left inside a row, column, or box — even though the cell it lands in still looks like it could hold several different digits.

Crosshatching a box is one flavor of this. But you can do the same thing along a row or a column, and beginners almost always forget to.

Look at column 5: 7, 9, ., 6, ., 2, ., 1, 8.

The column is missing 3, 4, and 5, and it has three empty cells: R3C5, R5C5, and R7C5. Where can the 4 go?

• R5C5? No — row 5 already contains a 4, at R5C1.
• R7C5? No — box 8 already contains a 4, at R8C4.
• R3C5? Nothing blocks it.

So R3C5 = 4. It's called hidden because R3C5 could still, on its own, have been a 3 or a 4. You only see the answer by asking about the digit rather than the cell.

Technique 3: The naked single

A naked single is a cell where the row, the column, and the box between them have already used up eight of the nine digits, leaving exactly one possibility.

Look at R5C5 — the dead center of the grid.

Put those together and you've used 1, 2, 3, 4, 6, 7, 8, and 9. The only survivor is 5.

R5C5 = 5.

And now column 5 has only one empty cell left, R7C5, and only one digit unaccounted for. R7C5 = 3. Free.

Watching the cascade

This is the part that hooks people. Solved cells create new solved cells, and a puzzle that looked frozen suddenly unzips.

We now know R7C5 = 3, which reshapes row 7 and box 8. Follow it:

R7C6. Row 7 holds 6, 2, 8 (and now 3). Column 6 holds 5, 3, 9, and the 8 we placed at R1C6. Box 8 holds 4, 1, 9, 8, and 3. Between them: 1, 2, 3, 4, 5, 6, 8, 9. R7C6 = 7.

R7C4. Row 7 now has 6, 3, 7, 2, 8. Column 4 has 1, 8, 4. Box 8 has 4, 1, 9, 8, 3, 7. What's left? R7C4 = 5.

R7C9. Row 7 is down to needing 1, 4, and 9. Column 9 already holds 3, 1, 6, 5, and 9 — which knocks out both 1 and 9. R7C9 = 4.

Six placements from one crosshatch. That chain reaction is what solving a sudoku actually feels like once the rules stop being the hard part.

Pencil marks: when and how

Pencil marks are the small candidate digits you write into a cell to record every value it could still take. They're the difference between finishing a medium puzzle and stalling out halfway.

The honest advice: don't reach for them too early.

• Easy puzzles: skip them. Crosshatch, place, repeat. Filling in candidates on an easy grid is busywork.
• Medium puzzles: start marking once scanning stops producing placements. Usually that's 20–30 cells in.
• Hard and above: mark everything, and mark it before you get stuck. Techniques like naked pairs and X-Wings are invisible without a complete candidate grid.

Two habits that save real time:

1. Write candidates in a fixed position inside the cell. Always put a 1 in the top-left corner, a 5 in the middle, a 9 in the bottom-right. After a week your eyes find patterns without reading anything.
2. Erase immediately. The instant you place a digit, strike it from every candidate list in that row, column, and box. Stale pencil marks are the number one cause of "impossible" puzzles that are actually just bookkeeping errors.

One free upgrade: the naked pair

Once you're marking candidates, this pattern shows up constantly and is worth knowing on day one.

If two cells in the same row, column, or box have identical candidate lists of exactly two digits, those two digits belong to those two cells — and can be erased from every other cell in that unit.

Say R4C2 and R4C7 both read {2, 9}. You don't know which is which, but between them they've claimed the 2 and the 9 for row 4. Every other cell in row 4 loses both. That often cracks a stubborn row wide open.

The same logic scales: three cells sharing three candidates form a naked triple.

Mistakes that trip up almost everyone

I've watched a lot of people learn this. The same five things go wrong.

• Solving cell by cell instead of digit by digit. Staring at one empty square and running through 1–9 is slow. Pick a digit, sweep the whole grid for it, then move on. This one change roughly halves solving time.
• Forgetting that boxes constrain columns and rows. People check the row and the column, place a digit, and only later notice it duplicates something two squares away in the same box. All three units, every time.
• Guessing after five minutes. A guess in ink poisons everything downstream, and you won't find out for twenty moves. If you're stuck, you've missed a scan — go back to digit 1 and sweep again.
• Ignoring the nearly-full units. A row with seven digits filled is the highest-value real estate on the board. Scan those first, always.
• Not erasing candidates. Covered above, but it's worth repeating, because a single stale pencil mark can make a perfectly good puzzle look unsolvable.

How difficulty levels actually work

Difficulty is not really about how many clues you're given. It's about which techniques the puzzle forces you to use.

Clue count is a weak proxy at best. A 30-clue puzzle can be gentler than a 34-clue one if the clues are placed generously. The proven minimum number of clues for a sudoku with a unique solution is 17 — that was settled by an exhaustive computer search published in 2012 — but 17-clue puzzles are research curiosities, not something you'll meet on a puzzle page.

A practice routine that actually builds skill

Playing a lot helps. Playing deliberately helps more.

1. Two weeks on easy, until you're consistently under five minutes. The goal isn't speed for its own sake — it's making crosshatching automatic so your attention is free for harder patterns later.
2. Move to medium and force yourself to use pencil marks. Even when you don't need them. Building the notation habit on puzzles you can already solve means it's ready when you meet one you can't.
3. Print a batch and work them away from a screen. Something about pencil on paper makes the patterns easier to see, and there's no "check" button to lean on. Our free puzzle maker generates fresh sudoku at easy, medium, or hard, prints one to six per page, and includes an answer key — handy for a stack you can chip away at over a week.
4. When you get stuck, don't reveal the answer. Walk away and come back. Stuck almost always means a scan you didn't finish, and fresh eyes find it in about ninety seconds.
5. After finishing a hard one, retrace the move that broke it open. Naming the technique you used is what turns a lucky solve into a repeatable one.

Frequently asked questions

Do you need to be good at math to play sudoku?

No. Sudoku involves no arithmetic at all — no adding, no multiplying, no comparing sizes. The digits 1–9 are just nine distinct symbols, and the puzzle would work identically with nine colors or nine letters. What it actually exercises is pattern recognition and constraint logic. (Killer sudoku, a variant, does add arithmetic — but that's a different game.)

Can a sudoku have more than one solution?

A properly constructed one can't. Multiple solutions is considered a defect, and reputable generators test for uniqueness by removing clues only while exactly one solution remains. If you find a puzzle with two valid answers, it was made badly — and it also means the puzzle can't be finished by logic alone, since at some point you'd have to guess.

Is guessing ever necessary in sudoku?

No, not in a well-made puzzle. Every placement in a unique-solution sudoku can be justified by a logical deduction, though at expert level the required deductions get long enough that some solvers use trial and error as a shortcut. If you find yourself guessing on an easy or medium grid, the fix is almost always to sweep the grid digit by digit again rather than to pick a value and hope.

How long should a sudoku take?

For a beginner, an easy puzzle takes 10–15 minutes at first and drops to under 5 within a couple of weeks. Medium puzzles run 8–20 minutes for a comfortable solver, and expert grids can take 45 minutes or more. Speed is a side effect of pattern recognition, not something to chase directly — accuracy first, because one wrong digit costs more time than a slow scan ever will.

What's the best first move in a sudoku?

Find the digit that already appears most often in the grid and scan for it. If there are six 4s on the board, those six 4s block a huge amount of space, which makes the remaining three easy to pin down. Then work down the list by frequency. Starting with the most common digit rather than starting at 1 is the single easiest habit to adopt.

Why does my sudoku suddenly become unsolvable partway through?

Nine times out of ten it's an error you made twenty moves earlier — usually a digit that duplicates in a box, or a candidate you forgot to erase. Scan each row, column, and box for duplicates before assuming the puzzle is broken. This is also a strong argument for pencil marks: with a complete candidate grid, a contradiction shows up as an empty cell almost immediately instead of hiding for half the puzzle.