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For beginners · the one rule · your first number

How to Play Sudoku

Sudoku has one rule: every row, every column and every 3×3 box must contain the digits 1 to 9 exactly once. There is no maths and no guessing. You solve a Sudoku by asking, again and again, “where is the only place this number can go?”

Show me the first move ↓

Your first puzzle takes about 10 minutes. Nothing to download, no sign-up.

By Dan Herlehy, author of Sudoku with a Secret · Updated

The rules of Sudoku, in one picture

The three parts of the one Sudoku rule
Every rowholds 1 to 9 once each. The highlighted row still needs a 1, 3 and 8.
Every columnholds 1 to 9 once each, top to bottom.
Every 3×3 boxholds 1 to 9 once each. The thick lines mark the nine boxes.

The printed numbers are called givens. They are correct and never change. Your job is to fill every empty square so that all three parts of the rule hold at once. The most common beginner slip is checking the row and the column and forgetting the box, so always look three ways.

Your first Sudoku, one certain number at a time

This is Puzzle 1 of the book, the same grid as in the video below. The first 8 moves are shown to you with the reason. After that the square is ringed and you choose the number.

Move 0 of 42

Press the gold button. The page will ring one square, mark the numbers that matter, and tell you why the answer is certain.

Every move of Puzzle 1, with the reason (42 moves, in order)

Puzzle 1 can be solved with only two kinds of move: “this number has only one place to go” (21 moves) and “this is the last gap” (21 moves). Rows are counted from the top, columns from the left.

  1. Where can the 6 go in the middle-left box? There is already a 6 in row 4, column 2 and column 3, so those squares are out. Only one empty square is left: row 5, column 1. That square is 6.
  2. Where can the 8 go in the bottom-left box? There is already an 8 in row 7 and row 9, so those squares are out. Only one empty square is left: row 8, column 1. That square is 8.
  3. Where can the 5 go in the bottom-middle box? There is already a 5 in column 5, column 6, row 8 and row 9, so those squares are out. Only one empty square is left: row 7, column 4. That square is 5.
  4. Where can the 7 go in row 8? There is already a 7 in column 6, so those squares are out. Only one empty square is left: row 8, column 5. That square is 7.
  5. Row 8 already holds every number except 9, and it has one empty square left (row 8, column 6). So that square is 9.
  6. Where can the 1 go in row 4? There is already a 1 in column 1 and column 2, so those squares are out. Only one empty square is left: row 4, column 3. That square is 1.
  7. Where can the 3 go in row 4? There is already a 3 in column 2, so those squares are out. Only one empty square is left: row 4, column 1. That square is 3.
  8. Row 4 already holds every number except 4, and it has one empty square left (row 4, column 2). So that square is 4.
  9. Where can the 6 go in row 1? There is already a 6 in column 4, column 7 and column 9, so those squares are out. Only one empty square is left: row 1, column 8. That square is 6.
  10. Where can the 7 go in row 1? There is already a 7 in column 7 and column 9, so those squares are out. Only one empty square is left: row 1, column 4. That square is 7.
  11. Where can the 1 go in the top-middle box? There is already a 1 in column 5 and row 3, so those squares are out. Only one empty square is left: row 2, column 4. That square is 1.
  12. Where can the 1 go in the centre box? There is already a 1 in column 4, so those squares are out. Only one empty square is left: row 5, column 6. That square is 1.
  13. The centre box already holds every number except 3, and it has one empty square left (row 5, column 4). So that square is 3.
  14. Column 4 already holds every number except 2, and it has one empty square left (row 3, column 4). So that square is 2.
  15. Where can the 2 go in the top-right box? There is already a 2 in column 9, row 2 and row 3, so those squares are out. Only one empty square is left: row 1, column 7. That square is 2.
  16. Row 1 already holds every number except 1, and it has one empty square left (row 1, column 9). So that square is 1.
  17. Where can the 1 go in the middle-right box? There is already a 1 in row 5, column 8 and column 9, so those squares are out. Only one empty square is left: row 6, column 7. That square is 1.
  18. Where can the 2 go in the middle-right box? There is already a 2 in column 7, column 9 and row 6, so those squares are out. Only one empty square is left: row 5, column 8. That square is 2.
  19. Where can the 9 go in row 6? There is already a 9 in column 8, so those squares are out. Only one empty square is left: row 6, column 9. That square is 9.
  20. Row 6 already holds every number except 3, and it has one empty square left (row 6, column 8). So that square is 3.
  21. Column 8 already holds every number except 4, and it has one empty square left (row 3, column 8). So that square is 4.
  22. Where can the 3 go in the top-right box? There is already a 3 in column 7, so those squares are out. Only one empty square is left: row 2, column 9. That square is 3.
  23. Row 2 already holds every number except 8, and it has one empty square left (row 2, column 5). So that square is 8.
  24. The top-right box already holds every number except 8, and it has one empty square left (row 3, column 7). So that square is 8.
  25. Where can the 8 go in the middle-right box? There is already an 8 in column 7, so those squares are out. Only one empty square is left: row 5, column 9. That square is 8.
  26. The middle-right box already holds every number except 4, and it has one empty square left (row 5, column 7). So that square is 4.
  27. Column 7 already holds every number except 9, and it has one empty square left (row 7, column 7). So that square is 9.
  28. The bottom-right box already holds every number except 4, and it has one empty square left (row 7, column 9). So that square is 4.
  29. Where can the 4 go in the bottom-left box? There is already a 4 in row 7, column 1 and column 2, so those squares are out. Only one empty square is left: row 9, column 3. That square is 4.
  30. Where can the 3 go in the bottom-left box? There is already a 3 in column 2 and column 1, so those squares are out. Only one empty square is left: row 7, column 3. That square is 3.
  31. Where can the 7 go in the bottom-left box? There is already a 7 in row 9, so those squares are out. Only one empty square is left: row 7, column 2. That square is 7.
  32. Where can the 7 go in the middle-left box? There is already a 7 in column 2, so those squares are out. Only one empty square is left: row 5, column 3. That square is 7.
  33. Column 3 already holds every number except 9, and it has one empty square left (row 3, column 3). So that square is 9.
  34. The top-left box already holds every number except 7, and it has one empty square left (row 3, column 1). So that square is 7.
  35. Column 1 already holds every number except 9, and it has one empty square left (row 9, column 1). So that square is 9.
  36. The middle-left box already holds every number except 9, and it has one empty square left (row 5, column 2). So that square is 9.
  37. Column 2 already holds every number except 2, and it has one empty square left (row 9, column 2). So that square is 2.
  38. Row 9 already holds every number except 3, and it has one empty square left (row 9, column 6). So that square is 3.
  39. Where can the 3 go in the top-middle box? There is already a 3 in column 6, so those squares are out. Only one empty square is left: row 3, column 5. That square is 3.
  40. The top-middle box already holds every number except 6, and it has one empty square left (row 3, column 6). So that square is 6.
  41. Column 5 already holds every number except 6, and it has one empty square left (row 7, column 5). So that square is 6.
  42. Column 6 already holds every number except 2, and it has one empty square left (row 7, column 6). So that square is 2.
Sudoku with a Secret, Volume 1: Crack the Keeper's Vault — book cover

Is the book all right for a beginner? Yes.

  • A “New to Sudoku? Start Here” page on page 5 teaches this exact method.
  • Seventy easy puzzles come first, before anything gets hard.
  • One big grid per 8.5 × 11 page, with room to write.
  • All the answers are in the back.
Get Volume 1 · $14.99 on Amazon ↗

Watch a real first-timer learn Sudoku

Fifteen unscripted minutes: the author teaching his uncle his first Sudoku on Puzzle 1 of the book. The pauses are the lesson. Watch on YouTube ↗

How to solve a Sudoku: the two questions

Every move in beginner Sudoku comes from one of two questions. Question one does most of the work; of the 42 moves in Puzzle 1, 21 come from it.

Question one: where can this number go?

Pick a box and a digit that box is missing. Every copy of that digit elsewhere on the grid “looks” along its row and its column, and no second copy can sit anywhere in that line. Cross off the squares it rules out. If exactly one empty square survives, the digit goes there. Sudoku players call this scanning, or cross-hatching.

1. Pick a box and a missing number. Here: the 6 in the tinted box.
2. Find every 6 that looks into that box along a row or column. Each one rules squares out.
3. One empty square is left. It is certain, so write the 6 in.

Where can the 6 go in the middle-left box? There is already a 6 in row 4, column 2 and column 3, so those squares are out. Only one empty square is left: row 5, column 1. That square is 6.

Question two: what can go in this square?

Now look at one empty square instead of one digit. Tick off every number already in its row, its column and its box. If only one number is left, write it in. The easiest case is the last gap: a row, column or box with eight numbers has room for only the ninth.

Row 8 already holds every number except 9, and it has one empty square left (row 8, column 6). So that square is 9.

Six moves, in order (said out loud at a kitchen table)

Everything below was said in September 2026 to someone who had never done a Sudoku. It worked.

01

Pick the fullest box

“I usually try to start with ones that are kind of mostly filled in.” A box with two or three gaps has two or three possibilities. A box with six gaps has too many.

02

Ask where a digit can’t go

Take one missing digit. Look along each empty square’s row and column, and remember the box itself. “It can’t, because there’s an eight here.” Cross those squares off.

03

One square left? Write it

“So we know a hundred percent — that’s an eight.” When exactly one square survives, the digit goes there. That is the only kind of move you ever make.

04

Two squares left? Leave it

“If you write the nine in, it still might be able to go there. So you don’t want to write it in yet.” Try a different digit. Come back later; it will have narrowed.

05

Stuck? Switch to a row

“Instead of thinking of it in a square, change your brain to try to finish that row.” A row with one gap is a free number, and every number you place removes a possibility somewhere else.

06

Fill the last gaps, and check

“That’s how you know you got it right — the last number works.” If a last number clashes with its row or column, something earlier was a guess. See how to recover.

What to do when you are stuck on a Sudoku

Being stuck on a Sudoku does not mean the puzzle is broken or that you must guess. It means a certain number is sitting somewhere you have not looked. Try these in order:

  1. Look for any row, column or box with only one or two gaps. Those are the cheapest numbers on the grid.
  2. Pick the digit that appears most often on the grid (count the 1s, the 2s...). The more copies there are, the fewer places the rest can go.
  3. Go round every box asking question one for that digit: where can it go in this box?
  4. Switch shape. If you have been working in boxes, ask the same question of a row or a column.
  5. Recheck the last number you placed: its row, its column and its box. New numbers open new moves right next to them.
  6. Still stuck on a harder puzzle? Pencil tiny notes into squares that have only two possible numbers, then go round again.

Made a mistake? How to recover on paper

A Sudoku mistake rarely shows up where it was made. It shows up later, as the same digit twice in one row, column or box. When that happens: find the pair of duplicates, then erase back to the last number you were completely sure of. With a book, check just that one box against the answer key in the back rather than reading the whole solution. This is the whole reason Sudoku is played in pencil.

Try it: spot the mistake. This finished box breaks the rule. Tap a number that is wrong.

When should a beginner use notes?

Later than most apps suggest. On an easy Sudoku the two questions are enough, and a grid full of tiny numbers hides more than it shows. Start using notes when the two questions stop producing moves, usually on medium puzzles. On paper, write small digits in the corner of a square only when that square has two possible numbers, and rub them out the moment one is ruled out. A stale note causes more mistakes than no note.

Sudoku myths and what is true

What beginners often thinkWhat is true
Sudoku is a maths puzzle.Sudoku uses no arithmetic. The digits are nine different symbols; letters, colours or fruit would work the same way.
Sooner or later you have to guess.A properly made Sudoku never needs a guess. Every number can be placed for a reason. Harder puzzles need more reasons than this page teaches, but never a coin flip.
The diagonals matter.In classic Sudoku only rows, columns and 3×3 boxes count. Diagonals are ignored.
A Sudoku can have several right answers.A valid Sudoku has exactly one solution. If two answers fit, the puzzle was badly made.
Writing small notes is cheating.Small pencilled notes are a normal part of Sudoku. Experts use them on every hard puzzle.
If I am stuck, the puzzle is broken.Being stuck in Sudoku almost always means there is a certain number you have not spotted yet, usually in a row or column you have not checked.

Key facts from this lesson

Grid9 rows × 9 columns = 81 squares, in nine 3×3 boxes
The rule1 to 9 exactly once in every row, column and box
Puzzle 1 of Sudoku with a Secret39 given numbers; solved in 42 moves using only the two beginner moves (21 “only place”, 21 “last gap”)
A first-timer’s timeabout nine minutes with coaching (filmed, September 2026); 15 to 30 minutes alone is normal
Toolsa pencil and an eraser

Teaching Sudoku to a child or a grandparent

Sit beside them, not across. Say the rule once, then point at a nearly full box and ask, “which numbers are missing?” Let them find where one goes; only point when they stall. Never fill a square for them. Use an easy puzzle with big squares, stop after one grid, and let the pauses happen. The video above is fifteen minutes of exactly that.

Practise on a real page

Free, in your browser

Play the puzzles online. No account, no app, and the easy ones come first.

On paper, with a pencil

Print the one-page cheat sheet with Puzzle 1, or print today’s free puzzle.

Then the secret

Hidden among the 200 puzzles in the book are shaded keys, and they open a vault on this site. Here is how the escape-room layer works.

Sudoku words, in plain English

Square (or cell)
One of the 81 small spaces in a Sudoku grid. Each square holds one digit.
Row
A line of nine squares running left to right. A Sudoku has nine rows.
Column
A line of nine squares running top to bottom. A Sudoku has nine columns.
Box
One of the nine 3×3 blocks marked by thicker lines. Also called a block or region.
Given
A digit printed in the grid at the start. Givens never change.
Only place (hidden single)
A digit that has just one square it can use inside a row, column or box. Experts call it a hidden single.
Last gap (full house)
The final empty square in a row, column or box. It takes whichever digit is missing.
Notes (pencil marks)
Small digits written in a square to record which numbers are still possible there.

Questions beginners ask about Sudoku

What are the rules of Sudoku?

Sudoku has one rule: fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. Some digits are printed to start you off. There is no arithmetic; Sudoku is solved by logic alone.

Is Sudoku maths? Do I need to be good with numbers?

No. Sudoku uses the digits 1 to 9 only as nine different symbols. Nothing is added, subtracted or multiplied. If you can tell whether a 7 is already in a row, you can solve a Sudoku.

Where do I start on a Sudoku?

Start a Sudoku in the 3×3 box, row or column that already has the most numbers, because it has the fewest possibilities. Take one missing digit and ask where in that box it can go. Write it in only when exactly one square is left.

Do you ever have to guess in Sudoku?

No. A properly made Sudoku has exactly one solution and every digit can be placed for a reason. If a digit could go in two squares, leave it and look elsewhere. Harder Sudoku puzzles need more advanced reasons than a beginner knows, but they never need a guess.

Do diagonals count in Sudoku?

No. In classic Sudoku only the rows, the columns and the nine 3×3 boxes must each contain 1 to 9. The diagonals can repeat digits freely.

What should I do when I am stuck on a Sudoku?

When a Sudoku stalls, look for a row, column or box with only one or two gaps, then take the most common digit on the grid and ask where it can go in each box. Change from boxes to rows or columns. A stuck Sudoku nearly always hides one certain number you have not checked yet.

I made a mistake in my Sudoku. Do I have to start again?

Usually not. Find the row, column or box that shows the same digit twice, then erase back to the last number you were completely sure of. On paper, check that one box against the answer key rather than the whole grid. This is why Sudoku is played in pencil.

How long should an easy Sudoku take a beginner?

A first easy Sudoku commonly takes a beginner fifteen to thirty minutes alone. With a little coaching, a complete first-timer finished Puzzle 1 of Sudoku with a Secret in about nine minutes. Times drop quickly with practice.

Which Sudoku book is good for a complete beginner?

Look for a Sudoku book with one large grid per page, a real run of easy puzzles at the start, and answers in the back. Sudoku with a Secret, Volume 1 opens with a Start Here page on page 5 and seventy easy puzzles before anything gets hard.

Start on page five

Volume 1 opens with a Start Here page, then 70 easy puzzles before anything gets hard. $14.99 in paperback.

Get Volume 1 on Amazon ↗

Or try the puzzles free first · or see the vault door the book unlocks.

The series

Sudoku with a Secret

Sudoku with a Secret, Volume 1: Crack the Keeper's Vault — book cover Volume 1 · Crack the Keeper's Vault
Out now ↗
Volume II — The Corsair's Chart
Volume II · The Corsair's Chart
In the works
Volume III — The Lost Love Letters
Volume III · The Lost Love Letters
In the works
Volume IV — The Silent Signal
Volume IV · The Silent Signal
In the works
Volume V — The Hollow Oak
Volume V · The Hollow Oak
In the works

Volume 1 has 200 puzzles, easy to hard, and a hidden escape room inside the book. Hear when Volume II opens — email is always optional.