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How to play sudoku

Sudoku is a logic puzzle, not a maths one. You never add, subtract or multiply anything — you only apply one rule, over and over, until the grid is full.

The rule

Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1–9 exactly once. That is the whole game.

Those 27 groups — 9 rows, 9 columns, 9 boxes — are called units. Every cell belongs to exactly three of them. Two cells that share a unit are peers: each cell has 20 peers, and no two peers may hold the same digit.

Cells printed in heavy type are givens. They are part of the puzzle and cannot be changed. Everything else is yours.

Why it isn't maths

You could replace 1–9 with nine colours, nine letters or nine Pokémon and the puzzle would be identical. The digits are labels, not quantities. What matters is only which cells must differ. This is why a good sudoku player rarely "calculates" anything — they compare sets.

How to talk about a grid

Every solver uses the same shorthand, and our hints use it too:

Notation	Meaning
R3C5	row 3, column 5 (row 1 is the top, column 1 is the left)
R3C5=7	put 7 in that cell
B4	box 4 — boxes are numbered 1–9 left to right, top to bottom
{2,6}	the candidates left in a cell: only 2 and 6 can still go there
R3C5 ≠ 4	eliminate 4 from that cell's candidates

The three questions

Most "how to play" guides give you a list of techniques. A list is hard to use when you are staring at a board. This is the same knowledge compressed into an order you can actually run in your head:

1. Is there a cell with only one candidate left? Scan each row, column and box: if eight digits are already visible, the ninth is the answer.
2. Is there a digit with only one possible home? Pick a digit and look at one box at a time: "where can the 7 go in this box?" If only one cell can take it, that cell is 7 even if it has other candidates.
3. Are two digits locked into two cells? If two cells in the same unit can only hold the same two digits, those digits cannot appear anywhere else in that unit — cross them out.

Repeat 1–3 until nothing moves. That is enough to finish every Easy puzzle on this site, and most of Medium.

When all three fail, you are not stuck — you have graduated. The next step is locked candidates, then pairs and triples, then fish and wings. Each one is a new question, not a new rule.

Watch the first move on a real board

This is an actual Medium puzzle from this site. Step through it: every step is exactly the kind of move the three questions produce.

The player replays the technique solver that rates our puzzles, so the moves you see are the same ones the hint button uses on a live game.

Two ways to scan — and when each one works

Every solver uses two scanning modes, and most beginners only use the weaker one.

Mode	How it works	Best for	Weakness
Cell scanning	look at one empty cell and ask “what can go here?”	the endgame, when few cells are left	early on, most cells have 4–6 candidates and nothing stands out
Digit scanning (cross-hatching)	pick a digit and sweep the grid: “where can 4 go in this band of boxes?”	the opening, and after every placement	needs discipline; you must check all three box-rows

The practical rule: start with digit scanning, finish with cell scanning. In the first half of a puzzle, digits are sparse and cells are crowded — so look for digits. In the last third, the board inverts: cells have two candidates and digits are everywhere, so look at cells.

A 60-second opening routine

Run this before you place anything. It finds most of the easy digits and costs about a minute:

1. Sweep 1–9 once. For each digit, scan the three box-rows looking for a box that can only take it in one cell. Write down any you find — do not place them yet.
2. Place the confident ones. Now fill those digits in; each one changes three units.
3. Re-sweep only the digits you just placed. A 6 placed in B2 often completes a 6 in the row below it.
4. If nothing moved, start candidates. That is the signal: singles are exhausted, so the board now needs eliminations.

This is also the routine to run when you come back to a half-finished puzzle: the digits you placed before the break are the ones that changed the board.

When to start writing candidates

There is a precise moment: as soon as question 1 and question 2 stop producing moves. Before that, pencil marks are busywork; after it, they are the only way to see what the board is telling you.

Write candidates in every empty cell at once, not one cell at a time — the whole point is to compare cells. Then, every time you place a digit, erase it from the peers you just updated. A stale candidate is the most common cause of a "wrong" move later.

Press N on any puzzle to toggle notes; the number pad then writes small digits instead of big ones.

A decision tree for "I'm stuck"

1. Re-scan the digit you just placed. A new digit changes its row, its column and its box — most "stuck" moments are one missed follow-up.
2. Switch units. If you were scanning rows, scan boxes; the same information looks different in a different direction.
3. Check for a locked candidate. Is a digit confined to one line inside a box (or one box inside a line)? That is an elimination you may have missed.
4. Count. Pick a unit with few blanks and list what each blank can be. The one with two candidates is usually the key.
5. Ask for a hint (H). It names the technique and explains it on your board instead of filling the cell, so you learn the move rather than receiving it.

Keyboard

Key	Action
↑ ↓ ← →	move the selection
1–9	place a digit (or toggle a candidate in notes mode)
N	notes mode on / off
Backspace	erase the cell
H	hint
Ctrl/⌘ Z	undo (add Shift to redo)
Space	pause the timer

Three myths

“You sometimes have to guess.”

A well-formed sudoku has exactly one solution and can be solved by logic alone. Guessing is what you do when a technique you don't know yet is required — not a property of the puzzle. (There is one legitimate exception: scanning is a guess you can verify instantly, which is why speed solvers do it.)

“Fewer clues means harder.”

Across the 893,916 puzzles we ship from, the median clue count for Medium, Hard and Extreme is 28, 28 and 28 respectively — essentially identical. Difficulty comes from which technique a puzzle forces, not from how many digits you are given. The full analysis is here.

“Sudoku is a memory game.”

You never need to memorise a grid. You need about six patterns, each of which fits on one line: single, hidden single, locked candidate, pair, triple, wing. Everything else is bookkeeping.

FAQ

Do diagonals have to contain 1–9?

No. Only rows, columns and 3×3 boxes. Diagonal constraints exist in a variant (Sudoku X), but they are not part of standard sudoku.

Can a digit repeat in a row?

Never. Each row, column and box contains each digit exactly once — that is the single rule of the game.

Is a 17-clue puzzle the hardest?

No — 17 is the fewest clues a puzzle can have and still be unique (proved in 2012), but many 17-clue puzzles are easy to solve. Clue count and difficulty are only loosely related.

How long should a puzzle take?

Roughly: Easy 3–6 minutes, Medium 6–12, Hard 12–25, Expert 20–45. If you are far outside that, the bottleneck is usually a missing technique rather than speed — see the practice ladder.

Do I need an account?

No. There is no sign-up, and your progress is stored in this browser only.