Guides · Sudoku

How to play Sudoku.

One rule, one way of reading the grid, and two habits that solve every beginner board. Every picture here is a real board from the app: the first one you get.

About 6 minutes · No experience needed

The board.

A Sudoku is a 9 by 9 grid of 81 squares. Some squares come filled in. Those are the givens, and they never change. Your job is to fill every empty square with a digit from 1 to 9.

A Sudoku board in Pocketful, Starter level 1: a 9 by 9 grid with the given digits filled in and the rest empty — Starter level 1 in Pocketful. The bold digits are givens. Everything else is yours to fill.

Rows, columns and boxes.

The grid is read three ways at once. A row runs left to right. A column runs top to bottom. And the thick lines split the grid into nine 3 by 3 boxes.

The same Sudoku board with one row highlighted across the grid — A row: nine squares, left to right.

The same Sudoku board with one column highlighted down the grid — A column: nine squares, top to bottom.

The same Sudoku board with one 3 by 3 box highlighted — A box: the 3 by 3 block inside the thick lines.

Every square belongs to exactly one row, one column and one box. Keep all three in mind and the puzzle opens up.

The rule of Sudoku.

Every row, every column and every box must contain the digits 1 to 9, each exactly once.

That is the whole game. No arithmetic, no guessing. A digit can never repeat inside a row, a column or a box, and each of those needs all nine digits by the end. Everything you do from here is just noticing where that rule leaves only one option.

The screen.

Tap an empty square to select it, then tap a digit on the keypad to place it. The app tints the square's row, column and box so you can see what it shares, and the small numbers under the keys count how many of each digit are still to be placed.

The Pocketful Sudoku screen with a square selected, its row, column and box tinted, and the number keypad and tools below the board — A selected square, with its row, column and box tinted. Below the board: the keypad, then the pencil, lock, eraser, hint and undo.

Pencil switches to notes, so a tap writes a small candidate digit instead of a big one.
Lock holds one digit so you can tap it into several squares in a row.
Eraser clears the selected square.
Hint fills a square for you when you are stuck.
Undo takes back your last move.

Habit one: find the missing digit.

Look at a box, ask "which digit is missing here?", and then ask where it can go. The digit can only sit in an empty square of that box, and it cannot sit in any row or column that already has one. Often that leaves exactly one square.

On this board the right-middle box has no 6. Step through it.

The right-middle box of the Sudoku board highlighted, with its three empty squares outlined as the possible places for the missing 6

A column of the Sudoku board highlighted, its existing 6 circled, two candidate squares crossed out and the one remaining square ringed

Habit two: fill the last gap.

Whenever a row, column or box has just one empty square, you can fill it straight away. Count which digit is missing and write it in. Placing a digit often finishes off a unit somewhere, so after every move, look for a new last gap.

A column of the Sudoku board with eight digits filled and its single empty square ringed

The Sudoku board after placing the 7 that completed the column

This is how beginner boards go: a chain of last gaps and missing digits, each one revealing another. When the chain stalls, go back to habit one on a different box or digit.

Notes, for when a square could be two things.

Sometimes a square has two or three digits that could still fit. Rather than hold that in your head, tap the pencil and write them in as small candidate digits. When one of them turns up elsewhere in the row, column or box, cross it off. When a square is down to one candidate, it is solved.

The Pocketful Sudoku screen in notes mode, with the candidate digits 2, 3 and 7 pencilled into one square — Notes on. This square could be a 2, 3 or 7, so all three are pencilled in. Placing a digit nearby will clear the candidates it rules out.

Finishing.

Keep going with the two habits until every square is filled. The board checks itself against the rule, so when the last digit goes in, you will know.

The completed Sudoku board, every row, column and box holding 1 to 9 once — The finished board. Every row, column and box holds 1 to 9 exactly once.

Starter and Easy boards never need more than what is on this page. Medium boards start to ask for notes. Harder boards use patterns with their own names, and those get their own guides.

Common questions.

Do you need to be good at math?

No. Sudoku uses digits, but there is no adding or multiplying. The digits could be letters or shapes and the puzzle would play the same. It is a logic game about where things can and cannot go.

Is it ever right to guess?

A proper Sudoku has exactly one solution and can be finished by reasoning alone, so a guess is never required. If you feel like guessing, it usually means there is a missing digit or a last gap you have not spotted yet. Notes help you find it.

What do the small numbers under the keypad mean?

How many of that digit are still to be placed. When a digit's count reaches zero it is finished for the whole board, and the key dims.

How long should a beginner board take?

A Starter board in Pocketful takes a few minutes once the two habits click. There is no clock pushing you, so take your time.