Sudoku:

What is Sudoku?

Sudoku is a logic number puzzle played on a 9×9 grid that is divided into nine 3×3 boxes. Your goal is to fill every cell with a digit from 1 to 9 so that every row, every column and every box contains each digit exactly once.

The rules

• Every row must contain the digits 1 to 9 exactly once.
• Every column must contain the digits 1 to 9 exactly once.
• Every 3×3 box must contain the digits 1 to 9 exactly once.
• The pre-filled clue numbers cannot be changed.

A valid puzzle always has exactly one solution. You never need to guess — every number can be deduced from the clues using logic alone.

How to play

The top toolbar

At the very top of the game screen you'll find the following items:

• Flame badge (leading, Challenge Mode only): a quick reminder that the running game was started in Challenge Mode.
• Question mark (leading): opens the help screen.
• Person icon (leading): opens the “Me” screen, where your settings live.
• Title (centred): shows the running puzzle’s difficulty so the level is always visible. On iPad there is room for “Sudoku · difficulty”; on iPhone the title is too narrow for both, so only the difficulty is shown.
• Circular arrows (trailing): starts a fresh puzzle at the current difficulty. If the running game has not yet finished, a confirmation dialogue is shown first; the unfinished game is recorded as unfinished in your statistics.

The board

Tap an empty cell to select it. Tap the selected cell again to clear the selection. Pre-filled clue cells cannot be selected — they stay fixed for the whole game.

If you tap a digit on the number pad without a cell selected, every cell that already holds that digit is highlighted, so you can see at a glance where the number still has room. Tap the same digit again, or tap any cell, to clear the highlight.

When the Auto-deselect after entry setting is on (see The Me screen below, under Board), the selected cell is released automatically after you write a solution digit into it. Pencil-note entries don’t trigger the deselection.

The board adapts automatically: in portrait the controls sit below the grid, in landscape they sit beside it.

Pencil, Undo, Erase

Below the board, three buttons sit in a row:

• Pencil (yellow): toggles pencil mode. While pencil mode is on, tapping a digit adds or removes that digit as a pencil note in the selected cell instead of writing the final value. Long-press the Pencil button to open a menu with Fill all pencil notes, which fills every empty cell with all of its possible pencil notes in one step. If the grid already contains pencil notes, you’ll be asked to confirm — the bulk fill is reversible with Undo.
• Undo (light blue): reverses your most recent move. Tap repeatedly to step back through this session’s moves. The undo history is not preserved across app restarts, so a resumed game can still carry digits and pencil notes from an earlier session that step-by-step undo can’t reach.
• Erase (red): clears the value (or all pencil notes) in the selected cell.

The number pad

The 3×3 number pad holds the digits 1 to 9. With a cell selected, tap a digit to fill it in — as a final value, or as a pencil note when pencil mode is on. With no cell selected, tapping a digit highlights every cell already holding that digit (see The board).

Any digit that has already been placed nine times in the grid appears dimmed on the pad — there’s nowhere left for it to go.

Hint, Check, Show

At the very bottom of the screen sit three orange buttons:

• Hint: highlights a cell where you can make progress towards the solution of the game. Cells you have already entered something into — right or wrong — are never suggested again. Each tap on Hint advances by one step towards the solution. The lightbulb shows as filled while you’re partway through a chain of hints. Long-press the Hint button to open a menu with Reset hint, which rewinds the hint engine to step one for the board you actually see — useful when you’ve stepped through several hypothetical hints without entering anything.
• Check: highlights every cell that currently holds a wrong digit. New mistakes are only flagged when you tap Check again.
• Show: fills in the correct number for the cell the next hint would point at. Use it sparingly — every Show tap is recorded in your statistics.

When Show hint strategy is on (see The Me screen), a small callout appears next to the hinted cell naming the technique that solves it. Tap Learn more inside the callout to open the full explanation. When Complete hint is on, every other cell that participates in a multi-cell technique (for example both cells of a naked pair or all four corners of an X-Wing) is highlighted alongside the focus cell.

Pencil notes themselves: the pencil-mode toggle on the row above switches digit input to pencil notes. As soon as a digit has been placed correctly nine times in the grid, its pencil notes are automatically removed from every remaining cell. With Remove notes in row/column/block on (see The Me screen below), matching pencil notes are also stripped from that cell’s row, column, and 3×3 block as soon as you enter the digit.

In Challenge Mode the three help buttons are disabled — you have to solve the puzzle without their help, and after three wrong entries the game ends.

For a recommended order in which to use Hint, Check, and Show, see When to use which helper button in the Solving strategies section.

Solving strategies

When to use which helper button

The app offers 3 helper buttons and 4 kinds of help. Let them build on each other rather than reaching for the strongest one first.

1. Hint without strategy. First try Hint with the strategy callout off. The suggested cell is highlighted, but you have to find the correct digit yourself. This is the gentlest nudge: it tells you where progress is possible, not how.
2. Hint with strategy. Turn on Show hint strategy in Settings under Board to also see which technique solves the highlighted cell. If the technique name alone is not enough, tap Learn more inside the callout to open the explanation right away.
3. Check. If you have already made entries and get stuck because you suspect a mistake, tap Check. Every cell that currently holds a wrong digit is flagged.
4. Show. Last resort. If you still can’t make progress, Show fills in the correct digit of the next cell so the puzzle moves forward.

Techniques

The techniques below range from very simple to advanced. Work through them in order. The first two methods alone will get you a long way on most puzzles, and the last few are only needed occasionally — but they can make the difference on very hard (Expert) puzzles.

Naked Single

The simplest technique. If a cell has only a single candidate left after eliminating every value that already appears in its row, column and box, that digit must go into the cell.

Hidden Single

A digit can only fit in one single cell of a row, column or box, even if that cell still has other candidates. For each digit from 1 to 9, scan every unit and look for the spots where that digit has just one possible home.

Naked Pair

If two cells in the same row, column or box share exactly the same two candidates, those two digits must occupy those two cells. You can strike both digits from the pencil notes of every other cell in that unit.

Hidden Pair

If two digits appear as candidates only in the same two cells of a unit, those digits must go in those cells — even if the cells still list other candidates. Every other candidate in those two cells can be removed.

Pointing Pair

If within a 3×3 box a digit only appears as a candidate inside cells of one row or column, then that digit must be placed in that row or column inside the box. You can eliminate the same candidate from the rest of that row or column outside the box.

Box/Line Reduction

The mirror image: if within a row or column a digit only appears as a candidate inside the cells of a single box, then it must be placed there. Every other candidate for that digit in the rest of the box can be eliminated.

X-Wing

If a digit appears in two rows only in the same two columns as a candidate, those four cells form an X-Wing. The digit must go diagonally into two of the four cells, which lets you remove the candidate from the rest of those two columns. The technique works the same way with rows and columns swapped.

Swordfish

The Swordfish extends X-Wing to three rows and three columns. If a digit appears in three rows using only three shared columns in total, each of those columns must contain the digit exactly once in those rows. The candidate can be removed from every other cell in those three columns.

XY-Wing

An XY-Wing is built from three cells with exactly two candidates each. One of them is the pivot and holds the pair XY. The other two are the pincer cells XZ and YZ, each of which shares a unit (row, column or box) with the pivot.

Whichever value the pivot ends up with, one of the pincer cells must end up holding the digit Z. It follows that any cell that sees both pincers cannot hold Z, so Z can be eliminated as a candidate there.

XYZ-Wing

A variation of XY-Wing. The pivot this time holds three candidates XYZ, one pincer holds XZ and the other YZ — so all three cells share Z as a common candidate.

The conclusion is a bit stronger: any cell that sees the pivot and both pincers at the same time cannot hold Z. In practice such a cell usually lives in the same box as the pivot.

Simple Coloring

A single-digit technique. Take a digit that appears as a candidate in only two cells of several units. Link each such pair together and colour the involved cells alternately with two colours, so that two linked cells never share a colour.

If two cells of the same colour end up seeing each other (sharing a unit), that colour is impossible — every cell of that colour can lose the digit as a candidate. You can also eliminate the digit from any cell that sees both a cell of one colour and a cell of the other.

Forcing Chains

The first real "what if" step, without actually guessing. Pick a cell with exactly two candidates and consider each value in turn. For each assumption, follow the chain of cells that are then forced to a single value.

If some other cell ends up with the same value in both scenarios, it must really hold that value — you can place it immediately. Likewise, any candidate that disappears in both scenarios can be eliminated. The actual choice in the starting cell never matters; you derive the result without committing to either branch.

The names and ideas of these techniques come from the wider Sudoku community and have been established there for a long time. The descriptions in this app are written in our own words. For more in-depth explanations with worked examples, see for instance Wikipedia or sudokuwiki.org.

The Me screen

Tap the person icon at the top-left of the game screen to open the “Me” screen. It groups all settings into Game, Board, iCloud, Appearance, and Tip jar.

Game

Difficulty

Determines the hardest solving techniques a puzzle will ever require.

• Simple: same as Easy, but with extra pre-filled digits.
• Easy: only naked and hidden singles.
• Medium: adds naked and hidden pairs.
• Hard: adds pointing pairs and box/line reduction.
• Expert: any puzzle with a unique solution is allowed. These puzzles may require any technique in the list, including advanced methods such as XY-Wing, coloring or forcing chains.

Changing the difficulty applies to the next game you start. The currently running game keeps the difficulty it was started with — you can see it at the top of the game screen.

Challenge Mode

Enables a more challenging game mode. All help actions (Hint, Check, Show) are disabled. Wrong entries are flagged immediately and counted. After 3 wrong entries the game ends.

A lost game counts as a completed game for the game counter.

Toggling Challenge Mode applies to the next game you start. The currently running game keeps the mode it was started with.

Board

Auto-deselect after entry

When turned on, the selected cell is released automatically after you write a solution digit into it. The next tap then has to pick a new cell first, which can prevent accidental overwrites in fast play. Pencil-note entries don’t trigger the deselection.

The setting is off by default.

Remove notes in row/column/block

When turned on, entering a digit into a cell automatically removes that digit from the pencil notes of every other cell in the same row, column, and 3×3 block. Pencil notes in cells outside those three units are left untouched.

The setting is off by default. Regardless of this toggle, the pencil notes for a digit are also cleared from every remaining cell as soon as that digit has been placed correctly nine times in the grid.

Complete hint

When turned on, hints whose underlying technique involves more than one cell — naked or hidden pair, pointing pair, box/line reduction, X-Wing, swordfish, and so on — highlight every cell that participates in the pattern instead of only the focus cell. With the setting off, only the single focus cell is highlighted.

The setting is off by default.

Show hint strategy

When turned on, requesting a hint shows a small callout next to the hinted cell naming the Sudoku technique the solver applied. Tap Learn more inside the callout to open the full explanation in this help screen.

The setting is off by default.

iCloud

Sync with iCloud

When enabled, your appearance, difficulty, and game-mode preferences are kept in sync across your iPhones and iPads via iCloud. Turn it on on one device and your other devices will adopt those values the next time they check in — the first device to enable sync wins, so other devices don’t overwrite its settings.

Your game history (games played, solved, lost, hints, shows) always syncs automatically, regardless of this toggle.

Turning the toggle off on any device stops syncing on all devices. Your last-seen settings remain on each device.

Appearance

Sets the app's colour scheme. System follows your device; Light and Dark override it.

Tip jar

Sudoku is free and ad-free. If you'd like to support its development, the tip jar offers a few one-time, in-app-purchase tips of varying sizes. Tipping is entirely optional, never unlocks features, and never affects your gameplay or statistics. A small heart appears next to a tip you've already left.

iCloud sync

What syncs

Sudoku uses your private iCloud to keep your stats and preferences in step across your devices. Nothing is shared with other users — the data lives in your own iCloud account.

• User settings (appearance, difficulty, auto-deselect, challenge mode, remove notes in row/column/block, show hint strategy, complete hint) are synced only when the Sync with iCloud switch in Settings is on.
• Your current in-progress board stays on the device you’re playing on. Each device resumes the game it last saved — puzzles are not carried over between devices.

You need to be signed in to iCloud on each device and have iCloud Drive enabled for Sudoku. On a device without an iCloud account, the app keeps working locally and syncs later once you sign in.