Sudoku on mobile

The settings for large digits, a left-handed pad and a colorblind-safe palette make a small screen easier to read. The game pauses when you switch apps or tabs and keeps your board in the browser, but your progress and statistics stay on that device only.

How do you enter digits on a touchscreen?

The basic move is two taps: tap an empty cell to select it, then tap a digit on the pad underneath the grid. The digit buttons are sized for thumbs, and the grid scales to the width of your screen with square cells. The toolbar has buttons for New, Restart, Undo, Redo, Erase, Notes, Auto-notes, Hint, Check and Pause. Every one of them is a tap target, so you never need a keyboard.

Two things help more than any other habit. The first is to keep your thumb near the pad and let your eyes do the scanning, since the cell you tap stays highlighted. The second is to use Undo freely. It covers notes as well as values, so a mistaken tap costs one more tap to reverse.

Know the difference between the three buttons that reset things. Erase clears the selected cell, both its value and its notes. Restart returns the current puzzle to its starting position, so you can try it again from the beginning. New loads a different puzzle at the level you have chosen. Because all three sit close together in the toolbar, take care that your thumb lands on the one you mean, especially if you are playing with one hand.

What is digit-first mode, and when should you use it?

Normally you pick the cell and then the digit. Digit-first mode reverses the order: you pick a digit and then tap cells to put it in. To switch it on, double-tap a digit on the pad. The digit button lights up, and every cell you tap afterwards receives that digit. Double-tap the same digit again to switch it off, or arm a different digit by tapping it.

This is the right tool when you have found where one digit goes in several places. For example, scanning the grid for the digit 7 might show that it belongs in three different boxes. Arming the 7 once and tapping three cells takes four taps. Selecting each cell and then the 7 takes six. If notes mode is on, digit-first mode places marks and not values, so you can tag one candidate across several cells in one sweep. Check the Notes button before you start, as the mode you are in decides what each tap does.

How do pencil marks work on a small screen?

The Notes button toggles notes mode, and its label shows whether it is on. In notes mode, tapping a digit adds or removes that digit as a mark in the selected cell. The Auto-notes button fills candidates for every empty cell from the digits already placed.

Marks are small on a phone, so a full grid of them can be hard to read. Two adjustments help. Turn on Large digits in the settings, and use selective marking, writing marks only in the cells and houses you are working on. The pencil marks guide explains both marking styles and a caution about pressing Auto-notes after you have removed candidates by reasoning. Also, by default the game removes a digit from the notes of its neighbors when you place it, so your marks stay accurate without extra taps.

Which settings matter most on a phone?

The Settings button opens a dialog with options that are particularly useful on small screens.

• Large digits. Makes the numbers in the grid bigger, which helps on a compact screen or in poor light.
• Left-handed pad layout. Moves the digit pad to suit a left-handed grip.
• Highlight row, column and box. Shades the cells that see the selected cell, so you can follow a house with your eye instead of counting across.
• Highlight matching digits. Marks every cell that holds the same digit as the selected one, which is the quickest way to spot where a digit is missing.
• Colors. Chooses between the default palette and a colorblind-safe one.
• Theme. Light, dark or match the device. Dark is easier on the eyes at night.
• Animation. Full, reduced or off. The game also respects your device’s reduced-motion preference.
• Sound effects. Switched off by default, so the game is quiet on a train or in bed.
• Show timer. Hide it if watching the clock makes you rush.

Will tapping the grid zoom or scroll the page?

The grid is set so that quick taps do not trigger the browser’s double-tap zoom. The page below the game scrolls normally, so you can scroll past the grid to the guide text without a conflict. If you ever find the page zoomed in, pinch to reset it. The game does not require any zoom gestures to play.

What happens when you switch apps or lose focus?

Phones interrupt constantly, and the game is built for that. When the page is hidden, for example when you switch to another app, or the window loses focus, the game pauses automatically and saves the board in the browser’s storage. The pause screen hides the grid, so pausing cannot be used to study the board with the clock stopped. Tap Resume to carry on.

Some points about that storage are worth knowing.

• It is local to one browser on one device. The statistics page says that statistics are stored only in this browser. Switching phones, clearing site data or using a different browser starts you from zero.
• Private browsing can limit it. If the browser blocks storage, the game falls back to keeping data in memory. In that case your board and settings last only until you close the page.
• Puzzles need a connection to load. A puzzle file is fetched when you choose a level. If it cannot be loaded, the game says so, and you can try again once you are back online.

How do hints and learning aids work on mobile?

The Hint button works exactly as on a desktop and has three levels. The first points at the area to look at, the second names the technique, and the third gives the move, which you can apply with one tap. The Check button flags any wrong digits on demand when mistake feedback is off. Hints are computed from the digits on the board and ignore your notes, so they are reliable even if your marks have drifted. They are unavailable in expert mode.

Use hints as small lessons: take the second level, find the move yourself, and look the technique up in the techniques hub afterwards.

What about larger boards on a small screen?

The 16×16 board has 256 cells and uses symbols 1 to 9 and A to G, so each cell is much smaller than on the classic grid, and the pad needs two rows. It plays best on a larger phone or tablet, with large digits on and notes used sparingly. The smaller 4×4 and 6×6 boards have larger cells and suit quick sessions on a small screen. The daily Sudoku is one puzzle per UTC day, with the same board for everyone, and works well as a short routine.

How do you read the highlights without relying on color?

Small screens and bright sunlight make color differences hard to judge, so the game pairs color with shape. Cells that share a row, column or box with the selected cell get a light shade. Cells holding the same digit as the selected one get a highlight color and their digit is underlined. A cell that repeats a digit within a row, column or region gets a wavy underline and an exclamation mark in its corner, and a digit that differs from the solution is marked with the same exclamation mark. You can read all of these without telling colors apart, and the colorblind-safe palette changes the colors as well.

What if something goes wrong on your phone?

• Digits turn into tiny marks. Notes mode is on. Tap the Notes button until it reads “Notes: off”.
• Every cell you tap fills with the same digit. Digit-first mode is armed, and the digit button is lit. Tap that digit again to turn it off.
• The game paused by itself. A notification, a call or a swipe to another app moved focus away from the page. Tap Resume to carry on.
• The grid is hidden behind a pause screen. That is the pause overlay doing its job. Resume restores the board.
• A puzzle will not load. The game could not fetch the puzzle file. Check the connection and try again.
• Your board is gone after closing the browser. Storage may be blocked, for example in private browsing, or site data may have been cleared.
• A digit is marked wrong or has an exclamation mark. With mistake feedback on, the game has found a conflict or a wrong digit. Use Undo to step back, or see the common mistakes guide for how to trace the cause.

How should you set up a comfortable session?

1. Pick a comfortable level on the home page. If you are new, start on Easy and read the guided first puzzle.
2. Open Settings and switch on Large digits if the grid feels tight. Pick your preferred theme and pad side.
3. Turn on instant mistake feedback while you learn, and move to Off when your errors are rare. The common mistakes guide explains the difference between a conflict and a wrong digit.
4. Play in short sessions, and let the game pause when you are interrupted.

What are the limits of this advice?

The game has been tested in emulated phone viewports and with automated browser tests, including a layout-shift check on a throttled phone profile. It has not yet been tested on a wide range of physical devices, so a particular handset or browser may behave differently from what is described here. If something misbehaves on your device, the contact page says how to report it. The controls page lists every control and keyboard shortcut, and the guides hub has the rest of the tutorials.