An offline Sudoku app is one that already has the puzzles inside it. Nothing downloads, nothing asks you to sign in, and the same grid comes up in flight mode as on wifi.
Most apps that advertise offline play mean half of it. The puzzles are bundled and the rest is not, so the game works until you close it, or the board works but the daily challenge is blank, or everything works until the app is killed from memory and a login screen comes back. One test separates them, it takes a minute, and you run it at home rather than at the gate.
Turn on airplane mode. Force-quit the app. Open it again. Start a new game at every difficulty it offers. The force-quit is the step people skip and it is the step that catches the failures.
Flying tomorrow? Sudoku Master carries 4,000 puzzles inside the app, so there is nothing to download and no account to make. Free, and no ads while you solve.
The flight-mode test, in order
Run these on the ground, in the order below. Each one proves a different thing, and the last two are the ones that matter on a long journey.
| Step | What it proves | What failure looks like |
|---|---|---|
| Install, then turn on airplane mode | Nothing yet | Nothing yet |
| Force-quit the app and reopen it | The app starts without a server | A login screen, a spinner that never ends, or a "check your connection" panel |
| Start a new game on the easiest grade | Easy puzzles are bundled | An empty grid, or a loading box |
| Start a new game on the hardest grade | All grades are bundled, not just the sample | Easy works and extreme does not, the most common half-offline app |
| Place a digit, add notes, undo, erase | The board logic is local | Nothing here should ever need a network, and it almost never does |
| Ask it to check the grid | The answer is stored or computable on the device | A check button that greys out with no signal |
| Open statistics and the streak | Your record is on the phone, not on a server | Empty screens, or last-synced numbers from a week ago |
| Open the daily or weekly challenge | Challenges are generated locally | A blank list, which is the second most common failure |
| Leave a game half solved, force-quit, reopen | The save is local | The board comes back empty |
Nine steps, one minute. If an app clears all nine, offline is real. If it clears the first four and drops the rest, you have a plane game and not much else, which may still be what you wanted.
Two failures deserve their own note because they look like bugs and are not. An app can be perfectly offline and still show today's date wrong after you fly across time zones, because a local challenge is picked from your phone's clock. And an app that has been open in memory for two days is not proof of anything. Session tokens are refreshed at cold start, so the app you have not killed since Tuesday will happily keep playing right up to the moment you kill it.
Sudoku is the cheapest thing there is to ship offline
A grid is 81 characters. Its solution is another 81. Here is a real one, 31 givens, exactly one solution.
.235..6..
1.726...5
.8...3.42
.7..5..96
3...8...7
6.2....5.
.5..3....
7..9.5.1.
....41...
And its answer, which an app can carry as a single line of text:
923574681147268935586193742874352196315689427692417853451836279768925314239741568
That is 162 characters for one puzzle and the means to mark it. A thousand puzzles is 162,000 characters. Four thousand is about 650 kilobytes, less than one photo taken on the phone you are reading this on. There is no art to stream, no level to fetch and no opponent to wait for.
So an app that needs a connection to hand you a puzzle is not fetching the puzzle. It is fetching an ad, a config file, or a session. That is worth knowing before you accept a "requires internet" line as a technical fact, because it is usually a business decision wearing a technical coat.
Checking your answer with no signal
This is the one offline feature people assume needs a server, and it is the one that explains why the solution file exists.
There are two different checks and apps conflate them.
A rule check looks at your grid and asks whether any digit repeats in a row, a column or a box. It needs nothing except the rules. Every app can do this offline and most do it whether you asked or not.
An answer check asks whether the digit you typed is the digit that belongs there. That needs either the stored solution or a solver run on the spot.
The gap between the two is much wider than it sounds. Take the grid above and
put a 3 in r6c4. Row 6 holds 6, 2 and 5, so no clash. Column 4 holds 5, 2 and
9, so no clash. Box 5 holds 5 and 8, so no clash. A rule check says the grid is
fine.
The true digit for r6c4 is 4, and the puzzle is now dead. Not hard. Dead. A
proper Sudoku has exactly one solution, so once the wrong digit is in, no
arrangement of the remaining cells finishes it.
Now the part that should worry you. Carry on from there, filling any cell with one candidate left and any digit with one home left in its row, column or box, and the grid raises no objection for a long while. I ran that from this exact position three thousand times, taking the available singles in a different order each run. The shortest run got 10 more digits down before a cell ran out of options. The longest got 45. The middle of the distribution sat at 28.
Twenty-eight cells of honest work on a puzzle that died twenty minutes earlier. That is what a rules-only checker saves you from, which is to say nothing.
An app holding the solution string flags the 3 at the moment you tap it. An app running a solver in the background can also catch it, because a backtracking solver returns nothing at all when a grid has no solution. Either way there is no network in the story. If an app tells you it needs a connection to check your answer, it means it never stored one.
No account, and what that actually costs
"Offline" and "no account" travel together because they fail together. An app that keeps your progress on a server needs you to be identifiable, and being identifiable needs a login, and a login needs a connection at cold start.
Here is the honest trade, both directions.
| Going without an account | Going with one |
|---|---|
| The app opens instantly, on a plane or a train | Progress survives a new phone |
| No password, no email, no phone number to hand over | Stats follow you between a phone and a tablet |
| No sign-in wall the day the wifi is down | Leaderboards and friends, if you want those |
| Nothing to delete later | Somebody can restore your streak when it breaks |
| Reinstall the app and your history is gone | Reinstall and it comes back |
That last row is the real cost and it is worth saying plainly. Local-only progress is exactly as durable as the app on your phone. Change phones and a four-month streak is a four-month streak that used to exist. If you care about the record more than the privacy, take the account. If your longest session is the Tube and you would rather not hand a puzzle app an email address, do not.
There is a middle position most people miss: an app with a local export or a backup file gives you the durability without the login. Look for it in settings before you decide the choice is binary.
Ads without a signal
Ads need a connection. That single fact explains most of what you see offline, and it produces one good outcome and one bad one.
The good one: an ad with nowhere to fetch from usually shows nothing, so a lot of apps are quieter at 30,000 feet than on wifi. Do not read that as a fix. An ad the app pulled down before you left the house still plays.
The bad one: an app that waits for an ad before letting you through. A screen that spins for eight seconds and then carries on is an ad request with a timeout on it, and it arrives every time you finish a puzzle. Time that spin during your flight-mode test. Once is a preview of every board you finish for the life of the app.
One thing to do before you lose signal. If the app offers a rewarded video that suppresses ads for a period, that video has to download, so watch it at the gate rather than in the air. The same goes for anything the app hands over in return for a video, none of which can work with the aerial off.
Ads are their own subject, and what to look for before you install goes through where they sit and which placements ruin a session.
Scanning and solving a paper grid offline
Text recognition runs on the phone in a modern scanning app, not in a data centre, which means a camera solver can work with the aerial off. That is the difference between a newspaper on a flight being solvable and being decoration.
Do not take it on faith, because some recognition libraries pull a model down the first time they run. Test it the same way as everything else: turn on airplane mode, photograph a printed grid, and see whether the digits come back. If they do, they will keep coming back. The detail of what reads well and what does not is in solving a Sudoku from a photo.
The solving half is not in question. A backtracking solver finishes a 9x9 grid in milliseconds on a phone from 2016. No app has a technical reason to send your grid anywhere, and how a solver works explains why the problem is so small.
What I built, and where it stops helping
I built Sudoku Master partly because the offline story in most Sudoku apps falls apart at exactly the moment you need it. Here it is run through the same nine steps, honestly.
All 4,000 puzzles are in the binary: 1,000 easy, 1,000 medium, 1,000 hard and 1,000 extreme, each stored with its solution. That is what offline means here, and it is why there is no download step and no account. Every grade opens with the aerial off, and so do the challenges, which are generated per window rather than fetched: one a day at each grade, three a week, five a month, each with an estimated time from 6 minutes at easy to 35 at extreme.
Statistics live on the phone. Games completed, games lost, best and average time, lowest mistakes, streaks and a heatmap, per difficulty. The camera reads digits on the device with Google ML Kit and the photo is not uploaded, so a scanned newspaper grid reads the same at cruising altitude as it does in the kitchen, and every digit it gets wrong can be corrected in the editor first.
Where it stops helping. Everything above is the trade this page described, taken at full price. No account means nothing to restore, so a reinstall or a new handset takes the streak, the heatmap and the best times with it. Screenshot them first if they matter to you. Classic 9x9 is the only shape it deals. And what the solver hands back is a solved grid and no working, which is the right trade for settling yesterday's paper on a train and the wrong one at cell forty of a puzzle you were enjoying.
Ads, because the flight-mode section above raises the question. They sit on the screens you cross to reach a puzzle, and on the exit from the scanner, and nowhere near the board. What to look for before you install has the placements in full.
Common mistakes
Running the test on the plane. By then the app is installed, the grade you wanted is missing, and the store is out of reach. Nine steps at home, once.
Trusting the store description. "Play offline" is written by the same person who wrote the feature list. The flight-mode test is written by you.
Forgetting the force-quit. An app that has been alive in memory since Tuesday proves nothing about a cold start over the Atlantic.
Assuming a rules check is an answer check. A grid with no repeated digit can still be dead, and it will stay quiet for another forty cells.
Downloading a huge puzzle pack you will never finish. A thousand puzzles is a couple of years at one a day. Pack size is not a feature, and what makes a grid hard matters far more than how many of them there are.
Treating a local streak as a saved streak. It lives on that phone and nowhere else. Take a screenshot before you migrate, if it matters to you.
Questions people ask
Does a Sudoku app use mobile data?
A bundled one uses almost none for the puzzles, because the puzzles are already there. Any data it does use is ads, analytics or sync. Check the per-app data figure in your phone's settings after a week and you will see which kind you installed.
How much storage does an offline Sudoku app need?
The puzzles themselves are tiny, in the region of a few hundred kilobytes of text for thousands of grids. Whatever else the app weighs is code, images and sound, not the content. An app claiming a large download because of its puzzle library is telling you something inaccurate.
Why does my Sudoku app say no connection when I open it?
Almost always a login, an ad request or a config fetch at launch rather than the puzzle. If the board works once you dismiss the message, the game is offline and the wrapper is not. If it will not let you past, the app is online-only whatever the listing said.
Can I play Sudoku on a plane?
Yes, on any app that passes the nine steps above. Do the test before you leave. And download or print a spare, because a printed grid has never once asked for a signal.
Will I lose my progress if I reinstall an offline app?
If there is no account and no export, yes. Local means local. Some apps offer a backup file or a device-level restore, which is the thing to look for if a long streak matters to you.
Do offline Sudoku apps still show ads?
Many do, and they simply show fewer with the aerial off because the ad cannot load. Watch for the opposite behaviour instead: a screen that pauses waiting for a request that will never arrive.
Can I scan a puzzle from a newspaper without a connection?
Yes, if the app recognises digits on the device rather than uploading the photo. Test it in airplane mode with one real grid. Scanning with a phone camera covers what makes a scan read cleanly.
Does an offline app still give me a new puzzle every day?
It can, if the daily is picked from the phone's clock out of a bundled library rather than fetched. The test is the one above: open the challenge list with the aerial off and see whether today's is there.
Keep reading
- Best Sudoku app: the seven tests to run before you keep one, because offline is only the fifth of seven things worth testing
- Sudoku apps and ads: what to look for before you install, because ad placement decides more sessions than any feature
- Sudoku on paper or on a phone: an honest comparison, if you are weighing a book against a screen for the same journey
- Scan a Sudoku with your phone camera, for turning the newspaper in your bag into a solvable grid
- A daily Sudoku habit that actually sticks, which is mostly a question of having a puzzle available when the gap appears
- The complete Sudoku guide, the short version of everything in the series
Get it: Sudoku Master, free on iPhone and Android. All 4,000 puzzles are already in the app, so it opens the same on a plane as it does at home.
