Where did all my mobile data go?

It's only the 20th of the month, and Liz's phone says she's out of mobile data.

“I barely did anything!” she says.

“Let's follow it,” says Dan.

See which apps used your data

Your phone keeps count. It lists every app and how much mobile data each one used, so you don't have to guess.

• On iPhone: open Settings, tap Mobile Data (it's called Cellular in the US), then scroll down. Each app has its number next to it.
• On a Google Pixel: open Settings, tap Network & internet, then SIMs, then App data usage. The apps are listed below the graph.
• On other Android phones, the menus have different names. Search Settings for “data usage”.

Two things to know before you read the numbers:

• An iPhone counts since the last reset, not since the start of your month. At the very bottom of the list there's a Reset Statistics button. If you've never pressed it, those numbers might cover months or years.
• Your phone's count and your carrier's count can differ. Google says so plainly: some carriers measure usage differently. When it's about your bill, your carrier's app or account page has the number that counts.

For Liz, the two biggest numbers belong to her video apps. Every morning and every evening, she watches shows on the bus.

Why video eats so much

Video is the heaviest thing most of us do on a phone. Worldwide, it's about three quarters of all mobile data traffic, according to Ericsson's latest mobility report.

How much is that for one person? Netflix publishes its own figures:

• High definition can use up to 3 GB an hour.
• On the phone app, the highest setting can use 1 GB every 20 minutes or more.
• The app's normal setting uses about 1 GB every 4 hours.
• The lowest setting, Save Data, uses about 1 GB every 6 hours.

So the same hour on the bus can use anything from about a sixth of a gigabyte to 3 GB, depending on one setting. YouTube doesn't publish figures like these, but the idea is the same: the sharper the picture, the more data it uses.

Texts, maps and email usually use far less. That's why Liz “barely did anything” and still ran out: the bus shows did almost all of it.

Keep watching, just use less

“So no more videos on the bus?” asks Liz.

“Keep watching!” says Dan. Three changes do most of the work.

Download at home, on Wi-Fi

Most video apps let you download shows to watch later. Download them at home on Wi-Fi, and watching them on the bus doesn't stream the video over mobile data.

Not everything can be downloaded. Some shows aren't allowed to be, and on YouTube, downloads are mostly a YouTube Premium feature. Netflix also has a Wi-Fi Only setting, so it never uses mobile data unless you let it.

Lower the quality on mobile data

Video apps let you pick one quality for Wi-Fi and another for mobile data:

• YouTube: in the app's settings, under Video quality preferences, choose Data saver for mobile networks. Keep the higher quality for Wi-Fi.
• Netflix: in the app's settings, set Mobile data usage to Save Data.

On a phone screen on a moving bus, most people can't tell the difference. The data meter can.

Check the “weak Wi-Fi” setting

Phones can quietly use mobile data when your Wi-Fi is weak, so things keep loading. On an iPhone it's called Connectivity Assist, and it's in the Wi-Fi settings (older iPhones called it Wi-Fi Assist, under Cellular). Apple says that with it on, you might use more mobile data. Many Android phones have a similar switch under their Wi-Fi settings, with a different name.

It's a trade-off, not a villain: it keeps your connection steady at home when the Wi-Fi is patchy. Apple also says it isn't used with some third-party video apps. If your data runs out and you're often on weak Wi-Fi, try turning it off for a month and compare.

Other things that use data quietly

If video isn't at the top of your list, look for these:

• App updates and backups over mobile data.
• Photos syncing to the cloud.
• Apps refreshing in the background, even when you're not using them.

An iPhone has one switch for most of these: Low Data Mode, in Settings, Cellular (or Mobile Data), Cellular Data Options. It turns off background app refresh and automatic downloads and backups, pauses iCloud Photos updates, and lowers the quality of streaming. Android phones have a similar mode, often called Data Saver.

You can also switch mobile data off for a single app, right from the list where you saw its number.

Next month: the 30th

Next month, it's the 30th, and Liz still has data. Same bus. Same shows.

Her sum-up: video eats her data, and her phone shows her which apps used it.

What you can do

• Look at the list: Settings, Mobile Data (Cellular in the US), scroll down. On Android, search Settings for “data usage”.
• On iPhone, reset the statistics on the day your plan renews, so the list shows this month only.
• Set a data warning on Android, so your phone tells you before you run out.
• Download shows at home, on Wi-Fi, before the trip.
• Set your video apps to a lower quality on mobile data.
• Check the setting that uses mobile data when Wi-Fi is weak, and try Low Data Mode if you're close to your limit.