How much data do you actually need?
Most people either buy far too much or run out on day three. The honest answer depends on one thing: whether you watch video on mobile data.
What actually uses data
The things travellers do most are surprisingly cheap. Maps and turn-by-turn navigation use very little — roughly a few megabytes an hour, because the map draws locally and only the route data comes over the network. Messaging, email and boarding passes are negligible. Web pages and social feeds add up slowly. What costs real money is video: streaming, autoplaying feeds and video calls run into the hundreds of megabytes an hour, and high definition can pass a gigabyte. Photo backup is the quiet one — a phone set to upload every picture to the cloud can empty a small plan in a day without you touching it.
Rough amounts for a week away
If you mostly navigate, message and look things up, and you use hotel or café Wi-Fi in the evenings, 1 to 3 GB covers a week comfortably. If you are on the phone through the day — maps, searching, social feeds, the occasional video call — 5 GB is the sensible middle, and it is the size most travellers land on. If you stream, video call daily, or work from the road, take 10 GB or more. For a longer trip or for tethering a laptop, 20 GB and up stops you thinking about it at all.
How to make a smaller plan go further
Download offline maps for the city before you fly — this alone removes the largest recurring use for most people. Turn off automatic photo and video backup, or restrict it to Wi-Fi. Stop autoplaying video in your social apps. Download music, films and podcasts at home rather than streaming them abroad. Use Wi-Fi where it is genuinely available, and check your phone’s data usage screen after two days: it will tell you whether you are on track long before you run out.
What happens if it runs out
The data stops, and that is all. Nothing renews automatically and nothing extra is charged — we hold no payment method on file, so there is no way for an unexpected bill to reach you. To carry on, buy another plan for the same destination: it arrives as a separate profile with its own QR code, which you install and switch to in your phone settings. Your original SIM is untouched throughout.
Buying a size up is usually the cheaper mistake
Per-gigabyte prices fall as plans get bigger, so the step from a small plan to the next one up is rarely proportional — on most destinations a 10 GB plan costs far less than two 5 GB plans. Running out mid-trip means buying a second plan at the small-plan rate, on airport Wi-Fi, in a hurry. If you are genuinely unsure, the larger plan is the better bet.