Most people buy too much. A few buy too little and discover it on day nine. Here is the arithmetic.
Per hour, per app
Maps with navigation: about 5 MB per hour. Messaging and email: about 10 MB. Instagram scrolling: about 150 MB. Music streaming: about 70 MB. Video calls: about 500 MB. Netflix in HD: about 3 GB.
A typical travel day
Two hours of maps, an hour of social, messaging all day and 20 photos uploaded: roughly 400–500 MB. That is about 3–3.5 GB a week and 12–15 GB a month.
The rule
Multiply your days by 0.5 GB and round up to the next plan size. If you are unsure, take the smaller plan and turn on auto-renew, or top up in the app when you run low.