In detail
The amount of mobile data you need depends far more on what you do than on how long you use your phone. Messaging, email, browsing and maps use only a few hundred megabytes a month, while an hour of HD video uses around 3 GB. That's why two people with the same screen time can need 5 GB or 100 GB.
As a rough guide, light users who are mostly on Wi-Fi get by with 5–15 GB a month. A typical mix of daily social media, music on the commute and occasional video lands around 20–50 GB. If you stream video for an hour or more a day on mobile data, use your phone as a hotspot or have no home broadband, 100 GB or an unlimited plan is the safer choice.
Social media is the most underestimated data user. TikTok, Instagram Reels and YouTube Shorts autoplay video continuously and use around 0.5–1 GB per hour — similar to SD video. Video calls on Zoom, FaceTime or WhatsApp use roughly 0.5–1.5 GB per hour depending on quality and the number of participants.
Before switching plans, check your actual usage: both iPhone (Settings → Mobile Data) and Android (Settings → Network → Data usage) show how much each app used in the current period. Compare that to the calculator's estimate and pick a plan with some headroom — running out a few days early usually costs more than the next tier up.
Is unlimited data worth it? Only if you regularly go past about 100 GB or rely on mobile data instead of home internet. Many 'unlimited' plans also slow you down after a fair-use threshold or cap hotspot data separately, so check those limits too.