Open the carrier app or bill and record the last three completed billing periods. Write down local data used, the highest month and any separately counted roaming or promotional allowance. A remaining-data screen halfway through the month is not a full month of usage.
On iPhone
Open Settings → Cellular/Mobile Data and look at the per-app totals. Check the date of the last reset before treating “Current Period” as this month. At the start of your next billing cycle, note the carrier balance and reset the phone statistics. With two SIMs, check the selected data line. Apple’s instructions explain the counters and per-app controls.
On Android
On Pixel, use Settings → Network & internet → SIMs → App data usage, then choose the relevant date range. Other Android phones use different names; search Settings for “data usage”. Set the cycle start to the carrier’s billing date. Google’s guide also explains warnings and hard limits. A hard limit can cut off your data, including a laptop hotspot, so use a warning first if interruption would be costly.
Turn the numbers into a buying decision
| What you find | What to do |
|---|---|
| 8GB, 10GB, 12GB in three ordinary months | A 20GB plan leaves 8GB above your recent peak. Check whether coming travel or hotspot work changes that pattern. |
| One 80GB month after replacing a laptop | Check downloads and hotspot traffic before paying for 100GB every month. A one-off top-up may be cheaper. |
| Phone says 40GB; carrier says 15GB | Match the dates, line and data pools. Save screenshots and ask the carrier to explain unexplained billed usage. |
After a week, review the largest apps. Move cloud backups and software downloads to Wi-Fi, download maps or entertainment before travelling, and lower video quality when the screen is small. Then measure again: reducing a background transfer is more useful than guessing that all streaming uses the same amount.