Broadly, anything carrying continuous video: video calls, video streaming and social apps that autoplay clips. Next come full-resolution photo uploads and large downloads. Text messaging and pre-downloaded maps sit at the light end. The real volume depends on quality settings and time spent, so your own phone's usage summary is the figure to trust.

Start from your own phone's mobile usage over one ordinary week, then adjust for the shape of the trip: more if you will navigate all day or work from the room, less if the accommodation has Wi-Fi and you are out only briefly. Then choose a size with headroom rather than an exact fit, and use our calculator to organise the numbers.

Usually it is background activity: automatic photo backup, file syncing, app updates and autoplaying clips in a social feed as you scroll past. At home all of that rides on Wi-Fi and nobody notices; on a trip it rides on your plan. Open the usage summary, find the heavy app and change that one setting.

Live navigation is light compared with video, but because people leave it open for hours a day while sightseeing, the total adds up. The effective fix is downloading the city area in advance on Wi-Fi and leaving only the genuinely live parts, such as traffic or transit times, to pull data.

Not necessarily. They measure at different points and reset at different times. The phone counts what the operating system sees over the cycle you set, while the supplier's balance is what has actually been deducted from the plan. Let the supplier's figure decide how much is left, and use the phone's counter to see which apps are heavy.

First check whether your plan can be topped up. If it can, do it while data remains or while you are on Wi-Fi rather than waiting for zero, since buying needs a connection. If it cannot, you will need a new plan. Meanwhile cut consumption by turning off autoplay video and photo backup, and if you get stuck, message us on LINE @esimonline or email esimonline.asia@gmail.com.