Use it as a starting point, then adjust for the two things it cannot know. First, whether you will really have usable Wi-Fi at your accommodation — if unsure, round up. Second, whether you are heading somewhere that makes buying more difficult; if so, round up as well. In a large city with reliable accommodation Wi-Fi, buying to the number is reasonable.

Because real trips have days that go off plan: maps open all day because a train was cancelled, a longer call home than usual, accommodation Wi-Fi that does not work. A small buffer costs less than solving the problem when the data runs out somewhere unfamiliar.

Only on plans that state they support top-ups, which the plan page tells you. Check that before buying if you intend to buy tight. If a plan cannot be topped up you can still buy another and install it as a second profile — but that needs a connection to complete, so act before you hit zero.

There is no fixed figure, because it depends on what your companions do. If they check maps and chat, the addition is small; if they watch video in transit, it can exceed your own usage. The direct approach is to ask what they normally do and add accordingly — or give everyone their own plan if the group will split up during the day.

Not quite, because nearly all unlimited plans have a daily full-speed ceiling. The question shifts from how many gigabytes to whether that daily ceiling covers your day — which still requires estimating daily use. The number from this page remains useful for comparing against the daily ceiling of the plan you are looking at.

Because real usage depends on things this page cannot see: background photo backup, the quality of your accommodation Wi-Fi, how many hours you spend out of the room, and video habits that vary enormously between people. The way to get a better number is to record this trip's total and use it as the base next time, which always beats an average.