Video bitrate and size
Before you press «export» there are three questions, and your editing program answers none of them: how big it will be, what bitrate you need to stay under the limit, and how many minutes fit on a card. Here the sum is done, and it also tells you whether it fits in an email.
The sum, and why it has to be done backwards
The size of a video is a multiplication: bitrate times duration, divided by eight because bits become bytes. The sum you actually need, though, is the other one: the portal takes 10 MB, the video runs four minutes, what bitrate do I set? Guessing means exporting, looking at the size, changing it, exporting again: half an hour thrown away each round. Here the number comes out straight away, and the audio is counted separately, because that takes up room too (and on a short video it weighs more than you would think).
A megabyte is not a megabyte
Whoever takes your upload counts 1 MB = one million bytes; Windows counts 1,048,576. The difference is 5%, and on an upload limit that is exactly the difference between «accepted» and «refused». Here the count is in millions, the way websites, mailboxes and the Mac count, and next to it you also get what it would be the other way, so you are not surprised when your computer shows a different number.
An email attachment grows by a third
This is what makes emails bounce. An attachment does not travel the way it sits on your disk: it is rewritten in a form made only of letters and numbers, and three bytes become four. With a mailbox that accepts 25 MB, the real file has to stay under 18-19 MB. Here the sum is already done both ways: how big the attachment will be, and how far under you have to stay so it does not bounce.
How much fits on a card
With your camera's bitrate (you find it in the quality menu) you immediately get how many minutes fit on a card, and that is the sum you do while packing. Watch out for one thing: a «128 GB» card does not have 128 GB free once formatted, it has somewhat less, so next to it you also get the cautious number. And remember that a long take can stop early because of the 4 GB per file limit of certain cards or cameras.
What it does not do
It compresses nothing and converts nothing: it is a calculator. To actually make a file lighter there is Compress video, to change format Convert video to MP4, and if the file already exists and you want to know its real bitrate, that is the job of What your video really is. There is no «right» bitrate in the absolute: it depends on how much the picture moves and on the codec (with H.265 or AV1 about half is enough compared with H.264 for the same quality).