100 GB
is 100,000 MB, or 100,000,000 KB or 100,000,000,000 bytes.
That's the SI definition.
If you want to work in binary-factor multiples, you need to talk about Gibibytes (GiB), being 1024 Mibibytes (MiB), each of those being 1024 Kibibytes (KiB), each of those being 1024 bytes. (Thus 100 GiB is 102,400 MiB, or 107andabit MB, being 107,374,182,400 plain old bytes.)
When quoting "what they will allow", it often behoves them ISPs to use the SI standards, much as HDD vendors will because it looks bigger than the equivalent "not quite X GiB" value (for X GB) or "only just X GiB" (for X.something GB). Although they may also try to confuse you with something like 8M
b/s (8 million
bits per second) speed rather than 1M
B/s (1 million
Bytes per second) if they can get away with it.
Not that it matters to most people. They get used to what they have (I've just reached over and picked up from my desk a 28,800 V.34/V.32bis PCMCIA card modem (
with XJACK(R)!!!), which was perfectly good enough back in its day) and if instead of getting 23 Hoojamaflips they now get 32 Hoojamaflips, they're happy, whether that's speed, total allowance, on-line storage space, etc.
Still, 100GB/month is 100 times what I use from a mobile machine[1] that regularly browses image-intensive stuff like Google Maps. So the near-constant video streaming (e.g. YouTube/iPlayer/whatever) and downloading (I suspect something Limewire-like, in which case I hope you have good AV) is what's eating it up, but even then it's probably going some.
[1] TBH, I have no idea what any of the other machines I use are up to, as they aren't so directly metered, although as an administrator of the firewall and someone able to add other devices, on demand, directly in-between my workplaces router and the first point of splitting (as well as into various links leading to various parts of the building, or even before/after individual local distributing switches and routers), I suspect I could find out. But all of that is in a great big building, with a mix of very IT-orientated people (who need to generate a lot of traffic in any case) and very ordinary people (who just generate a lot of traffic anyway, it just seems to be what they do), so I presume it's going to be a bloomin' cartload of bandwidth being burnt up during normal hours, and probably not a little off-hours traffic as well. Actually, the guy in charge of directly liaising with the ISP and paying those bills has essentially said that we
are doing that, so it's not much of a guess, although I've so far resisted asking for exact values .