Bits, Bytes and Bandwidth
The conversion nearly everyone gets wrong, and the reasons a real download never hits the advertised number.
BFCBrilliance · bfcbrilliance.com/tools/download-time-calculator
The unit that causes all the trouble
- 1 byte
- 8 bits
- Mb (small b)
- megaBITS — how plans are sold
- MB (capital B)
- megaBYTES — how files and downloads are shown
- Mbps to MB/s
- divide by 8
- 100 Mbps
- 12.5 MB/s at best
- 1 Gbps
- 125 MB/s at best
- The giveaway
- the capital letter, and nothing else
The other ambiguity — size units
- Operating systems
- 1 GB = 1024 MB (used here)
- Drive manufacturers
- 1 GB = 1000 MB
- Difference
- about 2.4% per step, and it compounds
- Why '1 TB' shows as 931 GB
- roughly 7% across four steps
Why the real download is slower
- Protocol overhead - a few percent before anything else
- The SERVER at the other end may be the limit, not your line
- Wifi is usually slower than the connection feeding it
- Everyone in the house shares one line
- Speed tests measure a nearby server under ideal conditions
- Distance to the server matters - a far one is slower at the same speed
- A cable instead of wifi is the biggest single fix for most people
Divide by eight
If you remember one thing: your connection speed in megabits, divided by eight, is the best you will ever see in a download manager. A 100 Mbps line moves 12.5 MB/s. When someone says their gigabit connection is 'only downloading at 100', they are usually looking at 100 megaBYTES per second, which is about 80% of the line and rather good.
This number is a floor
The calculation assumes full advertised speed for the whole transfer with nothing in the way. Nothing in the real path guarantees that, and several things guarantee otherwise. Use it to know whether a download is a coffee break or an overnight job — not to set a timer by.