Using the calculator
Type the file size, like `50 GB` or `700 MB`, and set your connection speed underneath. The time appears straight away, along with the working, which turns the size and the speed into bits and divides one by the other.
It works the same for uploads. Use your upload speed, which on most home broadband is far lower than the download speed.
To go the other way, switch to Size and time → Speed needed and write the size and the deadline with "in" between them: `50 GB in 2 hours` needs 55.56 Mbps. Times can be written as `2 hours`, `1h 30m`, `90 minutes` or `1:30:00`.
Bits, bytes and the factor of eight
Broadband is sold in megabits a second. File sizes are in megabytes. A byte is 8 bits, so a 100 Mbps line moves at most 12.5 megabytes a second, and that factor of eight is the usual reason a download seems slower than the advert.
The case of the b is the whole difference. Mb is megabits and MB is megabytes. The calculator reads a lowercase `mb` as megabytes, since that's how people type file sizes, and says so in the working. The Mbps to MB/s converter shows a speed in every unit at once.
Why real downloads take longer
The answer is the best case, with the whole line given to this one file the whole time. Real transfers lose some of that to protocol overhead, the headers every packet carries and the acknowledgements going back. Wi-Fi loses more. And a download can only go as fast as the server sends, which is often the real limit.
There's a quieter trap in the size. Windows counts in 1,024s but labels the result GB, so a file Windows calls 50 GB is 50 GiB, 7.4% bigger than 50 GB, and the working works that out too. The MB to GB converter explains the two systems.
A worked example
50 GB is 50 × 8,000,000,000 = 400,000,000,000 bits. A 100 Mbps line carries 100,000,000 bits a second. 400,000,000,000 ÷ 100,000,000 = 4,000 seconds, which is 1 hour, 6 minutes and 40 seconds.
Double the speed and the time halves: 33 minutes and 20 seconds at 200 Mbps. At 1 Gbps it's 6 minutes and 40 seconds.
The station wagon
"Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway," Andrew Tanenbaum wrote in his textbook Computer Networks. Amazon took him at his word in 2016 with Snowmobile, a truck that carried up to 100 petabytes. Put 100 PB through this calculator at 1 Gbps and it comes out at more than 25 years. The truck was quicker.
Questions
How long does 1 GB take to download at 100 Mbps?
1 minute and 20 seconds at best. 1 GB is 8,000 megabits, and 8,000 ÷ 100 = 80 seconds.
Why is my download in MB/s an eighth of my broadband speed?
Because broadband is measured in bits and downloads in bytes, and a byte is 8 bits. 100 Mbps is 12.5 MB/s.
Does it work for uploads?
Yes. Put in your upload speed instead. Most home connections upload much more slowly than they download.
What speed do I need to upload a file by a deadline?
Switch to Size and time → Speed needed and type something like 20 GB in 1 hour. Leave some headroom above the answer.
Sources
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