File Transfer Time

Size divided by throughput. A 700 MB image at a sustained 5 MB/s takes 140 seconds, and there is nothing more to the arithmetic. The difficulty is entirely in the units, and it costs people a factor of eight with impressive regularity.

Network rates are quoted in bits per second and file sizes in bytes, and the two are deliberately confusable. A 100 Mbit/s connection moves 12.5 MB/s at best, so a 4.7 GB image takes about 376 seconds rather than the 47 that a careless reading suggests. The lowercase b is bits, the uppercase B is bytes, and marketing departments have never once made that clearer than they had to. This page assumes megabytes and megabytes per second; if your rate is in megabits, divide it by 8 before entering it.

Then there is the difference between the rate on the box and the rate you get. Protocol overhead removes a few percent, TCP slow start costs you the first fraction of a second, and on a long link the window size may cap you far below the line rate regardless of how much bandwidth you bought. For anything that has to finish inside a window, measure a real transfer and use that number. And before buying a faster link, check whether compressing the payload is cheaper, because halving the file has exactly the same effect on the clock as doubling the pipe.