File Transfer Time Calculator

In short The advertised speed of a connection is not the speed a file moves at. Protocol overhead, TCP behaviour, disk speed at both ends and the difference between bits and bytes all take a share, and the last one alone catches people constantly: a 100 Mbit/s connection moves at most 12.5 MB/s, so a 1 GB file takes at least 80 seconds and not the 10 people expect. Realistic overhead is applied here rather than the theoretical maximum, because the theoretical figure is never achieved on a real link. The other large effect is file COUNT. Ten thousand small files transfer far more slowly than one large file of the same total size, because every file carries its own round trips for open, metadata and close, and on a high-latency link those round trips dominate completely — this is why archiving a folder before sending it can be several times faster even including the time spent compressing. Two further limits worth knowing: on a long-distance link, TCP window size and latency can cap throughput well below the line rate regardless of bandwidth, and on many home connections the upload speed is a fraction of the download speed, which is the number that matters when you are the one sending.