Estimate how long a download will take
4 GB at 100 Mbps is not 40 seconds — the bits-versus-bytes factor of eight surprises everyone once.
Tools used in this guide
The job
File sizes are bytes; connection speeds are bits — Mbps is megaBITS. 4 GB at 100 Mbps works out to 320 seconds, about five and a half minutes. Enter the size and speed, and the tool handles the ×8.
Picking the right units
The size dropdown includes binary units (MiB/GiB) because OS dialogs report those: what Finder calls 4.29 GB is 4 GiB. Speed options include KB/s and MB/s for tools that show bytes per second.
Why real downloads run slower
This is best-case math. Protocol overhead, Wi-Fi loss, server throttling, and congestion typically cost 10–30%; a slow evening can cost more. Use the estimate as a floor, not a promise.