Estimate how long a file takes to download.
Download time trips people up because file sizes are quoted in bytes and connections in bits, a factor of eight apart. A 100 Mbps link therefore delivers at most about 12.5 MB/s, so a 4.7 GB file cannot arrive in under roughly six minutes no matter how it feels. Two further details matter. Storage vendors and operating systems disagree on the gigabyte: a decimal GB is 1,000,000,000 bytes while a binary GiB is 1,073,741,824, a 7.4% difference that compounds on large transfers. And no link runs at its rated speed — protocol headers, TCP slow-start, server-side limits and Wi-Fi loss typically leave 80-90% of the nominal figure, which is why the efficiency factor is part of the calculation rather than an afterthought.
Transfer time from size and bitrate
Megabits = size in GB x bytes per GB x 8 / 1,000,000; effective Mbps = link speed x efficiency / 100; seconds = megabits / effective Mbps
The bottleneck may not be your link. Server-side rate limits, a congested peering path, a slow disk or a single-threaded transfer with high latency can all cap throughput well below your access speed.
Match whatever quoted the size. Download pages and storage vendors usually mean decimal GB; Windows file properties and most Linux tools report binary GiB while labelling it GB, which is why a file can look smaller than advertised.
Only while your link is the constraint. Past a few hundred megabits, latency, protocol overhead and the server become dominant, and doubling the plan may barely change a single-stream download.