Bits and bytes are different

Network rates normally use bits per second, while file sizes normally use bytes. Eight bits make one byte. A 100 Mbps connection therefore has a theoretical maximum of 12.5 MB per second before protocol and other overhead.

Decimal and binary file units

This tool treats MB and GB as powers of 1,000, while MiB and GiB are powers of 1,024. A decimal gigabyte is one billion bytes; a gibibyte is 1,073,741,824 bytes.

Why real transfers take longer

Advertised line speed is not guaranteed application throughput. Wi-Fi conditions, congestion, server capacity, protocol overhead, latency, storage speed, simultaneous traffic, retries, and speed variation can all increase elapsed time.

Work an ideal-time example

A decimal 10 GB file contains 80 gigabits. At a sustained 100 Mbps, divide 80,000 megabits by 100 megabits per second to get 800 seconds, or 13 minutes 20 seconds. This is an ideal lower-bound estimate. If a measured download averages 72 Mbps, entering 72 instead gives about 18 minutes 31 seconds. Using measured application throughput is usually more realistic than entering the access plan’s headline speed.

Diagnose an estimate that looks wrong

First check lowercase b versus uppercase B: 100 MB/s is eight times 100 Mbps. Then check decimal GB versus binary GiB and whether the displayed size includes all files. For a many-file transfer, setup latency and metadata work can dominate even when the byte total is small. Encrypted tunnels, browser processing, decompression, cloud-side preparation, and storage writes may also be limiting stages. Time a representative transfer, keep its unit, and compare sustained rather than momentary peak speed.

The result assumes a constant rate and no overhead. It is an ideal estimate, not a service-speed guarantee or a measurement of your current connection.