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.

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.