Bits and bytes are not the same unit
A byte is 8 bits. Internet and network speeds are conventionally advertised in bits per second (kbps, Mbps, Gbps) — a holdover from telecommunications convention — while file sizes and download progress are conventionally shown in bytes (KB, MB, GB). To convert an advertised speed into an actual download rate for a file measured in bytes, divide the Mbps figure by 8.
The formula and a worked example
Transfer time (seconds) = file size in bits ÷ speed in bits per second. A 1 GB file (8 billion bits) downloaded at 100 Mbps (100 million bits per second) takes 8,000,000,000 ÷ 100,000,000 = 80 seconds — about 1 minute 20 seconds. That same 100 Mbps connection is moving data at 12.5 MB per second (100 ÷ 8), which is why a download manager showing "12.5 MB/s" for a "100 Mbps" plan isn't underperforming — it's working exactly as advertised.
A second worked example — a larger file
A 5 GB file at the same 100 Mbps connection takes 400 seconds (6 minutes 40 seconds) — exactly 5 times longer than the 1 GB file, since transfer time scales directly with file size at a fixed speed. Doubling the file size doubles the download time; doubling the connection speed instead halves it — both follow directly from the same simple division.
Why this convention mismatch exists at all
Telecommunications and networking equipment historically measured and marketed connections in bits per second, tracing back to how data is physically transmitted (as a stream of individual bits) — while computer storage and file systems have long used bytes as the natural unit for measuring data quantity. Neither convention is "wrong," but the fact that internet plans are sold in one unit while everyday file-size numbers are shown in the other creates a persistent, ongoing source of confusion for anyone comparing an advertised speed against an actual observed download rate.
A quick way to sanity-check any advertised speed
Divide any advertised Mbps figure by 8 to get a rough MB/s ceiling — a "50 Mbps" plan tops out around 6.25 MB/s, a "500 Mbps" plan around 62.5 MB/s, and a "1 Gbps" (1,000 Mbps) plan around 125 MB/s. Real-world download speeds are often somewhat below this theoretical ceiling due to overhead and network conditions, but this division is the correct first step before comparing an advertised plan speed against an actual observed transfer rate.