Bits and bytes are not the same unit
File sizes are quoted in bytes, usually megabytes or gigabytes, while connection speeds are quoted in bits per second, almost always megabits. There are eight bits in a byte, so a 100 Mbps line moves about 12.5 megabytes per second, not 100. Most of the confusion around download estimates comes from that single missing factor of eight. The calculator does the conversion for you, but it is worth knowing which unit sits on each side.
A second source of error is the megabyte itself. Storage vendors and operating systems have historically disagreed about whether a megabyte is 1,000,000 bytes or 1,048,576. This tool treats a megabyte as 1,048,576 bytes, so a 1000 MB file means 1,048,576,000 bytes. Speed is handled in decimal megabits, 1,000,000 bits per second, which is how ISPs quote it.
The formula the page runs
Size in megabytes is multiplied by 1,048,576 to reach bytes, then by 8 to reach bits. That total is divided by the connection speed in megabits per second multiplied by 1,000,000. The result is seconds, formatted as seconds, minutes, hours or days depending on magnitude. For gigabytes the size is multiplied by 1024 first, and kilobytes are treated as one thousandth of a megabyte.
The estimate assumes a single uninterrupted stream running at the full quoted rate, with no protocol overhead, no competing traffic and a server that can feed you data as fast as you can take it. Real transfers fall short of that, which is why the answer is a floor rather than a prediction.
A 1000 MB file on a 20 Mbps line
Download a 1000 MB file on a 20 Mbps connection. The file is 1,048,576,000 bytes, which is 8,388,608,000 bits. Divide by 20,000,000 bits per second and you get 419.4 seconds, or just under seven minutes. Run the same file on a 100 Mbps line and the answer drops to 83.9 seconds, because the arithmetic is linear in speed. At 50 Mbps it is 167.8 seconds, or 2.8 minutes.
A larger case: a 4.7 GB disc image, which the tool reads as 4.7 x 1024 = 4812.8 MB. That is roughly 40.4 billion bits, so at 50 Mbps it takes about 807 seconds, or 13.5 minutes. If someone tells you the same file took them two hours, the bottleneck is not their line rate: a saturated Wi-Fi link, a throttled server or a slow disk will dominate the transfer long before the nominal speed does.
Limits, and what to compare against
Because the estimate excludes overhead, expect actual times to run longer. TCP and TLS handshakes add a fixed cost that matters for many small files and almost nothing for one large one, and packet headers shave a few percent off throughput on their own. Wi-Fi retransmissions, a busy household, a congested peering link or a server that caps per-connection speed can each cut the effective rate substantially. Uploads are usually slower than downloads on consumer connections, so use your measured upload figure if you are estimating a send rather than a receive.
To get the speed figure that belongs in this field, measure it rather than trusting the plan, with the internet speed test. If the question is really about how large a file is in another unit, data storage converter does the byte arithmetic, and speed calculator handles distance over time for the physical rather than the digital case.