Reference
File size and link speed, turned into a transfer time. Bits and bytes are not mixed up quietly.
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Quick answer
Download time in seconds = (file bytes × 8) / bits per second. A 500 MB file on a 100 Mbps link takes 40 seconds, because 500 MB is 4,000 megabits and 4,000 / 100 = 40.
Megabytes are not megabits
File sizes are bytes. Link speeds are usually bits. There are 8 bits in a byte, so a megabyte is 8 megabits. A “100 megabyte” file does not cross a “100 megabit” link in one second. It takes eight seconds, before protocol overhead. The calculator applies that factor and labels the units so the 8 does not disappear.
The formula
seconds = (size in bytes × 8) / bits per second. Decimal megabytes use 1,000,000 bytes. Decimal megabits use 1,000,000 bits. Binary mebibytes use 1,048,576 bytes. MB/s means megabytes per second, which is already bytes, so the extra × 8 converts it into the bit rate the formula divides by.
A few transfers worth memorizing
| Size | Link | Time |
|---|---|---|
| 500 MB | 100 Mbps | 40 seconds |
| 1 GB | 1 Gbps | 8 seconds |
| 1 GB | 100 Mbps | 1 min 20 sec |
| 10 MiB | 10 MB/s | 1.05 seconds |
1 GB at 1 Gbps is 8 seconds because a decimal gigabyte is 8 gigabits. 1 GB at 100 Mbps is 80 seconds, which is 1 minute 20 seconds. The 10 MiB row is the awkward one: a mebibyte is a little larger than a decimal megabyte, so 10 MiB at 10 MB/s takes 1.05 seconds, not 1.
Download speed calculator: Mbps to MB/s
Internet plans quote megabits per second (Mbps). Download managers show megabytes per second (MB/s). Divide Mbps by 8 to get MB/s: 100 Mbps is 12.5 MB/s. The Mbps to MB/s converter does it for any speed, and the bits to bytes converter explains the factor of 8.
Data transfer calculator
The same arithmetic runs the other way: data moved = rate × time. Decimal units, at the full line rate:
| Link | Time | Data moved |
|---|---|---|
| 10 Mbps | 1 hour | 4.5 GB |
| 100 Mbps | 1 minute | 750 MB |
| 100 Mbps | 1 hour | 45 GB |
| 1 Gbps | 1 hour | 450 GB |
Why the download is slower than this
The figure here is the wire time of the payload at the rate you typed. TCP overhead, encryption, a shared link, a slow disk, or a rate limit will add time. It will not add a factor of eight. If the estimate is off by about eight, someone mixed MB and Mb.
Questions
How long does 500 MB take at 100 Mbps?
40 seconds. 500 MB is 4,000 Mb, and 4,000 / 100 = 40.
What is the difference between MB/s and Mbps?
MB/s is megabytes per second. Mbps is megabits per second. 1 MB/s is 8 Mbps.
Does a gigabit link move a gigabyte in one second?
No. A decimal gigabyte is 8 gigabits, so a 1 Gbps link takes 8 seconds at line rate.
How many MB is 1 Mbps?
1 Mbps = 0.125 MB/s, which is 125 kilobytes per second. Divide Mbps by 8 for MB/s.
Are Mbps and MB/s the same?
No. Mbps is megabits per second and MB/s is megabytes per second. 1 MB/s is 8 Mbps. See the Mbps to MB/s converter.
How many MB is 500 Mbps?
500 Mbps = 62.5 MB/s in decimal units. At that rate a 1 GB file takes 16 seconds.
Is 20 MB/s slow?
20 MB/s is 160 Mbps. A 1 GB file takes about 50 seconds at that rate. That is quick for everyday downloads and slow for very large backups, so it depends on the file size.