How this calculator works
Bandwidth is the number of bits a link carries each second. A file size is measured in bytes, and one byte is eight bits, so the arithmetic starts by converting the file to bits. Divide those bits by the link speed to get seconds. A 100 Mbit/s plan carries 100 million bits each second, which is 12.5 million bytes, so a 5 GB file takes 400 seconds in ideal conditions. The two directions of a home connection are usually asymmetric, and Wi-Fi, protocol overhead and competing traffic reduce the usable speed; treat the answer as a best case.
Formula
transfer time (s) = file size (bytes) x 8 / bandwidth (bits per second). file size (bytes) = bandwidth (bits per second) x time (s) / 8. bandwidth (bits per second) = file size (bytes) x 8 / time (s).
Worked example
A 5 GB download on a 100 Mbit/s link: 5,000,000,000 bytes x 8 = 40,000,000,000 bits. Dividing by 100,000,000 bits per second gives 400 seconds, or 6 minutes 40 seconds. In binary units the same file is 4.66 GiB, but the transfer time does not change because the cable counts bits, not display units.
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Frequently asked questions
Why does my download finish later than this?
The advertised speed is a peak figure. Overhead from TCP, Wi-Fi and encryption, server limits and other devices on the network all take a share. Real throughput is often 70 to 90 percent of the plan speed, and the gap is largest on long-distance transfers and busy evenings.
Is a megabit the same as a megabyte?
No. One megabyte is eight megabits. A 100 Mbit/s plan is 12.5 MB/s. The capital B marks bytes and the lower-case b marks bits, which is why the two numbers differ by a factor of eight.
What is the difference between MB and MiB?
A decimal megabyte is 1,000,000 bytes and a mebibyte is 1,048,576 bytes. Drive makers and internet providers use decimal units, while many operating systems report binary ones, so a 5 GB file can appear as 4.66 GiB. Both describe the same file.
Should I use bits or bytes for the bandwidth?
Use whatever the speed is quoted in. This page converts both correctly, so 12.5 MB/s and 100 Mbit/s give the same result.