How this calculator works
A download streams bytes from a server over a link measured in bits per second. Multiply the file size in bytes by eight to get bits, then divide by the download speed. A 1 GB file on a 100 Mbit/s line takes about 80 seconds in theory. In practice the server, the route and the storage at both ends set a ceiling, and the browser or app often pauses between chunks, so the real time is longer. Downloads can also be limited by the plan during peak hours.
Formula
download time (s) = file size (bytes) x 8 / download speed (bits per second). file size = download speed x time / 8. download speed = file size x 8 / time.
Worked example
A 1 GB file at 100 Mbit/s: 1,000,000,000 bytes x 8 = 8,000,000,000 bits. Divide by 100,000,000 bits per second to get 80 seconds, which is 1 minute 20 seconds. At a measured 80 Mbit/s the same file takes 100 seconds, or 1 minute 40 seconds.
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Frequently asked questions
Why does my download never reach the plan speed?
The advertised figure is the best case on the provider network. Protocol overhead, Wi-Fi, the server's own upload limit and traffic on your line all take a share. A wired connection during a quiet period on a well-connected server typically reaches 80 to 90 percent of the plan speed.
Is 100 Mbit/s the same as 100 MB/s?
No. 100 Mbit/s is 12.5 MB/s, because one byte holds eight bits. A 1 GB file takes about 80 seconds at 100 Mbit/s but only 8 seconds at 100 MB/s. Speed test apps and internet plans use bits per second.
Do downloads resume where they left off?
Many do, using HTTP range requests. If a download resumes, the remaining time depends on the remaining bytes, not the whole file. This page assumes a single unbroken transfer from start to finish.
Does a faster plan always help?
Only if the bottleneck is your connection. A slow Wi-Fi link, an old hard drive or a busy server will cap the speed first. Check the measured speed during the download before paying for a faster plan.