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Arithmetic
Upload and download time calculator
Mostly here to make one point: your connection's advertised speed is in bits, your file is in bytes, and the factor of eight between them explains a lot of frustration.
Speeds are in megabits. Files are in megabytes. Divide by eight.
Result
Bits, bytes, and the missing eight
Connection speeds are quoted in megabits per second and file sizes in megabytes. There are eight bits in a byte, so a 40 Mbps connection moves about 5 megabytes per second at best. A 25 MB attachment therefore takes at least five seconds, not the fraction of a second the headline number suggests.
Then reality takes its cut. Protocol overhead, encryption, and the fact that no connection sustains its peak rate mean 80% of theoretical is a good real-world result, which is what the realistic figure below assumes.
The bigger asymmetry is upload against download. Most domestic connections are built for consuming rather than sending, and upload speeds are frequently a tenth of download. If a file feels slow to send but fast to receive, nothing is wrong; that is how the line was sold to you.
Common questions
Why is my upload so much slower than my download?
Because most broadband is asymmetric by design. Check your plan: a 100 down and 10 up connection will send a large file ten times slower than it receives one.
Does compressing first actually save time?
Almost always, and by exactly the proportion you saved. Halving a file halves the transfer. The savings calculator puts numbers on it.
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