Everyday

Data transfer time: why your 100 Mbps connection isn’t 100 MB/s

Data transfer time depends on file size and connection speed, and the two are usually quoted in different units: megabits per second for the connection, megabytes for the file. Internet speed is sold in bits per second, file size is shown in bytes - an exact 8x difference. A "100 Mbps" plan really moves data at 12.5 MB/s, not 100 MB/s.

What is the difference between megabits and megabytes?

ISPs advertise speed in megabits per second (Mbps) because it's a bigger, more impressive-looking number. File sizes and download progress bars show megabytes (MB) because that's how storage is measured. There are 8 bits in a byte, so "100 Mbps" is really 12.5 megabytes per second of actual throughput - a 4.7GB file at that speed takes about 6 minutes 16 seconds, not the 47 seconds you'd get from naively dividing 4700 by 100. This one mix-up explains more "why is my download slower than my internet speed" confusion than almost anything else.

Why is my download slower than my connection speed?

Even with the bits-to-bytes math right, real transfers rarely hit the full converted figure. Some of the connection carries protocol overhead rather than your file - packet headers, acknowledgements, encryption - which typically costs a few percent. Wi-Fi adds its own losses, other devices share the line, and the server on the far end has to keep up too. A practical rule: treat the calculated time as the best case, and expect real-world transfers to land somewhere modestly above it. If a transfer is running at half the calculated speed or worse, that's usually worth investigating - the common culprits are Wi-Fi distance, a busy server, or a plan speed lower than remembered.

Is this the same as a file transfer time calculator?

Yes. Whether you call it a data transfer time calculator, a file transfer time calculator, or a download time calculator, the arithmetic is identical: file size divided by connection speed, with the bits-to-bytes conversion handled properly. The name usually depends on context rather than the maths. People moving files between servers or drives tend to say file transfer, while people waiting on a download tend to say download time. Both land here, and both get the same answer.

Related tools

Common questions

Why is my 100 Mbps connection not 100 MB per second?

Connection speeds are quoted in megabits, file sizes in megabytes, and there are 8 bits in a byte. A 100 Mbps connection transfers about 12.5 MB per second at best.

How long does it take to download a file?

Divide the file size in megabits by the connection speed in megabits per second. A 5 GB file on a 100 Mbps connection takes roughly 7 minutes at full speed.

Need just one specific data conversion?

Each of these has its own page with the exact factor, a reference table and the working shown. The converter above handles any of them too - these exist so the specific answer is one click rather than a search.