Two conversions explain many apparently impossible download and upload estimates: eight bits make a byte, and a gigabyte is not the same unit as a gibibyte.
First separate a quantity from a rate
A file size describes an amount of data. A transfer rate describes an amount of data per second. “10 GB” is a quantity; “10 Mbps” is a rate. You cannot compare those numbers directly, just as a distance and a speed are different things. To find time, express the quantity and the rate in compatible units, then divide.
The capitalization matters. A lowercase b means bit, while an uppercase B means byte. Mbps is megabits per second. MB/s is megabytes per second. The slash in MB/s and the “ps” in Mbps both mean per second; they do not account for the factor-of-eight difference.
The factor of eight
In the units used here, one byte contains eight bits. Therefore 80 Mbps equals 10 MB/s before any allowance for throughput losses. A network interface reporting 80 Mbps and a file application reporting 10 MB/s may be describing exactly the same rate in different units. It is not evidence that seven-eighths of the connection has disappeared.
Try the Mbps-to-MB/s converter whenever an application and an internet plan appear to disagree. Convert first; investigate performance second. If the converted observed rate is still lower than expected, then it is time to consider the connection, competing traffic, and destination.
Decimal and binary prefixes are a separate issue
Decimal MB means 1,000,000 bytes and decimal GB means 1,000,000,000 bytes. Binary MiB means 1,048,576 bytes and GiB means 1,073,741,824 bytes. The NIST explanation of binary prefixes provides these definitions and the distinction between bits and bytes.
Some software displays a binary-derived number beside a decimal-looking label. If a deadline matters, inspect the exact byte count in file properties when available, or check how that application defines its display. Upload Compass labels GB and GiB separately so that you can make the choice explicitly rather than relying on a hidden assumption.
A worked transfer example
Take a 10 GB file and a sustained rate of 100 Mbps. The file contains 10,000,000,000 bytes, or 80,000,000,000 bits. The rate is 100,000,000 bits per second. Dividing gives 800 seconds: 13 minutes, 20 seconds. These are idealized transfer seconds at that sustained rate, not a promise about a particular internet plan.
Now change only the file unit to 10 GiB. That represents 10,737,418,240 bytes. At the same 100 Mbps, the transfer takes about 859 seconds, or 14 minutes, 19 seconds. The binary-sized file is about 7.37% larger. This is a different effect from the eight-to-one bits/bytes conversion; do not apply both corrections indiscriminately.
What “100 Mbps” does not guarantee
Converting a rate is exact arithmetic. Predicting a real transfer is a model. A subscription's quoted rate may not equal the sustained throughput available to one file at one moment. Other traffic, wireless conditions, remote service behavior, and interruptions can all change the result. Changing unit labels cannot explain every gap.
In the deadline planner, efficiency is a planning input rather than a fixed law. If you use a quoted 100 Mbps rate with 80% efficiency, the model starts with 80 Mbps. If you then reserve 10 Mbps for other uploads, 70 Mbps remains for the file. But if you enter an already-observed file throughput of 70 Mbps, set efficiency to 100% unless you deliberately want another conservative adjustment.
A three-question habit
- What is being measured? Is this the file's size, the export's bitrate, the link capacity, or the observed transfer rate?
- Which exact units are shown? Check b versus B, decimal versus binary, and seconds versus minutes.
- Is the rate sustained for this task? A short speed test is evidence about that test, not a guarantee for every destination.
Write down the units alongside each number before sharing an estimate. “About 15 minutes for 10 GB at 90 Mbps effective upload” is much more useful than “about 15 minutes on fast internet.” It exposes the assumptions and gives the next person something they can reproduce, challenge, or update when the file changes.