Internet and Networking · 5 min read
Mbps vs. MB/s: Calculate Download Time for a 1 GB File
Convert Mbps to megabytes per second and calculate ideal download times. Compare decimal GB with binary GiB using clear 100 Mbps examples.
100 Mbps is 12.5 MB/s before overhead
The lowercase b in Mbps means bits. The uppercase B in MB/s means bytes. There are eight bits in a byte, so a 100-megabit-per-second connection has an ideal decimal transfer rate of 12.5 megabytes per second: 100 divided by eight.
Mixing these units can make a download estimate eight times too optimistic or pessimistic. Before comparing a connection-speed label with a download application's display, check both the letter case and whether the application uses decimal or binary units.
The formula: file bits divided by bits per second
For a decimal 1 GB file, the size is 1,000,000,000 bytes. Multiply by eight to get 8,000,000,000 bits. At 100,000,000 bits per second, the ideal time is 80 seconds. Equivalently, divide 1,000 MB by 12.5 MB/s.
| File size | At 25 Mbps | At 100 Mbps | At 1,000 Mbps |
|---|---|---|---|
| 100 MB | 32 seconds | 8 seconds | 0.8 seconds |
| 1 GB | 320 seconds | 80 seconds | 8 seconds |
| 10 GB | 3,200 seconds | 800 seconds | 80 seconds |
At the same sustained speed, doubling the file size doubles the ideal time. Doubling the sustained speed halves it. These comparisons are useful for checking whether a result has the right scale before considering real-network conditions.
Why 1 GiB takes about 85.9 seconds instead
Binary sizes use powers of 1,024. One gibibyte, written GiB, contains 1,073,741,824 bytes. At 100 Mbps, its ideal transfer time is 85.89934592 seconds. It is a larger file than a decimal 1 GB file, even though software sometimes labels both with GB.
NIST's binary-prefix reference distinguishes decimal mega and giga from binary mebi and gibi. The numerical difference is a unit convention, not lost bandwidth.
| Quantity | Decimal interpretation | Binary interpretation |
|---|---|---|
| Byte rate | 12.5 MB/s | About 11.92 MiB/s |
| One file-size unit | 1 GB = 1,000,000,000 bytes | 1 GiB = 1,073,741,824 bytes |
| Ideal time for that file | 80 seconds | About 85.9 seconds |
How the CalQuanta file-size inputs work
Connection-speed inputs use decimal multiples: 1 Mbps is 1,000,000 bits per second. The current file-size options labeled KB, MB, and GB use powers of 1,024. In other words, selecting a file size of 1 GB in this tool models a GiB-sized file.
To reproduce the 85.9-second model, choose file size 1 GB and speed 100 Mbps; the rounded display is about 1 minute 26 seconds. To model an exact decimal 1 GB file, enter approximately 0.9313225746 in the tool's GB field with the same speed. That represents 1,000,000,000 bytes and produces about 80 seconds. Stating the unit convention makes the two calculations consistent.
Why an actual download can take longer
The table assumes a constant useful transfer rate for the entire file. Real transfers may share capacity with other activity, pause, negotiate a connection, or be limited by the sending service, local network, or storage. A connection's advertised rate is not a measurement of the sustained speed of one particular download.
For a planning scenario, replace the headline rate with a sustained rate you have actually observed under similar conditions. For example, at 60 Mbps, an exact decimal 1 GB file has an ideal time of approximately 133.3 seconds. That is a changed input to the same formula, not a separate conversion rule.
Does upload use a different formula?
No. Use the upload rate available for the transfer rather than substituting the download rate. The same size-divided-by-rate calculation applies.
For more unit examples, see the bandwidth and internet speed guide, or try the download-time calculator with a different file size.