Data Transfer Time Formula
Divide file size by transfer rate (mind the bits-vs-bytes factor of 8).
t = size / rate
How it works
Almost every error in transfer-time arithmetic comes from one place: network speeds are quoted in bits per second and file sizes in bytes, and there are eight bits in a byte. A 100 Mbps connection therefore delivers 12.5 MB/s at best, not 100. The capital B and lower-case b are doing all the work in those abbreviations, and it is a genuine ambiguity rather than a trick: internet plans have been sold in bits since the days of dial-up, while operating systems have always reported file sizes in bytes. Working an example through: a one-gigabyte file is 8,000,000,000 bits, so over a perfect 100 Mbps link it takes 80 seconds. If that gigabyte is actually a gibibyte, as your operating system probably means, it is 8,589,934,592 bits and takes 85.9 seconds. Neither figure is what you will observe, because none of the line is available to your data. TCP, TLS and HTTP framing consume roughly 5 to 10% of it, latency limits throughput on long links regardless of bandwidth, and a shared connection is rarely yours alone. Treating the theoretical figure as a floor and adding 10 to 20% is a more useful planning habit than trusting the arithmetic exactly.
Variables
- tTime (seconds)
- sizeFile size in bits or bytes
- rateTransfer rate (matching unit/sec)
Worked examples
| Input | Result |
|---|---|
| 1 GB over 100 Mbps | 1 GB × 8 = 8 Gb; 8,000 Mb / 100 = 80 s |
| 10 GB over 1 Gbps | 10 GB × 8 = 80 Gb / 1 = 80 s (theoretical) |
Practical applications
- Working out whether a backup or sync will finish inside its overnight window
- Sizing an internet plan against what a household or office actually moves
- Estimating bandwidth cost when data is billed by the gigabyte
- Judging whether a video bitrate will stream without buffering on a given connection
Common mistakes
- Treating Mbps and MB/s as the same. They differ by a factor of eight.
- Ignoring protocol overhead. TCP, TLS and HTTP framing take a real share of the line.
- Mixing decimal and binary units. A gigabyte is 10⁹ bytes; a gibibyte is 2³⁰, about 7.4% more.
- Assuming bandwidth alone determines speed. On a high-latency link, round trips can matter more than capacity.
- Planning to the theoretical figure with no margin. Real throughput is reliably lower.
Used in
Use it in a tool
FAQs
Why is my real download slower?+
Headers, retransmission, encryption overhead, and your ISP usually cap below the line rate. Plan for ~70% of theoretical.
Is Mbps the same as MB/s?+
No — divide Mbps by 8 to get MB/s. 100 Mbps ≈ 12.5 MB/s.