Bandwidth Calculator
Estimate how long a file takes to transfer at a given connection speed — computed in your browser
Examples
Download a 1.5 GB file over 100 Mbps
size: 1.5 GB
speed: 100 Mbps2m (exact: 120 s)1.5 × 10⁹ × 8 / 10⁸ = 120 s. The exact seconds value is shown alongside the human-readable duration.
Same link, MB/s instead of Mbps
size: 1.5 GB
speed: 12.5 MB/s2m (exact: 120 s)12.5 MB/s × 8 = 100 Mbps, so the time matches — a quick sanity check that the two unit systems are handled correctly.
2 GiB file over a 1 Gbps link
size: 2 GiB
speed: 1 Gbps17s (exact: 17.18 s)Using the binary GiB (1024³) instead of SI GB (1000³) gives a slightly larger byte count and a slightly longer time.
About this tool
The Bandwidth Calculator estimates how long a file takes to transfer at a given connection speed, working entirely in your browser. It is built around the two conversions most back-of-envelope estimates get wrong: connection speeds are measured in bits per second (Mbps), file sizes in bytes (MB), and the two differ by exactly 8× — a mix-up that turns a 2-minute download estimate into a 16-minute one.
Size units cover both conventions: SI prefixes (KB, MB, GB, TB) use powers of 1000, while binary prefixes (KiB, MiB, GiB, TiB) use powers of 1024. Speed units cover both bit rates (Kbps, Mbps, Gbps) and byte rates (KB/s, MB/s, GB/s), with the tool applying the 8× factor automatically so you can compare quoted ISP speeds (Mbps) against actual bytes-on-the-wire (MB/s) without leaving the page.
The result is shown both as a human-friendly duration (`2m`, `12m 32s`, `11h 6m`) and as the exact number of seconds, useful for budgeting or capacity planning. The estimate assumes a sustained ideal rate — real transfers are slower due to protocol overhead, congestion and disk speed — so treat the number as a lower bound, not a promise.
How to use
Pick a file size and unit
Enter the number of bytes (or KB, MB, GB…) and pick the matching prefix. Use KiB/MiB/GiB when the source reports binary units, KB/MB/GB for SI.
Pick a connection speed and unit
Enter the link rate. ISP quotes are usually in Mbps or Gbps; NAS copies are often in MB/s. The tool handles both.
Read the duration
Look at the human-readable time on the right (e.g. `2m`, `12m 32s`) and the exact seconds value below it for downstream calculations.
Compare Mbps vs MB/s
Switch the speed unit to see how the same physical link is reported differently by the two conventions — the difference is exactly 8×.
Use cases
Planning a large backup or migration window
Pick the file size and the slowest link in the chain to estimate the worst-case duration before scheduling a maintenance window.
Comparing ISP plans in real terms
Translate a quoted Mbps into a real MB/s figure, then estimate how long a typical 4.7 GB DVD rip or 50 GB game download will take.
Estimating build-artifact transfer times in CI
Size the artifact, pick the network speed between the build agent and the artifact store, and budget the upload step accordingly.
Teaching the bits-vs-bytes confusion
Toggle Mbps and MB/s on the same size to show students that the difference is a factor of 8, not a different metric.
Common mistakes
Mistake:Treating MB and MiB as interchangeable.
Fix:MB is 1,000,000 bytes (SI) and MiB is 1,048,576 bytes (binary). On a 1 TB drive the difference is roughly 10% — pick the prefix that matches the source reporting the size.
Mistake:Assuming the estimate includes protocol overhead.
Fix:The calculation is a sustained ideal-rate lower bound. Real TCP transfers lose 5–15% to headers, ACKs, and congestion — multiply by ~1.1 for a more realistic budget.
Mistake:Quoting a 1000 Mbps link as 1000 MB/s.
Fix:Mbps is megabits per second. Divide by 8 to get megabytes per second: 1000 Mbps ≈ 125 MB/s, not 1000 MB/s.
Mistake:Entering the size in bytes when the unit is already MB.
Fix:Pick the prefix that matches the number — a file reported as '4.7 GB' should be entered as `4.7` with unit `GB`, not as `4700000` with unit `B`.
Frequently asked questions
Related guides
References & standards
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