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Transfer Time Calculator
Work out how long moving a given volume of data takes over a given link — with realistic protocol overhead and, crucially, the latency limit that actually decides a WAN transfer. A single TCP stream can only carry one window of data per round trip, so a fat pipe sits half idle across distance. Model the round-trip, the TCP window, and parallel streams, then compare link speeds to see exactly where an upgrade stops buying you anything.
Sets the overhead derate below.
Line rate → sustained throughput.
A single TCP stream can only carry one window of data per round trip, so on a high-latency path the window — not the link — sets the ceiling. This is why a WAN copy crawls on a pipe that looks empty.
Robocopy /MT, rsync, aria2, S3 CLI…
Latency-limited. At 30 ms round trip, 4 streams of a 256 KB window tops out at 279.6 Mbps — about 40% of your 700 Mbps of usable link. To fill the pipe, raise the window to roughly 641 KB per stream, or run about 11 parallel streams.
15h 53m will not fit in a single overnight window. Stage the copy across several nights, seed it beforehand and sync the delta, or move the data on physical media.
279.6 Mbps used of 700 Mbps usable (1,000 Mbps line rate)
| Link | Effective | Transfer window |
|---|---|---|
| 100 Mbps | 8.8 MB/s | 2d 15h |
| 1 Gbps | 35 MB/s· capped | 15h 53m |
| 2.5 Gbps | 35 MB/s· capped | 15h 53m |
| 10 Gbps | 35 MB/s· capped | 15h 53m |
| 25 Gbps | 35 MB/s· capped | 15h 53m |
| 40 Gbps | 35 MB/s· capped | 15h 53m |
| 100 Gbps | 35 MB/s· capped | 15h 53m |
Click a row to set that link speed. “Capped” means latency, not the link, is the limit — a faster pipe buys nothing there.
- Link speeds are decimal bits: 1 Gbps = 109 bits/s. Data volumes in GB/TB are decimal bytes; TiB is binary. Mixing the two is the usual source of a wrong answer.
- The overhead derate covers TCP/IP and protocol overhead plus the gap between line rate and sustained throughput. 70% is fair for large files on a healthy LAN.
- With latency on, a single stream is capped at window ÷ round-trip regardless of link speed — the long-fat-network problem. Parallel streams or a larger TCP window (window scaling) raise the ceiling.
- 64 KB is the classic un-scaled window; modern operating systems scale well past it, but appliances, VPNs, and old stacks often do not. Set it to what your slowest hop actually allows.
- The figure assumes the bottleneck is the link or the window. If source disks, target disks, or a single-threaded copy are slower, they set the ceiling instead. Everything runs in your browser.
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