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Line rate is not throughput. Framing, headers and real-world efficiency all take a cut. Enter the volume and the link and this gives both the theoretical floor and a realistic estimate.
Line rate versus throughput. A 100G link runs at 100 gigabits per second on the wire, but not all of that carries your data. Ethernet framing costs about 1.6% with standard 1500-byte frames and rather less with jumbo frames. TCP adds its own headers, and on a long path a single stream is limited by the window size and the round-trip time long before it is limited by the link.
Why a single stream underperforms. TCP throughput is bounded by window size divided by round-trip time. Over a 100 km path with a 1 ms round trip, a 64 KB window gives you about 0.5 Gb/s no matter how fast the link is. The fixes are window scaling, parallel streams, or a protocol designed for long fat networks. This is why bulk transfers use many connections at once.
Sizing the link. Work backwards from the window you have. If a nightly backup must finish in six hours and there are 40 TB to move, you need about 15 Gb/s of real throughput — call it a 25G link at 80% efficiency, or a 100G link if the window might shrink.
About 13 minutes at full line rate, and around 15 minutes at a realistic 90% efficiency. Getting anywhere near that requires parallel streams and tuned TCP windows — a single default-configured stream over a long path will take far longer.
Storage vendors quote TB (10¹²) and operating systems usually report TiB (2⁴⁰), which is about 10% larger. Both are in the dropdown. If the numbers do not match what your file system says, that 10% is usually why.
For a single TCP stream, enormously — see the note about window size above. For a well-parallelised transfer, barely at all: the round trip only costs you at the start. Our latency calculator gives you the round trip for your route.
One-way and round-trip delay from route length and fibre refractive index
Open tool →Total channel loss, power budget and pass/fail margin for single-mode and multimode links
Open tool →Convert optical power between logarithmic and linear units, in both directions
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.