TCP will not send more than one window before it hears back, so throughput is capped at window divided by round-trip time no matter what the line is rated at. A 64 KB window over a 10 ms hop is 52 Mbps; over a 150 ms ocean crossing it is 3.5 Mbps.
People treat latency as something that only affects how snappy a page feels, and bandwidth as a separate number on the bill. They are coupled. The amount of data in flight at any moment is speed multiplied by round-trip time — the bandwidth-delay product — and if the window is smaller than that, the link is not full, it is idle waiting for acknowledgements. This is why the same file moves faster to a server down the road than to one across an ocean on the identical connection.
It stops being the binding limit as soon as the window covers the pipe, and modern systems scale windows automatically, so on a well-tuned path the link itself is what constrains you. Parallel streams also sidestep it entirely, since each stream carries its own window — which is why download managers open several at once.
Estimate with the rule, then check it against the calculator that models it properly.
Open Data Transfer Time Calculator →TCP will not send more than one window before it hears back, so throughput is capped at window divided by round-trip time no matter what the line is rated at. A 64 KB window over a 10 ms hop is 52 Mbps; over a 150 ms ocean crossing it is 3.5 Mbps. People treat latency as something that only affects how snappy a page feels, and bandwidth as a separate number on the bill.