Convert a round-trip time into the furthest that server can possibly be.
Time spent in routers and at the far end rather than travelling. Leave at 0 for the absolute ceiling.
How much longer the fibre path is than the straight line. Terrestrial routes are typically 1.4 to 2.
20 ms is at most about 1,339,073 km away
Physics sets a hard floor on latency that no amount of bandwidth changes. Light in fibre covers roughly 200 km per millisecond, and cable never runs in a straight line, so a realistic figure is 100 to 150 km of geography per millisecond of round trip. This is why a server across an ocean cannot answer in 5 ms however it is provisioned, and why a claimed location that would need faster-than-light propagation is simply wrong. Read the result as a ceiling rather than an estimate: real round trips include queueing, forwarding and the far end taking time to reply, all of which add milliseconds that have nothing to do with distance.
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