Network latency calculator

Light in fibre travels at about two thirds of its speed in vacuum, so a signal covers roughly 200 km per millisecond. London to New York is about 5,570 km, giving a round-trip floor near 56 ms before any equipment is involved.

This number is a floor, not a prediction. Real latency adds switching, routing, queuing and the fact that fibre does not run in straight lines — actual London to New York round trips run 70 to 80 ms against the 56 ms physics allows. What the floor is good for is knowing when optimisation is pointless: if your API is 90 ms away and physics says 56, there is 34 ms of engineering to attack. If it says 56 and you are getting 58, the only remaining move is to put the data closer to the user, which is what a CDN is. It also explains why interactive applications feel different across an ocean no matter how good the servers are.