Fibre Latency Calculator (Minimum Ping by Distance)
The physical minimum round-trip time over an optical fibre route, from its length, compared with light in vacuum.
Results
Round-trip time (ping)
9.793
ms
One-way delay
4.897
ms
Round trip at the speed of light in vacuum
6.671
ms
Speed of light in the fibre
204,218
km/s
Delay per kilometre
4.897
µs/km
What this tool does
Light in glass travels about a third slower than in vacuum, roughly 204 000 km/s, so every 100 km of fibre adds about 1 ms to a ping. No upgrade can beat that floor; it is why trading firms paid for straighter cables and microwave links between Chicago and New York. Real cables rarely follow the great-circle line: a transatlantic route can be 10 to 30 % longer than the map distance, and routers, switches and the far server all add their own delay on top. So if a ping is much higher than this figure, the cause is routing or queues, not distance.
Formula
one-way delay = distance × n_g ÷ c + equipment delay · RTT = 2 × one-way
Variables
| Symbol | Meaning | Unit |
|---|---|---|
dk | Length of the cable route | km |
ng | Group index of the fibre (about 1.468) | — |
eq | Equipment delay, each direction | ms |
RT | Round-trip time (ping) | ms |
OW | One-way delay | ms |
VR | Round trip at the speed of light in vacuum | ms |
SP | Speed of light in the fibre | km/s |
PK | Delay per kilometre | µs/km |
Worked example
- Length of the cable route1000 km
- Group index of the fibre (about 1.468)1.468
- Equipment delay, each direction0 ms
- Round-trip time (ping)9.793 ms
- One-way delay4.897 ms
- Round trip at the speed of light in vacuum6.671 ms
- Speed of light in the fibre204,218 km/s
- Delay per kilometre4.897 µs/km
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.