How fiber latency is calculated
Light travels through conventional optical fiber at roughly 200,000 kilometers per second. A useful planning approximation is 5 ns per meter, or 5 µs per kilometer. The exact value depends on the fiber and wavelength. Distances are converted using 1 mile = 1,609.344 meters and 1 foot = 0.3048 meters.
Hollow-core fiber guides light through an air-filled core. We use an approximate 3.4 ns per meter (3.4 µs per kilometer), based on Relativity Networks’ published propagation rate. This is a planning estimate; use the specifications for your particular cable when available. For a route mixing fiber types, calculate each segment separately and add the delays.
One-way delay (µs) = cable length (km) × 5
Round-trip delay (µs) = cable length (km) × 10
For a 10 km cable using standard glass fiber, the one-way propagation estimate is 50 µs. An equal-length return path makes the round-trip estimate 100 µs.
Fiber latency by distance
| Cable length | Standard fiber | Hollow-core fiber |
|---|---|---|
1 m 0.000621 mi · 3.281 ft | One way: 5 ns Round trip: 10 ns | One way: 3.4 ns Round trip: 6.8 ns |
100 m 0.062137 mi · 328.084 ft | One way: 500 ns Round trip: 1 µs | One way: 340 ns Round trip: 680 ns |
1 km 0.621371 mi · 3,280.84 ft | One way: 5 µs Round trip: 10 µs | One way: 3.4 µs Round trip: 6.8 µs |
10 km 6.213712 mi · 32,808.399 ft | One way: 50 µs Round trip: 100 µs | One way: 34 µs Round trip: 68 µs |
100 km 62.137119 mi · 328,083.99 ft | One way: 500 µs Round trip: 1 ms | One way: 340 µs Round trip: 680 µs |
1,000 km 621.371192 mi · 3,280,839.895 ft | One way: 5 ms Round trip: 10 ms | One way: 3.4 ms Round trip: 6.8 ms |
How much latency does hollow-core fiber save?
Using the rates in this calculator, hollow-core fiber reduces propagation latency by approximately 32% over the same cable length: (5 − 3.4) ÷ 5 × 100 = 32%. Light travels through its air-filled core faster than through the solid glass core of conventional fiber.
Over 10 km, that means about 34 µs one way instead of 50 µs, saving 16 µs in one direction or 32 µs on an equal-length round trip. The percentage applies to fiber propagation; the reduction in total network latency depends on equipment delays and how much of the route uses hollow-core fiber.
Why measured network latency can be higher
A ping measures round-trip time across a working network. It includes the outward and return routes, packet processing and other delays. Fiber propagation is one part of that result. Switches, firewalls, optical transport equipment and congestion can add latency; the two directions can also take different routes.
Browse individual equipment estimates in the device latency table, or combine them using the network latency calculator. Add each component once for the path you are modeling and check whether any carrier-provided latency figure is one-way or round trip.
Does faster fiber bandwidth reduce propagation delay?
A faster link can reduce the time needed to put a packet onto the link, called serialization delay. It does not by itself shorten the cable or make light travel faster through the same fiber. This calculator models propagation delay only.
Methodology and sources
The standard-fiber approximation of 5 µs per kilometer is described in Cisco’s explanation of network round-trip delay. The hollow-core approximation of 3.4 µs per kilometer comes from Relativity Networks’ hollow-core fiber technology overview. For the broader model and equipment-data limitations, read About Latency Lens.