Fiber Latency
Convert the distance of a fiber run into propagation delay: one-way and round-trip (RTT) in µs and ms. Light in fiber travels at c/n (≈4.90 µs/km in single-mode), slower than in vacuum. Everything in your browser.
The delay in all units
| Magnitude | one-way | round-trip |
|---|
Fiber vs vacuum (radio / line of sight)
| Medium | n | µs/km | one-way |
|---|
In vacuum light travels at c (≈3.336 µs/km);
in fiber it drops to c/n. The penalty is the factor n.
How it is calculated · speed of light and index
1. In vacuum light travels at
c = 299,792.458 km/s, that is
≈3.336 µs/km (1e6 / c).
2. In a medium with refractive index
n the velocity drops to v = c / n. Single-mode
fiber has n ≈ 1.4682.
3. One-way delay:
t = distance · n / c. In µs:
distance · n / c · 1e6 ≈ 4.90 µs/km for
n = 1.4682.
4. The round-trip (RTT) is there and back:
RTT = 2 · one-way. It is the link's minimum physical
latency; it does not include processing, queuing or serialization.
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How it works
The calculator converts the distance of a fiber optic span into its propagation delay: the physical minimum time light needs to traverse the link, one-way and round-trip (RTT), expressed in µs and ms. It also shows the resulting propagation speed and a comparison against vacuum (radio or line of sight).
The physics is straightforward: in vacuum light travels at c = 299,792.458 km/s (≈3.336 µs/km), but inside glass it slows to v = c ÷ n, where n is the group refractive index. For typical single-mode fiber n ≈ 1.4682, which yields the classic rule of ≈4.90 µs per kilometer one-way; RTT is twice that. You can pick single-mode, multimode (n≈1.48), air, vacuum or a custom index. This delay excludes processing, queuing and serialization: it is the floor no optimization can lower.
Example: how much latency do 100 km of single-mode fiber add?
- One-way:
100 × 1.4682 ÷ 299,792.458 × 10^6 ≈ 489.7 µs, that is ≈0.49 ms. - RTT (what a ping would see, propagation only):
2 × 489.7 ≈ 979.5 µs ≈ 0.98 ms. - Comparison: in vacuum the same path would take
100 × 3.336 = 333.6 µs; fiber pays a penalty factor of n ≈ 1.47.