Network Latency Calculator (propagation, transmission)

Understanding the network latency calculator — concept and principle

Network latency is the time a packet takes to get from one place to another. This calculator works out its two physical building blocks: propagation delay, set by distance and the speed of light in fibre, and transmission (serialisation) delay, the time needed to push a packet's bits onto the link. It also gives the minimum possible round-trip time (RTT). Use it to sanity-check ping times between cities, set realistic expectations for trading, gaming or video calls, and see whether distance or bandwidth is your real limit.

Over long distances the travel time of light dominates; a faster link barely shortens it.

How it is calculated

Propagation delay (one way) = distance ÷ (c × velocity factor), with c = 299,792.458 km/s. Multiply by 1,000 for milliseconds.
Minimum RTT = 2 × propagation delay.
Transmission delay = packet size (bytes) × 8 ÷ link speed (bit/s).

The velocity factor is the fraction of c at which signals travel in the medium. For common single-mode fibre it is about 0.68, i.e. roughly 4.9 µs per km.

The principle

Glass has a refractive index of about 1.47, so light inside it travels at roughly c ÷ 1.47, about two-thirds of its speed in vacuum. No amount of bandwidth can beat that limit: distance sets a floor on latency. Transmission delay is different: it shrinks as links get faster, since a 1,500-byte packet is 12,000 bits whichever way it travels. Total real latency adds processing and queueing delay in routers on top of these two parts.

Worked example

Inputs: 9,000 km, velocity factor 0.68, 1,500-byte packet, 100 Mbps.
Speed in fibre: 299,792.458 × 0.68 = 203,858.871 km/s.
One-way propagation: 9,000 ÷ 203,858.871 = 0.044148 s = 44.148 ms.
Minimum RTT: 2 × 44.148 = 88.296 ms.
Transmission: 1,500 × 8 = 12,000 bits ÷ 100,000,000 bit/s = 0.12 ms.

Each point is the calculator re-run with only “Fibre velocity factor” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Fibre velocity factor	Propagation delay one-way (ms)
0.17	176.5927563
0.34	88.29637814
0.51	58.86425209
0.68	44.14818907
0.85	35.31855126
1.02	29.43212605
1.36	22.07409454

How to read the result

Measured RTT is always higher than this minimum, because cables follow coastlines and sea routes rather than straight lines, and routers add their own delay. A measured RTT around 1.3 to 2 times the straight-line minimum is common. If it is much higher, look for long detours or congestion. Transmission delay matters mainly on slow links: at 1 Mbps the same packet takes 12 ms.

Common mistakes

Using straight-line distance and expecting ping to match it.
Confusing one-way delay with RTT, which is what ping shows.
Expecting a faster plan to lower latency to a distant server; it only cuts transmission delay.

Keep in mind: This gives physical lower bounds only. Routing, queueing, Wi-Fi and server processing add delay that the formula does not include.

Frequently asked questions

How much latency does 1,000 km of fibre add?

About 4.9 ms one way, or roughly 10 ms round trip.

Is microwave or satellite faster than fibre?

Line-of-sight microwave travels close to c, so it can beat fibre over land. Geostationary satellites add roughly 240 ms one way because of their altitude.

Does a bigger packet increase latency?

Only its transmission delay, which grows with size ÷ link speed.

Sources: BIPM, SI Brochure (defined value of the speed of light, 299,792,458 m/s) · ITU-T G.652, Characteristics of a single-mode optical fibre and cable

Checked 8 Nov 2026. Standard formulas and RFC/IEEE definitions, computed locally in your browser — nothing you type is sent anywhere.

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