What this calculator does
Propagation delay is distance divided by signal speed, and signals travel slower in cable than in vacuum. Through optical fibre at a velocity factor of 0.67, 100 km takes 0.498 ms one way.
The figure worth remembering is about 5 microseconds per kilometre in fibre. It is why a round trip to a server 1,000 km away cannot beat about 10 ms no matter how much bandwidth is bought, and why latency-sensitive systems are placed near their users rather than given faster links.
The formula
The distance is divided by the speed of light multiplied by the velocity factor of the medium. Velocity factor accounts for signals travelling slower through a dielectric than through vacuum: about 0.67 in fibre and 0.59 to 0.66 in copper. Round trip is simply twice the one-way figure.
| Term | Meaning |
|---|---|
| Propagation delay | Time for the signal to traverse the distance. Independent of bandwidth. |
| Velocity factor | Signal speed as a fraction of the speed of light in vacuum. |
| Round trip time | Twice the one-way delay, which is what a ping measures. |
| Latency floor | The minimum delay physics allows, which no amount of bandwidth can reduce. |
The inputs explained
| Field | What to enter |
|---|---|
| Distance (km) | Distance in kilometres, along the actual cable route rather than straight-line distance. |
| Medium | The medium, which sets the velocity factor. |
When to use it
Estimating network latency
Propagation sets the floor below which no link can go, whatever its bandwidth.
Choosing a data centre location
Distance to users translates directly into unavoidable latency.
Understanding a traceroute
Comparing measured hop times against the propagation floor shows how much is queueing and processing.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How far is how long?
A range of distances through the same medium.
| Distance | One-way propagation delay | Round trip | Delay per kilometre |
|---|---|---|---|
| 10 km | 0.050 ms | 0.100 ms | 4.98 µs/km |
| 100 km | 0.498 ms | 0.996 ms | 4.98 µs/km |
| 1,000 km | 4.979 ms | 9.957 ms | 4.98 µs/km |
| 10,000 km | 49.786 ms | 99.571 ms | 4.98 µs/km |
Questions
Does more bandwidth reduce propagation delay?
No. Propagation is distance divided by speed and has nothing to do with bandwidth. Extra bandwidth reduces transmission delay, the time spent clocking bits onto the line, but the signal still takes the same time to cross the distance.
Why is fibre slower than the speed of light?
Because light travels more slowly through glass than through vacuum, by the refractive index of the fibre core. That gives a velocity factor around 0.67, so signals move at about two thirds of the vacuum speed of light.
Why is my ping higher than this calculation?
Because propagation is only the floor. Real latency adds transmission delay, queueing in routers, processing at each hop, and the fact that cable routes are far from straight. A figure two or three times the propagation floor is entirely normal.
Is satellite different?
Substantially. A geostationary satellite sits about 36,000 km up, so the round trip through the satellite is around 480 ms before anything else. Low earth orbit constellations are far closer, which is precisely why they can offer much lower latency.
For the time spent clocking bits onto the link, see the transmission delay calculator. For memory timings, see the RAM latency calculator.