How this instrument works
Propagation delay is the time a signal takes to physically travel from one end of a link to the other, determined entirely by distance and how fast the signal moves through that specific medium. It's distinct from other sources of network latency — queuing delay, processing delay, transmission delay — which depend on equipment and traffic, not on physics; propagation delay is the one component that exists purely because the signal has to cross real distance at a finite speed.
No signal travels faster than the speed of light in vacuum, c = 299,792,458 m/s, an exact SI-defined constant with no measurement uncertainty. Every real medium is slower than that, which is why this calculator uses a velocity factor — the fraction of c a signal actually achieves in a given medium. Standard fiber-optic cable typically runs a velocity factor around 0.66 to 0.69 depending on the cable's construction, disclosed by the manufacturer rather than fixed by physical law, while a line-of-sight radio or laser link in open air runs close to the full 1.0.
Even at the speed of light, distance adds up to real, measurable delay over long enough links — this is why a satellite hop or a transoceanic cable run introduces noticeably more delay than a link across a single building, no matter how well the equipment on either end performs. Propagation delay sets a hard floor under total network latency: no amount of faster hardware, better software, or more bandwidth can push a link's latency below what the distance and medium's velocity factor require.
- Enter Distance (km) — the physical length of the link the signal travels across, one way.
- Enter Velocity factor of the medium (fraction of c) — the fraction of light speed the signal achieves; use a manufacturer's disclosed figure for fiber (commonly around 0.66-0.69), or 1.0 for free-space/vacuum paths.
- Read Propagation delay (ms) directly — this is the one-way time for a signal to cross the given distance at that velocity.
- For round-trip time, double the result — propagation delay alone only accounts for one direction of travel.
- Distance must be zero or greater, and velocity factor must be greater than zero and no more than 1 — nothing travels faster than light in vacuum.
Worked example — 1000 km of fiber at velocity factor 0.66
Enter Distance (km) = 1000 and Velocity factor of the medium (fraction of c) = 0.66 — a typical disclosed figure for standard fiber-optic cable. The signal's actual speed is c x 0.66 = 299,792,458 x 0.66 ≈ 197,822,822 m/s, and dividing the 1,000,000-meter distance by that speed gives roughly 0.005054 seconds. Propagation delay reads 5.0540014424 ms.
That's the one-way delay; a round trip across the same 1000 km fiber run would take roughly double, about 10.1 ms, before any switching, routing, or processing time at either end is added on top. For comparison, the same 1000 km covered at the full speed of light (velocity factor 1.0, as in a line-of-sight radio path) would take only about 3.34 ms — the fiber's slower velocity factor adds real, physically unavoidable delay purely from the medium itself.
Questions
What's the difference between propagation delay and total network latency?
Propagation delay is only the time a signal spends physically crossing a distance, set by that distance and the medium's velocity factor. Total latency also includes transmission delay (time to push all the bits onto the link), queuing delay (time waiting behind other traffic), and processing delay (time routers and switches spend handling the signal) — propagation delay is the one piece of that total that pure physics fixes, regardless of equipment or congestion.
Why is fiber's velocity factor less than 1.0 if it uses light?
Light travels slower inside a medium like glass than it does in a vacuum, because the glass's refractive index slows it down. A velocity factor of 0.66 means the signal moves at 66% of the vacuum speed of light — a property of the specific fiber's material and construction, disclosed by the manufacturer, not a universal constant that applies to every fiber type equally.
How do I get round-trip time (RTT) from this calculator's result?
Double the one-way propagation delay this calculator returns. RTT also typically includes processing and queuing delay at both ends in real network measurements, so a measured RTT (like a ping time) will usually run somewhat higher than double the pure propagation delay alone.
Why can't propagation delay ever reach zero?
Because it requires dividing a real distance by a finite speed, and no signal travels instantaneously — even at the full speed of light, crossing any nonzero distance takes some nonzero amount of time. Only a distance of exactly zero would give a delay of zero, which isn't a meaningful network link.
Why does satellite internet have noticeably higher latency than fiber?
Largely because of the much greater distance signals travel — a geostationary satellite sits about 35,786 km above the equator, so a round trip up and back down covers roughly 71,500 km even at the full speed of light, versus a much shorter terrestrial fiber path for the same two endpoints. That extra distance, not equipment quality, is the main source of satellite internet's characteristically higher latency.