Propagation Delay Calculator
Enter a signal path length, pick a transmission medium, and this calculator gives you the one-way propagation delay in picoseconds, nanoseconds, and milliseconds. It covers coax, Ethernet, fiber, PCB microstrip and stripline, and free-space wireless links. You also get the effective signal speed and velocity factor so you can cross-check your timing budget instantly.
What is propagation delay?
Propagation delay is the time it takes a signal to travel from its source to its destination through a physical medium. It depends on two things: the path length and the speed at which the signal moves through that medium. In vacuum or open air, electromagnetic signals travel at the speed of light (approximately 299,792,458 m/s). Inside a cable or optical fiber, the signal slows down because the material interacts with the electromagnetic field, and the ratio of actual speed to the speed of light is called the velocity factor (VF). A copper coax cable filled with polyethylene, for example, has a VF around 0.66, meaning signals move at 66% of the speed of light. A fiber optic cable with glass core has a VF near 0.68. Understanding propagation delay is essential for network timing, PCB signal integrity, and any system where simultaneous signal arrival matters.
How this calculator works
Select a transmission medium from the dropdown. The calculator looks up its velocity factor (or lets you enter a custom one), then multiplies the speed of light by that factor to find the signal speed. It divides your path length by the signal speed to produce the one-way propagation delay. Results appear in picoseconds, nanoseconds, and milliseconds so you can match whichever unit your data sheet uses. The reverse-solve mode works backwards: enter a target delay in nanoseconds, and the calculator returns the maximum cable or trace length that stays within that budget. The delay-per-metre output (ps/m) is especially useful for PCB length matching, where you need to know how many millimetres to add or trim to hit a skew target.
Propagation delay in PCB design
On a printed circuit board, signal layers are sandwiched between dielectric material. The effective dielectric constant of FR4 is roughly 4.3 for microstrip (one reference plane) and around 4.5 for stripline (two planes), giving velocity factors near 0.54 and 0.49 respectively. Propagation delay for a typical FR4 microstrip trace is around 6 ps/mm, or about 150 ps for a 25 mm trace. High-speed buses such as DDR memory and PCIe require matched trace lengths so signals arrive within a fraction of a unit interval. Engineers tune trace lengths in the layout tool using the ps/m figure to translate timing margin into physical distance.
Network latency and propagation delay
In telecommunications, propagation delay is one component of total link latency alongside serialization delay, processing delay, and queuing delay. A 1 Gbit/s Ethernet link over 100 m of Cat5 cable adds roughly 500 ns of propagation delay. A transcontinental fiber link spanning 5,000 km adds about 24 ms one-way. Satellite links in geostationary orbit at 35,786 km add approximately 250 ms each direction. These figures matter for real-time protocols: TCP acknowledgements must arrive within round-trip time budgets, VoIP requires end-to-end latency below about 150 ms to remain comfortable, and algorithmic trading systems count individual nanoseconds. The formula is always the same: delay equals distance divided by propagation speed.
Velocity factors and delay rates for common media
| Medium | Velocity factor (VF) | Delay (ps/m) | Delay (ps/ft) |
|---|---|---|---|
| Air / free space | 1.000 | 3.336 | 1.017 |
| Air-spaced PTFE coax | 0.850 | 3.925 | 1.197 |
| Air-spaced polyethylene coax | 0.820 | 4.068 | 1.240 |
| Foam polyethylene coax | 0.780 | 4.277 | 1.304 |
| Solid PTFE coax | 0.695 | 4.800 | 1.463 |
| Polyethylene / Cat5 UTP | 0.660 | 5.054 | 1.540 |
| Fiber optic (glass) | 0.680 | 4.906 | 1.495 |
| FR4 PCB microstrip | 0.540 | 6.178 | 1.883 |
| FR4 PCB stripline | 0.490 | 6.807 | 2.075 |
Typical values. Actual VF depends on exact cable construction and manufacturer tolerances.
Frequently asked questions
What is the propagation delay formula?
Propagation delay equals path length divided by signal speed, where signal speed equals the speed of light multiplied by the velocity factor of the medium. In symbols: T = d / (c x VF). For example, a 3 m Ethernet cable with VF = 0.66 has a delay of 3 / (299,792,458 x 0.66) = approximately 15.2 nanoseconds.
What is velocity factor and why does it matter?
Velocity factor (VF) is the ratio of a signal's actual speed through a medium to the speed of light in vacuum. It is always between 0 and 1. Air has VF = 1.0 (the theoretical maximum). Solid polyethylene dielectric coax is typically 0.66, meaning signals travel at 66% of the speed of light. A lower VF means more delay per metre of cable. You can find VF on the data sheet for any coaxial cable, and it is used to tune antenna feed lines to the correct electrical length.
How much delay does 1 metre of Ethernet cable add?
Cat5 and Cat6 Ethernet cables use polyethylene-based dielectric with a velocity factor near 0.66, giving a delay of about 5.05 ns/m (or 1.54 ns/ft). A 10 m patch cable adds roughly 50 ns of propagation delay. For Gigabit Ethernet this is negligible compared to serialization delay, but at 10 Gbit/s and beyond, even centimetres of length difference matter for timing.
What is the propagation delay of fiber optic cable?
Glass optical fiber has a refractive index around 1.47, giving a velocity factor of about 0.68 and a delay near 4.9 ns/m (or 1.5 ns/ft). A transatlantic fiber link of 6,000 km would have a one-way propagation delay of approximately 29 ms, not counting repeater and routing delays. Plastic optical fiber has a slightly higher index and a lower VF, so it is slower per metre than glass.
How do I use propagation delay for PCB trace length matching?
Find the delay rate for your stackup (typically 5 to 7 ps/mm for FR4 microstrip). Calculate the maximum skew your timing budget allows in picoseconds. Divide the skew by the delay rate to find the maximum allowable length difference in millimetres. For example, if your DDR4 interface can tolerate 30 ps of byte-lane skew and your board uses FR4 microstrip at 6 ps/mm, traces within a byte lane must be within 30 / 6 = 5 mm of each other.
What is the difference between propagation delay and latency?
Propagation delay is purely the travel time of a signal through a physical medium, determined by distance and signal speed. Total latency in a network link also includes serialization delay (time to push all bits onto the wire), processing delay (time spent in routers and NICs), and queuing delay (wait time when buffers are full). Propagation delay is constant for a given path length; the other components vary with load and hardware.
How does signal delay in air compare to inside a cable?
In air or free space, signals travel at the full speed of light, about 300,000 km/s, so 1 metre of path adds only 3.34 ns. Inside a cable the signal slows down depending on the dielectric material. A solid polyethylene coax adds about 5.05 ns/m, roughly 51% more than air. PCB FR4 stripline, with higher dielectric constant, adds about 6.8 ns/m, more than double the air figure. Choosing a lower-loss dielectric with a higher velocity factor is one way to reduce delay in high-speed designs.