Calculator
Formula used
Propagation delay is the time a signal takes to travel through a medium:
- One-way delay:
t = d / v - Round-trip delay:
RTT = 2t - Velocity factor:
v = VF × c - Refractive index:
v ≈ c / n - Permittivity (μr≈1):
v ≈ c / √εr
This tool also supports slack (% extra length) and a fixed extra one-way delay.
How to use this calculator
- Enter the link distance and choose a unit.
- Add slack if the route is longer than straight-line.
- Select a speed method: preset medium, VF, n, εr, or custom speed.
- Optionally add fixed delay for hardware along the path.
- Press Calculate to view results and download files.
Example data table
| Distance | Medium | Speed (m/s) | One-way delay | Round-trip delay |
|---|---|---|---|---|
| 1 km | Optical fiber | 2.042183e+8 | 4.897 µs | 9.793 µs |
| 100 m | Coax (VF 0.66) | 1.978630e+8 | 505.4 ns | 1.011 µs |
| 50 m | Twisted pair (VF 0.70) | 2.098547e+8 | 238.26 ns | 476.52 ns |
| 10 km | Air | 2.997025e+8 | 33.366 µs | 66.733 µs |
| 5 mi | FR-4 PCB trace | 1.462837e+8 | 55.008 µs | 110.015 µs |
Values are illustrative. Use manufacturer data for precise work.
FAQs
1) What is propagation delay?
Propagation delay is the travel time of a signal through a medium. It depends on path length and propagation speed, not on packet size or bandwidth.
2) How is it different from transmission delay?
Transmission delay is the time to push bits onto the link, roughly size ÷ bitrate. Propagation delay is the time the signal needs to move across the medium.
3) Which speed method should I use?
Use a preset for quick estimates. Use velocity factor for copper cables. Use refractive index for fiber or optical paths. Use εr for board traces and dielectrics.
4) What does “slack” represent?
Slack models extra length from routing, bends, trays, and service loops. If a cable run is 10% longer than the straight line, set slack to 10%.
5) Why include an extra fixed delay?
Real links may add latency from transceivers, connectors, repeaters, or conversion stages. Add this as a fixed one-way delay when you know the hardware contribution.
6) Can the result exceed the speed of light?
No. If the calculator warns that speed exceeds c, it usually means a unit mismatch or an unrealistic custom speed entry.
7) How accurate are the presets?
Presets are typical values for common materials. Actual speeds depend on dielectric, wavelength, frequency, temperature, and construction. For engineering decisions, prefer datasheets.
8) What should I report: one-way or round-trip?
One-way is best for timing budgets and control loops. Round-trip is common for network tests like ping. Report both when troubleshooting latency.