Nothing travels faster than light, and signals in cables travel slower still. Propagation delay is the time a signal needs to cover the distance between sender and receiver – the physical, unavoidable part of network latency. A radio link from New York to London (about 5,567 km) needs 18.6 ms just for the waves to cross; a fibre route of the same length takes about 27 ms because light in glass moves at only about two-thirds of c.
The slowdown is described by the velocity factor (VF), the signal speed as a fraction of the speed of light. It is set by the insulation: VF = 1 ÷ √εr. Solid polyethylene coax has VF ≈ 0.66, foam dielectrics reach 0.8–0.85, and twisted-pair Ethernet cable is around 0.64–0.70 – a 100 m run adds about half a microsecond. On circuit boards the same effect makes signals cover only about 15 cm per nanosecond, which is why DDR memory and high-speed buses need length-matched traces.
In networking, propagation delay is only one piece of latency. Pushing a packet onto the wire takes the transmission delay – packet size divided by link speed – and switches add queuing and processing time. The calculator can include the transmission delay and the bandwidth-delay product, the number of bits “in flight” on the link.
How to use the Propagation Delay Calculator
- Distance
- Medium
- Velocity factor
- Relative permittivity (εr)
- Add transmission delay for a packet?
- Packet size
- Link speed
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the distance, choose the medium (or a custom velocity factor or permittivity) and optionally a packet size and link speed. The calculator multiplies the speed of light by the velocity factor and divides the distance by that speed; packet size ÷ link rate gives the transmission delay.
Worked example
A 100 m Ethernet run with VF 0.65: the signal travels at 299,792,458 × 0.65 = 1.949 × 10⁸ m/s, so the propagation delay is 100 ÷ 1.949 × 10⁸ = 513 ns. A 1,500-byte packet at 100 Mbit/s adds 12,000 ÷ 10⁸ = 120 µs of transmission delay – far more than the propagation time on such a short link. A radio signal across 5,567 km of air takes 18.57 ms.
Velocity factors
| Medium | Velocity factor | Delay per metre |
|---|---|---|
| Vacuum, air | 1.00 | 3.34 ns |
| Foam PE coax | ≈ 0.80 | 4.17 ns |
| Solid PE coax (RG-58) | 0.66 | 5.05 ns |
| Cat 6 Ethernet | ≈ 0.65 | 5.13 ns |
| Optical fibre | ≈ 0.68 | 4.90 ns |
| FR-4 stripline | ≈ 0.48 | 7.0 ns |
Inputs explained
| Field | What to enter |
|---|---|
| Distance | Units: m, km, mi, ft, cm |
| Medium | Choose one: Vacuum or air – VF 1.00, Air-spaced PTFE coax – VF ≈ 0.85, Air-spaced polyethylene coax – VF ≈ 0.84, Foam polyethylene coax – VF ≈ 0.80 … |
| Velocity factor | A number |
| Relative permittivity (εr) | A number |
| Add transmission delay for a packet? | Choose one: No, Yes |
| Packet size | Units: bytes, kB, KiB, bits |
| Link speed | Units: Mbit/s, Gbit/s, kbit/s, bit/s |
What each result means
- Propagation speed = 299792458×[Velocity factor]
- Propagation delay = [Distance]÷[Propagation speed]
- Delay per length = 1÷[Propagation speed]
- Round-trip propagation time = 2×[Propagation delay]
- Transmission delay = [Packet size]÷[Link speed]
- Total one-way delay (propagation + transmission) = [Propagation delay] + [Transmission delay]
- Bandwidth–delay product (bits in flight) = [Link speed]×[Propagation delay]
Formula
Propagation speed v = c × VF (VF = 1 ÷ √εr) · propagation delay = distance ÷ v · transmission delay = packet size ÷ link rate · total = propagation + transmission
Worked example
Inputs — Distance: 100 m; Medium: Ethernet twisted pair (Cat 5e/6) – VF ≈ 0.65; Add transmission delay for a packet?: No.
Results — Velocity factor = 0.65; Propagation speed = 194,865,097.7 m/s; Propagation delay = 0.51318 µs; Delay per length = 5.13176 ns/m; Round-trip propagation time = 1.02635 µs.
Key terms
- Propagation delay
- Time for a signal to travel a distance through a medium.
- Velocity factor (VF)
- Signal speed as a fraction of the speed of light.
- Transmission delay
- Packet size divided by the link bit rate.
- Bandwidth–delay product
- Link rate × delay: the amount of data in transit on the link.
Frequently asked questions
What is propagation delay?
The time a signal needs to travel from sender to receiver: distance ÷ propagation speed.
How fast do signals travel in Ethernet cable?
About 65% of the speed of light – roughly 5.1 ns per metre, or 0.51 µs over a 100 m run.
What is the velocity factor?
The signal speed in a medium as a fraction of the speed of light, 1 ÷ √εr for a cable dielectric.
How long does light take through 1,000 km of fibre?
About 4.9 ms (VF ≈ 0.68), so about 9.8 ms round trip before any equipment delay.
What is the difference between propagation and transmission delay?
Propagation is travel time and depends on distance; transmission is the time to put all bits on the wire and depends on packet size and link speed.
Why does propagation delay matter for gaming and trading?
It sets a hard floor on latency – no faster hardware can beat the speed of light over a given distance.