# Propagation Delay Calculator URL: https://speedycalculator.com/calculators/other/propagation-delay/ Last updated: 2026-10-30 > The propagation delay calculator finds how long a signal takes to travel a given distance through air, coax, Ethernet cable, optical fibre or a PCB, from the medium's velocity factor, and can add the transmission delay of a packet for a given link speed. ## Inputs - Distance (m, km, mi, ft, cm) - Medium - Velocity factor - Relative permittivity (εr) - Add transmission delay for a packet? - Packet size (bytes, kB, KiB, bits) - Link speed (Mbit/s, Gbit/s, kbit/s, bit/s) ## Results - Velocity factor - Propagation speed - Propagation delay - Delay per length - Round-trip propagation time - Transmission delay - Total one-way delay (propagation + transmission) - Bandwidth–delay product (bits in flight) ## Formula ``` Propagation speed v = c × VF (VF = 1 ÷ √εr) · propagation delay = distance ÷ v · transmission delay = packet size ÷ link rate · total = propagation + transmission ``` ## Explanation 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 MediumVelocity factorDelay per metreVacuum, air1.003.34 nsFoam PE coax≈ 0.804.17 nsSolid PE coax (RG-58)0.665.05 nsCat 6 Ethernet≈ 0.655.13 nsOptical fibre≈ 0.684.90 nsFR-4 stripline≈ 0.487.0 ns ### Inputs explained FieldWhat to enterDistanceUnits: m, km, mi, ft, cmMediumChoose 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 factorA numberRelative permittivity (εr)A numberAdd transmission delay for a packet?Choose one: No, YesPacket sizeUnits: bytes, kB, KiB, bitsLink speedUnits: 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] ## 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. ## FAQ **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. ## Sources - [Propagation delay – Wikipedia](https://en.wikipedia.org/wiki/Propagation_delay) - [Velocity factor – Wikipedia](https://en.wikipedia.org/wiki/Velocity_factor)