Propagation Delay Calculator

Find how long a signal takes to travel a link — enter the distance and medium to get propagation delay, transmission delay, total one-way delay, and round-trip time.

Quick Facts

Formula
Propagation delay = Distance ÷ (c × velocity factor)
c = 299,792.458 km/s in a vacuum; a medium's velocity factor is set by its dielectric (about 0.64–0.70 for cable, 1.0 in free space).
Total link delay
Total delay = Propagation delay + Transmission delay
Transmission delay = data size ÷ bandwidth; round-trip time is approximated as 2 × total one-way delay, excluding processing and queuing delay.

Your Results

Calculated
Propagation delay
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Signal travel time (distance ÷ speed)
Transmission delay
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Time to push the data onto the link
Total one-way delay
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Propagation + transmission delay
Round-trip time (RTT)
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≈ 2 × total delay, excludes queuing/processing

Ready

Enter distance, medium, data size, and bandwidth, then press Calculate.

Understanding Propagation Delay

Propagation delay is the time a signal takes to travel across a physical link — copper cable, fiber-optic strand, or open air — from sender to receiver. It is determined entirely by two things: how far the signal has to go, and how fast it moves through that particular medium. Propagation delay is one of the two components of end-to-end network delay you can compute directly from physics; the other is transmission delay, covered below.

The formula

Propagation delay is distance divided by propagation speed:

  • Propagation delay = Distance ÷ Propagation speed
  • Propagation speed = c × velocity factor (VF), where c is the speed of light in a vacuum, 299,792.458 km/s (about 186,282 miles/s).

The velocity factor accounts for how much a real medium slows an electromagnetic signal compared to a vacuum. It is set by the medium's relative permittivity (dielectric constant) εr: VF = 1/√εr. Typical values used by this calculator:

  • Fiber-optic cable: ~0.68c (glass core, refractive index ≈1.47)
  • Copper twisted pair (Ethernet cable): ~0.64c
  • Coaxial cable: ~0.66c
  • Free space / radio / satellite link: 1.0c — radio waves in air or vacuum travel at essentially the full speed of light

Transmission delay and total delay

Sending a message also takes time to push every bit onto the link, called transmission delay (or serialization delay): Transmission delay = Data size ÷ Bandwidth. The total one-way delay a message experiences is Total delay = Propagation delay + Transmission delay. This calculator reports both pieces plus an approximate round-trip time, computed as RTT ≈ 2 × Total delay — a useful estimate that ignores processing and queuing delay at routers and switches, which real, measured round-trip times also include.

Worked example

With the default inputs — a 1,000 km fiber link (VF 0.68), a 12,000-bit (1,500-byte) packet, and 100 Mbps of bandwidth — propagation delay works out to about 4.91 ms (1,000 km ÷ (299,792.458 × 0.68) km/s), transmission delay to about 0.12 ms (12,000 bits ÷ 100,000,000 bps), for a total one-way delay near 5.03 ms and an approximate round trip near 10.05 ms.

Not the same as gate propagation delay

"Propagation delay" is also used in digital electronics to describe how long a logic gate takes to respond after its input changes (often specified as tPLH and tPHL on a datasheet). That is a device-specific switching-time figure, not the wire-length delay this calculator computes — check the component's datasheet for that number instead.

Frequently Asked Questions

What is propagation delay?
Propagation delay is the time a signal needs to travel from sender to receiver across a physical medium such as copper cable, fiber-optic cable, or open air. It equals distance divided by propagation speed, where propagation speed is the speed of light in a vacuum multiplied by the medium's velocity factor.
What is a velocity factor and why is it less than 1?
A velocity factor (VF) is the fraction of the speed of light in a vacuum that a signal actually achieves in a given medium. It equals 1 divided by the square root of the medium's relative permittivity. Glass and plastic dielectrics slow electromagnetic waves down, so fiber runs about 0.68c, copper twisted pair about 0.64c, and coaxial cable about 0.66c. Only free space or a vacuum reaches a velocity factor of 1.0.
How is propagation delay different from transmission delay?
Propagation delay is travel time over distance and depends only on distance and velocity factor. Transmission delay (serialization delay) is the time needed to push all the bits of a message onto the link, equal to data size divided by bandwidth. More bandwidth shortens transmission delay but does not change propagation delay at all — a 10 Gbps link across the same distance has essentially the same propagation delay as a 1 Mbps link.
Why does satellite internet usually have higher latency than fiber?
Even though radio waves travel at the full speed of light (velocity factor 1.0) while fiber signals travel at roughly 0.68c, a geostationary satellite link covers a much longer round-trip distance — tens of thousands of kilometers up and back — than a terrestrial fiber route between the same two points, so its total propagation delay is typically far larger.

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