Cable Latency Calculator
Even at the speed of light, distance costs time. The propagation delay through a cable is the minimum latency physics imposes on a signal, set by the cable length and how fast the signal travels in that medium. This calculator computes it: enter the cable length in kilometres and the cable's velocity factor from its data sheet, and it returns the one-way and round-trip propagation delay in milliseconds and microseconds, plus the actual signal speed. It models cable propagation only; real networks add switching and processing delays on top of this floor.
Cable latency formula
Signal speed = 299,792.458 km/s * velocity factor
One-way delay (s) = length (km) / signal speed (km/s)
One-way delay (ms) = one-way delay (s) * 1,000
Round-trip delay = one-way delay * 2
The speed of light is the exact SI constant 299,792,458 metres per second, or 299,792.458 kilometres per second. The velocity factor scales it down to the actual speed in the cable, and dividing the length by that speed gives the delay.
Network latency context
- The speed of light in vacuum is exactly 299,792,458 metres per second by SI definition.
- Optical fibre has a velocity factor around 0.67; check the manufacturer's data for your cable.
- Round-trip time is twice the one-way propagation delay for a single cable run.
- This is the physical minimum; switches, routers, and queuing add further latency.
- A rough rule of thumb is about 5 microseconds per kilometre of fibre, consistent with a 0.67 velocity factor.
Cable latency: frequently asked questions
What is propagation latency in a cable?
Propagation latency is the time a signal takes to travel along a cable. It equals the cable length divided by the signal's propagation speed. That speed is the speed of light multiplied by the cable's velocity factor, which accounts for the signal travelling slower than light in a vacuum.
What is the velocity factor?
The velocity factor is the fraction of the vacuum speed of light at which a signal travels in a given medium. Optical fibre is around 0.67, and many coaxial and twisted-pair cables fall between about 0.6 and 0.7. Because it depends on the cable, you enter it from the manufacturer's specification.
Why is round-trip latency twice the one-way value?
Round-trip time is the time for a signal to travel to the far end and back. For a single cable the return path has the same length, so the round trip is exactly twice the one-way propagation delay. Real network round-trip times also add switching and processing delays not modelled here.
Does this include equipment and processing delay?
No. This calculator models only the propagation delay through the cable itself, the time set by the speed of light and the cable length. Real systems add latency from switches, routers, serialisation, and queuing. The cable propagation delay is the irreducible minimum imposed by physics.
How fast does light travel in a vacuum?
The speed of light in a vacuum is exactly 299,792,458 metres per second, a defined constant in the SI system. The signal in a cable travels at this speed multiplied by the velocity factor, which is always less than one, so cable signals are always slower than light in free space.
Official sources
- NIST: Speed of light in vacuum.
- IETF RFC 2330: Framework for IP Performance Metrics.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.