Latency Calculator
Estimate one-way network latency from distance, transmission medium, and processing delay.
Inputs
Router/switch processing, serialization, and queuing delay
One-Way Latency
21.614ms
Propagation Delay
19.614ms
Estimated Round-Trip Time
43.227ms
Signal Speed
203,940km/s
Step by step
Values used
Distance = 4,000 km; Transmission Medium = Fiber optic cable (n ≈ 1.47); Processing / Queuing Delay = 2 ms
One-way latency
latency = distance / (speed_of_light / refractive_index) + processing_delay
One-Way Latency
= 21.614 ms
Propagation Delay
= 19.614 ms
Estimated Round-Trip Time
= 43.227 ms
Signal Speed
= 203,940 km/s
How it works
One-way network latency is the sum of propagation delay (the time for a signal to physically travel the distance at a speed determined by the transmission medium) and processing delay (time spent in router/switch queues, serialization, and packet handling). Propagation speed is the speed of light divided by the medium's refractive index — light travels slower in fiber (n≈1.47) or copper (n≈1.5) than in free space (n=1.0). Round-trip time (RTT) is approximately twice the one-way latency when the path is symmetric.
Formula
One-way latency
latency = distance / (speed_of_light / refractive_index) + processing_delay
- d
- Distance the signal travels
- c
- Speed of light in vacuum (299,792.458 km/s)
- n
- Refractive index of the transmission medium
- t_{proc}
- Processing, serialization, and queuing delay
Frequently Asked Questions
Why is fiber optic signal speed slower than the speed of light?
Light travels slower in a medium than in a vacuum, by a factor equal to the medium's refractive index. For typical fiber, that index is about 1.47, so light travels roughly 68% as fast as it does in a vacuum.
Is RTT always exactly double the one-way latency?
Only approximately, and only for symmetric paths. In practice, upload and download paths often differ (asymmetric routing, different congestion in each direction), so measured RTT can deviate from a clean 2x multiple.
What contributes to processing delay besides distance?
Router and switch forwarding lookups, packet serialization onto the physical link, queuing behind other traffic, and any middlebox processing (firewalls, NAT, deep packet inspection) all add processing delay on top of pure propagation delay.
Why does satellite (geostationary) latency feel so high?
Geostationary satellites orbit about 35,786 km above the equator, so a signal travels roughly 71,572 km round-trip to the satellite and back just one way, adding over 200ms of pure propagation delay even though the signal moves at nearly the speed of light.