How the calculation works
Route length = distance × route factor (miles are converted at 1 mile = 1.609344 km).
One-way propagation delay = route length × 4.89 µs per km. Light travels through fiber at the speed of light in a vacuum (299,792.458 km/s) divided by the fiber’s group index. The calculator uses a group index of 1.467, a typical value for standard single-mode fiber at 1,550 nm, which gives about 204,000 km/s, or 4.89 µs per km. The exact index varies a little by fiber type and wavelength.
One-way latency = propagation delay + equipment allowance. Round-trip time (RTT) = 2 × one-way latency.
Assumptions you can change:
- Route factor, default 1.5. Fiber follows roads, railways and rights of way, and passes through carrier buildings on the way, so the cable is longer than a straight line between the sites. 1.5 is a planning assumption, not a measured figure. Use 1 when the distance you enter is a known fiber route length, such as one from a carrier’s route map or a wavelength quote.
- Equipment allowance, default 1 ms each way. Routers, switches, transponders and regeneration add delay along the path. 1 ms is a planning assumption.
The result is a floor for a quiet network. It doesn’t include queuing on busy links, the last mile, Wi-Fi, VPNs or the time a server takes to respond.
Worked example
New York to Chicago. The straight-line distance between the city centers is about 1,144 km. With the default route factor of 1.5 and 1 ms of equipment each way:
- Route length = 1,144 × 1.5 = 1,716 km
- Propagation, one way = 1,716 × 4.8934 µs = 8.397 ms
- One-way latency = 8.397 + 1 = 9.397 ms (9.4 ms rounded)
- Round trip = 2 × 9.397 = 18.79 ms
With fiber on the straight line and no equipment delay, the round trip would be 11.2 ms: the physical floor for this pair. If a carrier quotes a route of a known length, enter it with a route factor of 1 to compare routes.
To see what that latency does to a single TCP transfer, enter the RTT in the TCP window calculator.
City-to-city examples
The distance column is the straight-line (great-circle) distance between city centers, computed with the haversine formula on a sphere of radius 6,371 km from approximate coordinates rounded to 0.01°, so treat them as approximate. The last column uses the calculator’s defaults: route factor 1.5 and 1 ms of equipment each way.
| City pair | Distance | RTT, straight line | RTT, defaults |
|---|---|---|---|
| New York – Ashburn, VA | 350 km (217 mi) | 3.43 ms | 7.14 ms |
| New York – Chicago | 1,144 km (711 mi) | 11.2 ms | 18.79 ms |
| Chicago – Dallas | 1,295 km (805 mi) | 12.67 ms | 21.01 ms |
| Los Angeles – Seattle | 1,546 km (961 mi) | 15.13 ms | 24.7 ms |
| New York – Los Angeles | 3,935 km (2,445 mi) | 38.51 ms | 59.77 ms |
| London – Frankfurt | 638 km (396 mi) | 6.24 ms | 11.37 ms |
| New York – London | 5,570 km (3,461 mi) | 54.51 ms | 83.77 ms |
| Los Angeles – Tokyo | 8,817 km (5,479 mi) | 86.29 ms | 131.43 ms |
Real routes differ from both columns: a direct low-latency route can come close to the straight-line figure, and an internet path that detours through a distant exchange point can be well above the estimate. Use a carrier’s route length or a measured ping for a specific pair.
