Data centres / Fibre round trip / Milan–Paris
Milan to Paris: minimum fibre round trip
Light in fibre needs at least 8.0 ms to go from Milan to Paris and back over the mapped route. An active-active application depends on the carrier route; synchronous storage replication and a stretched database cluster are out of reach.
Which replication set-ups fit
| Set-up | Round-trip limit | On this pair |
|---|---|---|
| Synchronous storage replicationArray-to-array mirroring where every write waits for the second site. Usually kept within one metro. | 1 ms | Out of reach |
| Stretched database clusterSynchronous commit between database nodes in two sites, one cluster across both. | 5 ms | Out of reach |
| Active-active applicationBoth sites serve users and keep shared state in step. | 10 ms | Depends on the carrier route |
Fits: the mapped round trip stays inside the limit even on the longest typical carrier path. Depends on the carrier route: the mapped floor is inside the limit, a longer path may not be. Out of reach: even the floor is above the limit. Real carrier paths are typically 10 to 40% longer than the mapped route. Asynchronous replication has no round-trip limit and works on any pair.
From distance to round trip
- Straight line, city centre to city centre
- 639 km
- Shortest mapped fibre route
- 817 km
- Route over straight line
- 1.28×
- Minimum one-way time, mapped route
- 4.0 ms
- What the fibre detour costs
- 1.7 ms per round trip
Worked example. Light in single-mode fibre travels at the speed of light divided by the fibre’s group index, 1.4688, about 204,107 km per second. 816.9 km ÷ 204,107 km/s = 4.00 ms one way, and twice that is 8.00 ms for the round trip.
The two markets
Data-centre projects of 5 MW or more in our pipeline tracker, counted when this page loads.
Nearest alternatives
Minimum round trips on mapped fibre routes to the other hubs in the calculator, nearest first.
All market pairsAny two cities in the calculator
Method and limits
The figures are the time light needs in single-mode fibre, at a group index of 1.4688, over the shortest mapped land fibre route between two city centres, and over the straight line for comparison. They are physical floors.
- City centre to city centre. Data-centre campuses sit 15 to 40 km outside most centres, and on short pairs that distance can add more than the route's detour.
- No equipment. Transponders, error correction, regeneration and switching add delay of their own, which matters most against the tightest limit.
- No protection path. Synchronous replication has to be designed to the backup route a circuit fails over to, which can be much longer than the working route.
- No queueing or congestion: the figures hold for an idle path.
- The limits are common engineering rules of thumb for each set-up; vendors and workloads set their own.
- Real carrier paths are typically 10 to 40% longer than the mapped route, so every figure here is a floor, not what a circuit will show.
Route lengths come from a public map of operational terrestrial fibre, joined to Great Britain and Ireland by submarine and tunnel links of published length (lengths only). City coordinates are city centres. No carrier route is modelled.
ScienceShot calculation.
Cite this page
ScienceShot. (2026). Milan–Paris Minimum Fibre Round Trip [Online calculator]. https://scienceshot.com/data-centres/latency/milan-paris
@misc{scienceshot-milan-paris-2026,
author = {{ScienceShot}},
title = {{Milan–Paris Minimum Fibre Round Trip}},
year = {2026},
note = {Online calculator},
url = {https://scienceshot.com/data-centres/latency/milan-paris}
}