Fibre Latency Calculator for European Data Centres
Minimum round-trip and one-way delay in optical fibre between 20 European data-centre hubs, from Frankfurt and London to Madrid and Helsinki, with the physics shown.
What is the lowest latency physics allows between Frankfurt and London, or Dublin and Milan? Light in optical fibre covers about 204,000 km per second, so distance alone sets a floor no network can beat. Pick two hubs to see that floor, and adjust the route factor to allow for real cable paths.
Fibre latency between European data-centre hubs
- Light in standard single-mode fibre travels at about 204,107 km/s, so every 1,000 km of fibre adds at least 4.90 ms one way and 9.80 ms to a round trip.
- Among these 20 hubs the shortest straight-line round trip is Amsterdam to Brussels, 1.7 ms; the longest is Helsinki to Lisbon, 32.9 ms.
Minimum round trip between hubs, milliseconds
| From \ to | Amsterdam | Berlin | Brussels | Copenhagen | Dublin | Frankfurt | Helsinki | Lisbon | London | Madrid | Marseille | Milan | Munich | Oslo | Paris | Prague | Stockholm | Vienna | Warsaw | Zurich |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Amsterdam | – | 5.6 | 1.7 | 6.1 | 7.4 | 3.6 | 14.7 | 18.3 | 3.5 | 14.5 | 9.9 | 8.1 | 6.5 | 8.9 | 4.2 | 7.0 | 11.0 | 9.2 | 10.7 | 6.0 |
| Berlin | 5.6 | – | 6.4 | 3.5 | 12.9 | 4.1 | 10.8 | 22.7 | 9.1 | 18.3 | 11.6 | 8.2 | 4.9 | 8.2 | 8.6 | 2.7 | 8.0 | 5.1 | 5.1 | 6.5 |
| Brussels | 1.7 | 6.4 | – | 7.5 | 7.6 | 3.1 | 16.1 | 16.8 | 3.1 | 12.9 | 8.3 | 6.8 | 5.9 | 10.6 | 2.6 | 7.0 | 12.5 | 9.0 | 11.4 | 4.8 |
| Copenhagen | 6.1 | 3.5 | 7.5 | – | 12.1 | 6.6 | 8.7 | 24.3 | 9.4 | 20.3 | 14.4 | 11.4 | 8.2 | 4.7 | 10.1 | 6.2 | 5.1 | 8.5 | 6.6 | 9.4 |
| Dublin | 7.4 | 12.9 | 7.6 | 12.1 | – | 10.7 | 19.8 | 16.1 | 4.5 | 14.2 | 13.8 | 13.9 | 13.5 | 12.4 | 7.7 | 14.3 | 16.0 | 16.5 | 17.9 | 12.1 |
| Frankfurt | 3.6 | 4.1 | 3.1 | 6.6 | 10.7 | – | 14.8 | 18.5 | 6.2 | 14.2 | 7.8 | 5.1 | 3.0 | 10.8 | 4.7 | 4.0 | 11.6 | 5.9 | 8.7 | 3.0 |
| Helsinki | 14.7 | 10.8 | 16.1 | 8.7 | 19.8 | 14.8 | – | 32.9 | 17.8 | 28.9 | 22.4 | 19.0 | 15.6 | 7.7 | 18.7 | 12.8 | 3.9 | 14.1 | 9.0 | 17.4 |
| Lisbon | 18.3 | 22.7 | 16.8 | 24.3 | 16.1 | 18.5 | 32.9 | – | 15.5 | 4.9 | 12.9 | 16.5 | 19.3 | 26.8 | 14.2 | 22.0 | 29.3 | 22.5 | 27.0 | 16.9 |
| London | 3.5 | 9.1 | 3.1 | 9.4 | 4.5 | 6.2 | 17.8 | 15.5 | – | 12.4 | 9.8 | 9.4 | 9.0 | 11.3 | 3.4 | 10.1 | 14.0 | 12.1 | 14.2 | 7.6 |
| Madrid | 14.5 | 18.3 | 12.9 | 20.3 | 14.2 | 14.2 | 28.9 | 4.9 | 12.4 | – | 8.0 | 11.6 | 14.5 | 23.4 | 10.3 | 17.4 | 25.4 | 17.7 | 22.4 | 12.2 |
| Marseille | 9.9 | 11.6 | 8.3 | 14.4 | 13.8 | 7.8 | 22.4 | 12.9 | 9.8 | 8.0 | – | 3.8 | 7.1 | 18.5 | 6.5 | 10.0 | 19.4 | 9.9 | 15.0 | 5.1 |
| Milan | 8.1 | 8.2 | 6.8 | 11.4 | 13.9 | 5.1 | 19.0 | 16.5 | 9.4 | 11.6 | 3.8 | – | 3.4 | 15.8 | 6.3 | 6.3 | 16.2 | 6.1 | 11.2 | 2.1 |
| Munich | 6.5 | 4.9 | 5.9 | 8.2 | 13.5 | 3.0 | 15.6 | 19.3 | 9.0 | 14.5 | 7.1 | 3.4 | – | 12.8 | 6.7 | 2.9 | 12.9 | 3.5 | 7.9 | 2.4 |
| Oslo | 8.9 | 8.2 | 10.6 | 4.7 | 12.4 | 10.8 | 7.7 | 26.8 | 11.3 | 23.4 | 18.5 | 15.8 | 12.8 | – | 13.1 | 10.9 | 4.1 | 13.2 | 10.4 | 13.7 |
| Paris | 4.2 | 8.6 | 2.6 | 10.1 | 7.7 | 4.7 | 18.7 | 14.2 | 3.4 | 10.3 | 6.5 | 6.3 | 6.7 | 13.1 | – | 8.6 | 15.1 | 10.1 | 13.4 | 4.8 |
| Prague | 7.0 | 2.7 | 7.0 | 6.2 | 14.3 | 4.0 | 12.8 | 22.0 | 10.1 | 17.4 | 10.0 | 6.3 | 2.9 | 10.9 | 8.6 | – | 10.3 | 2.5 | 5.1 | 5.2 |
| Stockholm | 11.0 | 8.0 | 12.5 | 5.1 | 16.0 | 11.6 | 3.9 | 29.3 | 14.0 | 25.4 | 19.4 | 16.2 | 12.9 | 4.1 | 15.1 | 10.3 | – | 12.2 | 7.9 | 14.4 |
| Vienna | 9.2 | 5.1 | 9.0 | 8.5 | 16.5 | 5.9 | 14.1 | 22.5 | 12.1 | 17.7 | 9.9 | 6.1 | 3.5 | 13.2 | 10.1 | 2.5 | 12.2 | – | 5.4 | 5.8 |
| Warsaw | 10.7 | 5.1 | 11.4 | 6.6 | 17.9 | 8.7 | 9.0 | 27.0 | 14.2 | 22.4 | 15.0 | 11.2 | 7.9 | 10.4 | 13.4 | 5.1 | 7.9 | 5.4 | – | 10.2 |
| Zurich | 6.0 | 6.5 | 4.8 | 9.4 | 12.1 | 3.0 | 17.4 | 16.9 | 7.6 | 12.2 | 5.1 | 2.1 | 2.4 | 13.7 | 4.8 | 5.2 | 14.4 | 5.8 | 10.2 | – |
Straight-line fibre between city centres, route factor 1. Real routes are longer, so measured round trips are higher.
Round trips along mapped fibre routes, milliseconds
| From \ to | Amsterdam | Berlin | Brussels | Copenhagen | Dublin | Frankfurt | Helsinki | Lisbon | London | Madrid | Marseille | Milan | Munich | Oslo | Paris | Prague | Stockholm | Vienna | Warsaw | Zurich |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Amsterdam | – | 6.4 | 1.8 | 6.8 | 8.1 | 3.8 | – | 20.1 | 4.0 | 15.7 | 11.4 | 9.2 | 7.2 | 11.6 | 4.4 | 8.1 | 12.4 | 10.8 | 11.9 | 7.0 |
| Berlin | 6.4 | – | 7.1 | 3.9 | 14.5 | 4.7 | – | 25.4 | 10.4 | 20.9 | 13.4 | 9.5 | 5.4 | 8.8 | 9.7 | 3.1 | 9.5 | 5.9 | 5.6 | 7.7 |
| Brussels | 1.8 | 7.1 | – | 8.5 | 8.7 | 3.5 | – | 18.5 | 3.5 | 14.1 | 9.8 | 8.9 | 6.8 | 13.4 | 2.8 | 7.8 | 14.1 | 10.5 | 12.4 | 6.7 |
| Copenhagen | 6.8 | 3.9 | 8.5 | – | 14.4 | 7.1 | – | 26.8 | 10.8 | 22.4 | 15.8 | 12.7 | 9.2 | 4.9 | 11.1 | 7.0 | 5.6 | 9.7 | 8.9 | 10.5 |
| Dublin | 8.1 | 14.5 | 8.7 | 14.4 | – | 12.0 | – | – | 5.2 | 20.2 | 16.1 | 16.1 | 15.1 | 15.3 | 9.1 | 16.2 | 19.1 | 18.8 | 20.1 | 14.0 |
| Frankfurt | 3.8 | 4.7 | 3.5 | 7.1 | 12.0 | – | – | 21.2 | 7.0 | 16.8 | 8.8 | 5.6 | 3.3 | 11.9 | 5.9 | 4.4 | 12.7 | 7.0 | 9.5 | 3.4 |
| Helsinki | – | – | – | – | – | – | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
| Lisbon | 20.1 | 25.4 | 18.5 | 26.8 | – | 21.2 | – | – | 19.6 | 5.2 | 15.5 | 19.7 | 21.8 | 31.7 | 15.7 | 24.6 | 32.4 | 25.0 | 30.7 | 18.8 |
| London | 4.0 | 10.4 | 3.5 | 10.8 | 5.2 | 7.0 | – | 19.6 | – | 15.1 | 11.0 | 10.9 | 10.0 | 15.0 | 4.0 | 11.3 | 16.4 | 13.7 | 15.9 | 8.8 |
| Madrid | 15.7 | 20.9 | 14.1 | 22.4 | 20.2 | 16.8 | – | 5.2 | 15.1 | – | 10.3 | 14.5 | 17.3 | 27.3 | 11.3 | 20.1 | 28.0 | 20.6 | 26.2 | 14.3 |
| Marseille | 11.4 | 13.4 | 9.8 | 15.8 | 16.1 | 8.8 | – | 15.5 | 11.0 | 10.3 | – | 4.3 | 8.4 | 20.7 | 7.0 | 11.8 | 21.4 | 11.0 | 17.4 | 6.3 |
| Milan | 9.2 | 9.5 | 8.9 | 12.7 | 16.1 | 5.6 | – | 19.7 | 10.9 | 14.5 | 4.3 | – | 4.1 | 17.5 | 8.0 | 7.5 | 18.3 | 6.7 | 13.1 | 2.2 |
| Munich | 7.2 | 5.4 | 6.8 | 9.2 | 15.1 | 3.3 | – | 21.8 | 10.0 | 17.3 | 8.4 | 4.1 | – | 14.1 | 7.0 | 3.5 | 14.8 | 3.8 | 10.0 | 3.0 |
| Oslo | 11.6 | 8.8 | 13.4 | 4.9 | 15.3 | 11.9 | – | 31.7 | 15.0 | 27.3 | 20.7 | 17.5 | 14.1 | – | 16.0 | 11.9 | 4.3 | 14.6 | 13.8 | 15.3 |
| Paris | 4.4 | 9.7 | 2.8 | 11.1 | 9.1 | 5.9 | – | 15.7 | 4.0 | 11.3 | 7.0 | 8.0 | 7.0 | 16.0 | – | 9.2 | 16.7 | 10.8 | 15.1 | 5.9 |
| Prague | 8.1 | 3.1 | 7.8 | 7.0 | 16.2 | 4.4 | – | 24.6 | 11.3 | 20.1 | 11.8 | 7.5 | 3.5 | 11.9 | 9.2 | – | 12.6 | 2.8 | 6.7 | 6.1 |
| Stockholm | 12.4 | 9.5 | 14.1 | 5.6 | 19.1 | 12.7 | – | 32.4 | 16.4 | 28.0 | 21.4 | 18.3 | 14.8 | 4.3 | 16.7 | 12.6 | – | 15.3 | – | 16.1 |
| Vienna | 10.8 | 5.9 | 10.5 | 9.7 | 18.8 | 7.0 | – | 25.0 | 13.7 | 20.6 | 11.0 | 6.7 | 3.8 | 14.6 | 10.8 | 2.8 | 15.3 | – | 6.4 | 6.2 |
| Warsaw | 11.9 | 5.6 | 12.4 | 8.9 | 20.1 | 9.5 | – | 30.7 | 15.9 | 26.2 | 17.4 | 13.1 | 10.0 | 13.8 | 15.1 | 6.7 | – | 6.4 | – | 12.2 |
| Zurich | 7.0 | 7.7 | 6.7 | 10.5 | 14.0 | 3.4 | – | 18.8 | 8.8 | 14.3 | 6.3 | 2.2 | 3.0 | 15.3 | 5.9 | 6.1 | 16.1 | 6.2 | 12.2 | – |
Shortest mapped route along land fibre, with the submarine and tunnel links that join Great Britain and Ireland at their published length, 169 of 190 pairs. A dash means the map only links the two round a sea, as with Helsinki and some Baltic pairs, whose real traffic uses undersea cables the map omits.
Method
Distance is measured between city centres. Delay is the fibre path divided by the speed of light in single-mode fibre, about 204,000 km/s; the path is the straight line times the route factor, or the mapped route. It is a physical lower bound: real routes and equipment add delay.
Why the floor sits where it does: why light in optical fibre is slower than light in a vacuum.
Minimum round trip between data centre markets
How long light in fibre needs to go between two data centre markets and back, over the shortest mapped fibre route, and which replication set-ups that leaves room for. 21 market pairs, each with its own page.
| Pair | Mapped route | Minimum round trip | Strictest set-up that fits |
|---|---|---|---|
| Amsterdam–Frankfurt | 392 km | 3.8 ms | Active-active application |
| London–Paris | 409 km | 4.0 ms | Active-active application |
| Amsterdam–London | 411 km | 4.0 ms | Active-active application |
| Amsterdam–Paris | 453 km | 4.4 ms | Active-active application |
| Dublin–London | 529 km | 5.2 ms | Active-active application |
| Frankfurt–Milan | 573 km | 5.6 ms | Active-active application |
| Frankfurt–Paris | 599 km | 5.9 ms | Active-active application |
| Frankfurt–London | 712 km | 7.0 ms | Active-active application |
| Milan–Paris | 817 km | 8.0 ms | Active-active application, depending on the route |
| Amsterdam–Dublin | 831 km | 8.1 ms | Active-active application, depending on the route |
| Dublin–Paris | 929 km | 9.1 ms | Active-active application, depending on the route |
| Amsterdam–Milan | 939 km | 9.2 ms | Active-active application, depending on the route |
| London–Milan | 1,115 km | 11 ms | Asynchronous only |
| Madrid–Paris | 1,149 km | 11 ms | Asynchronous only |
| Dublin–Frankfurt | 1,221 km | 12 ms | Asynchronous only |
| Madrid–Milan | 1,480 km | 15 ms | Asynchronous only |
| London–Madrid | 1,542 km | 15 ms | Asynchronous only |
| Amsterdam–Madrid | 1,600 km | 16 ms | Asynchronous only |
| Dublin–Milan | 1,642 km | 16 ms | Asynchronous only |
| Frankfurt–Madrid | 1,712 km | 17 ms | Asynchronous only |
| Dublin–Madrid | 2,057 km | 20 ms | Asynchronous only |
Pairs whose two cities both have a market page and a mapped land fibre route, shortest round trip first.
The limits
- Synchronous storage replication, 1 ms. Array-to-array mirroring where every write waits for the second site. Usually kept within one metro.
- Stretched database cluster, 5 ms. Synchronous commit between database nodes in two sites, one cluster across both.
- Active-active application, 10 ms. Both sites serve users and keep shared state in step.
- Asynchronous replication. No round-trip limit; works on any pair.
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.
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). Fibre Latency Calculator for European Data Centres [Online calculator]. https://scienceshot.com/data-centres/latency
@misc{scienceshot-latency-2026,
author = {{ScienceShot}},
title = {{Fibre Latency Calculator for European Data Centres}},
year = {2026},
note = {Online calculator},
url = {https://scienceshot.com/data-centres/latency}
}