How fast can light
cross the sea floor?
Pick two cities and estimate the best-case latency of a fiber route between them — the physics floor under every video call, game and web page. Submarine cables like those linking Southeast Asia, the Middle East and Europe carry most of the world’s internet traffic.
Estimate latency
Fiber latency estimator
Light in optical fiber travels at roughly two-thirds of its speed in a vacuum, and cables never run in a straight line. Adjust the route factor to see how detours add delay.
- Great-circle distance
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- Estimated cable length
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- One-way in fiber
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- Round trip in a vacuum
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Numbers behind the estimate
Why real-world ping is always higher than the physics floor.
| Factor | Typical value | Effect on latency |
|---|---|---|
| Speed of light in fiber | ≈ 204,000 km/s (refractive index ≈ 1.47) | About 4.9 µs per km, one way. |
| Route factor | 1.2–1.6× great-circle distance | Cables follow coastlines, avoid hazards and stop at landing stations. |
| Terrestrial backhaul | Tens to hundreds of km | Data still has to reach the landing station and the data center inland. |
| Routing and equipment | A few ms | Each router, switch and amplifier adds a little processing delay. |
| Congestion | 0 to 100+ ms | Busy links queue packets; this is where “lag” spikes usually come from. |
A short guide to submarine cables
More than 95% of intercontinental data travels through cables laid on the ocean floor. A modern cable is about the thickness of a garden hose; inside are a handful of hair-thin glass fibers, each carrying many wavelengths of laser light. Repeaters every 50–100 km boost the signal, powered through a copper conductor in the cable itself.
Why routes zig-zag
Cables connect landing stations along the way to serve many countries, steer around undersea mountains, volcanic zones and busy anchorages, and sometimes follow political boundaries. That is why a cable from Southeast Asia to Europe can be considerably longer than the straight-line distance.
When cables break
Ship anchors, fishing gear and earthquakes damage cables dozens of times a year. Traffic is rerouted over other systems, which is why a single break usually means slower connections rather than a total outage. Repair ships lift the damaged section and splice in new cable, a job that can take days to weeks.
Frequently asked questions
Is this the official SEA-ME-WE 4 website?
No. This is an independent educational site. It is not affiliated with, endorsed by or operated for any cable system, consortium or telecom operator.
How is the latency estimated?
We take the great-circle distance between the two cities, multiply it by a route factor for real cable paths, and divide by the speed of light in glass fiber (about 204,000 km/s). Doubling that gives the round-trip time.
Why is my real ping higher than the estimate?
The estimate is a physics floor. Real connections add terrestrial backhaul, routing equipment, congestion and sometimes detours through other countries, so measured ping is usually 10–60% higher.
What route factor should I use?
For long subsea routes 1.3–1.5 is typical. Direct terrestrial fiber between nearby cities can be closer to 1.2; routes that go around continents can exceed 1.6.
Are the city coordinates exact?
They are rounded city-center coordinates, which is precise enough for latency estimates; a few kilometers make a difference of only microseconds.
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