Free tool · China & network
Fiber latency calculator: TCP throughput
See the physical minimum round-trip time between two cities over fiber, and the TCP throughput one stream can reach. Physics only, not our network.
Type to search: capitals and cities of a million people or more. 1.0 is the straight great-circle line; real fiber routes run longer.
- Great-circle distance
- 6,427 km
- Round-trip floor, straight fiber
- 64.3 ms
- Round trip with path factor 1.5
- 96.9 ms
Physics only, not our network: measured latency on any network is higher.
TCP throughput of one stream
Filled from the city pair; change it to test other latencies.
- Bandwidth-delay product
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- Window limit
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- Loss limit (Mathis)
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- Best one stream can do
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Limited by packet loss: lower loss or use several streams.
How this works and its limits
Runs in your browser: what you enter stays on your device and is never uploaded.
The floor is the great-circle distance between the two cities multiplied by a typical path factor, at the speed of light in fiber, plus a small allowance for equipment. Throughput uses the window size and the Mathis approximation for packet loss.
Physics only, not our network: real routes are longer than the great circle and add queueing, so measured latency is always higher than the floor. City coordinates come from an open gazetteer, credited below.
Data sources and licences
Operator names, brands and codes shown here are public reference data compiled from national regulators and other public sources. Their appearance does not indicate any partnership, endorsement, agreement or connectivity between unicomam.com and the named operators. All trademarks belong to their owners. Data is provided “as is” with no warranty; verify against the official source before relying on it for routing, billing or compliance decisions.
Regulatory information is for reference only and is not legal advice; check the regulator link and the “last verified” date.
City coordinates: GeoNames (opens in a new tab), licensed under CC BY 4.0 (opens in a new tab). Subset of cities15000; data modified.
Why latency has a physical floor
No network can beat the speed of light. In optical fiber, light travels at roughly two-thirds of its speed in a vacuum, so every kilometer of fiber adds about 5 microseconds in each direction. The shortest possible round trip between two cities follows directly from the great-circle distance between them: twice the distance at 5 microseconds per kilometer.
Real routes are longer than the great-circle line. Cables follow coasts, borders and existing rights of way, and traffic passes through equipment that adds a little delay at every hop. A path factor captures this: 1.0 is the straight line, while real routes commonly run 1.2 to 2 times longer. Comparing a measured round-trip time with the floor shows how much of the delay is distance and how much is the route and the network.
Latency also caps throughput. A single TCP connection cannot have more data in flight than its window, so it sends at most one window per round trip: a 64 KB window on an intercontinental path carries only a few megabits per second, however fast the link is. The amount of data needed to keep a link busy, its bandwidth multiplied by the round-trip time, is the bandwidth-delay product; a window smaller than that leaves the link partly idle.
Packet loss lowers the ceiling further. The Mathis approximation estimates the best a standard TCP stream can sustain as the segment size divided by the round-trip time, multiplied by about 1.22 and divided by the square root of the loss rate. Long paths make every loss more costly, because each recovery takes a full round trip.
Long-distance links therefore need larger windows, modern congestion control, or several parallel streams to reach their bandwidth. Use the floor to set expectations for an application between two regions, then measure the real path before deciding on the connectivity.