Bandwidth-Delay Product Calculator (TCP Window)
How much data must be in flight to fill a network link, and the throughput a single TCP connection can reach with a given window size.
Results
Data in flight needed to fill the link
5,000,000
B
The same in megabytes
5.000
MB
Most a single connection can reach with that window
13.11
Mbit/s
What this tool does
A connection can only send a window's worth of data before it waits for an acknowledgement, so its speed is capped at window ÷ round-trip time no matter how fast the line is. With the old 64 KiB window and 40 ms of latency that cap is about 13 Mbit/s — which is why a gigabit link can feel slow to a distant server. Modern systems scale the window automatically, but this is still the first thing to check when long-distance transfers are slow while local ones are fast.
Formula
BDP = bandwidth × RTT · max throughput of one connection ≈ window ÷ RTT
Variables
| Symbol | Meaning | Unit |
|---|---|---|
bw | Link bandwidth | Mbit/s |
rt | Round-trip time (ping) | ms |
wk | TCP window in use | KiB |
B | Data in flight needed to fill the link | B |
MB | The same in megabytes | MB |
T | Most a single connection can reach with that window | Mbit/s |
Worked example
- Link bandwidth1000 Mbit/s
- Round-trip time (ping)40 ms
- TCP window in use64 KiB
- Data in flight needed to fill the link5,000,000 B
- The same in megabytes5.000 MB
- Most a single connection can reach with that window13.11 Mbit/s
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.