Bandwidth-Delay Product Calculator
Calculate the bandwidth-delay product (BDP) of a link and the TCP window size needed to keep it fully utilised. The BDP is the amount of data that can be 'in flight' on the wire before the first acknowledgement returns.
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How to use this calculator
- Enter the link bandwidth in megabits per second (Mbps).
- Enter the round-trip time (RTT) in milliseconds — a ping time is a good estimate.
- Read the optimal TCP window (in bytes and KB); set your socket buffers at least this large to reach full throughput.
How it works
Bandwidth is converted to bits per second (Mbps × 1,000,000) and RTT to seconds (ms ÷ 1000). BDP in bits = bandwidth × RTT. Since a TCP window is measured in bytes, the optimal window = BDP ÷ 8. The result is shown in bytes and in kilobytes (bytes ÷ 1024).
Worked example
Worked example. A 100 Mbps link with a 40 ms round-trip time: 100,000,000 bits/s × 0.04 s = 4,000,000 bits of BDP. Divided by 8 that is 500,000 bytes ≈ 488.28 KB — the TCP window needed to keep the pipe full.
Common mistakes
- Mixing up bits and bytes — bandwidth is in bits per second but the window is in bytes, so remember to divide by 8.
- Using one-way latency instead of the round-trip time; the window must cover the full there-and-back delay.
- Assuming the default 64 KB window is enough on high-latency links — without TCP window scaling, throughput is capped well below the link speed.
Frequently asked questions
What is a 'long fat network' (LFN)?
A path with both high bandwidth and high latency, giving a large bandwidth-delay product — for example satellite or intercontinental links. These need TCP window scaling to avoid the classic 64 KB window bottleneck.
Why does my fast link feel slow over long distances?
If the TCP window is smaller than the BDP, the sender must stop and wait for acknowledgements before the pipe is full, so throughput is limited by window ÷ RTT rather than by the link's bandwidth. Enlarging the window (window scaling) fixes it.
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