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Bandwidth-Delay Product Calculator

Compute the bandwidth-delay product and the throughput a given TCP window can reach over a round trip — and the window size needed to fill the link.

Link bandwidth

Round-trip time (RTT)

TCP window size

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How it works

BDP (bytes) = bandwidth ÷ 8 × RTT · Throughput = window ÷ RTT

The bandwidth-delay product is the amount of data "in flight" that a link can hold — its bandwidth multiplied by the round-trip time. A TCP sender can only have one window of unacknowledged data outstanding, so if the window is smaller than the BDP the sender stalls waiting for ACKs and never fills the pipe. Setting the window at least equal to the BDP is how you get full throughput on long-fat networks (high bandwidth, high latency links).

Worked example

On a 100 Mbps link with 50 ms RTT, the BDP is 100,000,000 ÷ 8 × 0.05 = 625,000 bytes ≈ 625 KB. A 64 KB window only carries 65,536 ÷ 0.05 = 1,310,720 bytes/s ≈ 10.5 Mbps — about 10% of the link. To fill it you would raise the window to roughly the 625 KB BDP.

Frequently asked questions

Why can’t I fill a fast, far-away link?

Because throughput is capped at window ÷ RTT. A default 64 KB window over a 50 ms path tops out around 10 Mbps no matter how fast the link is; only enlarging the window (TCP window scaling) to match the bandwidth-delay product removes the ceiling.

What is a "long-fat network"?

A link with both high bandwidth and high latency — for example intercontinental or satellite paths. Its large bandwidth-delay product needs a correspondingly large TCP window, which is why window scaling (RFC 7323) matters most there.

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This calculator is for educational purposes. Double-check important results before acting on them.