The calculation
A worked example
A 1,000 Mbps path with a 40 ms round-trip time has a BDP of 40,000,000 bits: 5,000,000 bytes, or about 4.77 MiB.
Use round-trip time, not one-way latency
Bandwidth-delay product describes the amount of data that could occupy the path over one round trip at the selected rate. With TCP, a sufficiently large usable window is one prerequisite for filling such a path, but it is not a guarantee of throughput.
What this number does not tell you
Loss, congestion control, receiver limits, application behavior and competing traffic all affect performance. The receive window and congestion window are different constraints. A high BDP is not a recommendation to indiscriminately enlarge every buffer: excess queues can make interactive traffic less responsive.
Use a representative round-trip time to the destination, not latency to a nearby speed-test server. The calculator reports bits and bytes explicitly, and provides both decimal MB and binary MiB. It is a conceptual planning aid, not automated operating-system tuning advice.
Source & further reading
RFC 7323: TCP extensions for high performance
Model reviewed October 25, 2026. Read our methodology and limitations.