Bandwidth-Delay Product Calculator — TCP Window Size
The bandwidth-delay product (BDP) is the amount of data that can be "in flight" in a network link at any moment. It equals bandwidth × round-trip time and determines the minimum TCP receive window size needed to keep a link fully utilised. Enter your link speed and latency to get the BDP in bits, bytes, and KB.
Bandwidth unit
ms
KB of data "in flight" in the network at any instant — minimum TCP window size for full utilisation
- 1
RTT in seconds
50 ms ÷ 1000 = 0.05 - 2
BDP in bits
100,000,000 × 0.05 = 5,000,000Bandwidth × RTT is how many bits are 'in flight' in the network at any instant. - 3
BDP in bytes
5,000,000 ÷ 8 = 625,000 - 4
BDP in kilobytes
625,000 ÷ 1024 = 610.4
How does this calculator work?
BDP (bits) = bandwidth_bps × RTT_seconds. This is how many bits are "in the pipe" at once. For full link utilisation, the TCP receive window must equal at least BDP / 8 bytes. A 100 Mbps link with 50 ms RTT needs a 625 KB window — the legacy 64 KB default only achieves ~10% utilisation on that link.
Formula
How this is calculated
When a sender transmits data using TCP, it cannot send more data than the receiver's advertised window allows without waiting for an acknowledgement. If the window is smaller than the amount of data in transit at any instant (the bandwidth-delay product), the sender runs out of window space before the ACK returns — the pipe sits idle and throughput suffers.
BDP = bandwidth × RTT. For a 100 Mbps link with a 50 ms round-trip time: BDP = 100 × 10⁶ × 0.05 = 5 × 10⁶ bits = 625 KB. The TCP receive window must be at least 625 KB for this link to run at 100% utilisation. The legacy TCP default window of 64 KB — specified in RFC 793 — was designed for local area networks with very low latency and becomes a severe bottleneck on high-bandwidth, high-latency (long-fat) links.
Modern operating systems support TCP window scaling (RFC 1323), which allows windows up to 1 GB. Linux and Windows typically auto-tune their buffers to match BDP, so this problem is mostly solved on up-to-date systems. The BDP is still useful for sizing dedicated application buffers, tuning satellite links, diagnosing throughput problems, and understanding why a fast WAN link can still deliver disappointing speeds if latency is high.
Frequently asked questions
Geostationary satellites have RTTs of around 600 ms. For a 100 Mbps link: BDP = 100 × 10⁶ × 0.6 = 7.5 MB. If the TCP window is smaller than 7.5 MB, throughput is capped well below 100 Mbps. TCP window scaling and performance-enhancing proxies (PEPs) are used by satellite ISPs to work around this.
Run ping <server-hostname> from a terminal. The "time=" value in each reply is the RTT in milliseconds. For file transfers, use the RTT to the specific server you are transferring to or from — RTT to Google may differ from RTT to your cloud storage provider.
A long-fat network (pronounced "elephant") has a large bandwidth-delay product — either a very fast link, a very long-latency link, or both. Intercontinental 1 Gbps links or satellite broadband are classic examples. Standard TCP defaults were not designed for LFNs and require tuning (window scaling, larger buffers) to achieve good throughput.
Also known as
TG we-Calculate Editorial Team. (2026). Bandwidth-Delay Product Calculator — TCP Window Size [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/bandwidth-delay-product-calculator
TG we-Calculate Editorial Team. "Bandwidth-Delay Product Calculator — TCP Window Size." TG we-Calculate. 2026. https://we-calculate.com/calculator/bandwidth-delay-product-calculator.
TG we-Calculate Editorial Team, "Bandwidth-Delay Product Calculator — TCP Window Size," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/bandwidth-delay-product-calculator
@misc{wecalculate_bandwidth_delay_product_calculator, title = {Bandwidth-Delay Product Calculator — TCP Window Size}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/bandwidth-delay-product-calculator}}, year = {2026}, note = {TG we-Calculate} }
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