TCP Window Size Calculator
Calculate optimal TCP window size from bandwidth-delay product for maximum throughput.
About TCP Window Size Calculator
To fully utilize a network link, the TCP window must be at least as large as the bandwidth-delay product (BDP) — the amount of data that can be in flight before the first acknowledgment returns. The base TCP header only supports a 16-bit window field, capping it at 65,535 bytes, so any link with a BDP above that threshold needs the TCP window scaling option (RFC 1323), which the Linux kernel enables by default via net.ipv4.tcp_window_scaling.
Formula
Bandwidth-delay product
window = bandwidth × RTT
- B
- bandwidth in bytes/sec
- T
- round-trip time in seconds
Worked example
With the default values below, TCP Window Size Calculator returns Optimal Window (Bandwidth-Delay Product) of 10,000,000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Link Bandwidthminimum 0 Mbps
- 1,000 Mbps
- Round-Trip Time (RTT)minimum 0 ms
- 80 ms
Results
- Optimal Window (Bandwidth-Delay Product)
- 10,000,000 bytes
- Optimal Window
- 9,765.63 KB
- TCP Window Scaling Required
- true
- Recommended Scale Factor
- 8
- Max Window With Scale Factor
- 16,776,960 bytes
Step by step
- 1Values used
- 2Bandwidth-delay product
- 3Optimal Window (Bandwidth-Delay Product)= 10,000,000 bytes
- 4Optimal Window= 9,765.63 KB
- 5TCP Window Scaling Required= Yes
- 6Recommended Scale Factor= 8
- 7Max Window With Scale Factor= 16,776,960 bytes
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What happens if the TCP window is smaller than the bandwidth-delay product?
Throughput gets capped well below the link's actual capacity, because the sender must stop and wait for acknowledgments before the pipe is full — this is the classic 'long fat network' problem on high-bandwidth, high-latency links.
Is TCP window scaling enabled by default on Linux?
Yes, modern Linux kernels enable window scaling by default (net.ipv4.tcp_window_scaling = 1), and it is negotiated automatically during the TCP handshake if both endpoints support it.
How do I tune the actual buffer sizes on Linux?
The relevant sysctls are net.core.rmem_max/wmem_max (system-wide caps) and net.ipv4.tcp_rmem/tcp_wmem (min/default/max per-socket buffer sizes) — set the max values at or above your calculated optimal window for the link.