What TCP Window Calculator helps you do

Calculate the bandwidth-delay product, the suggested TCP receive-window size, the required scaling shift and the number of packets in flight. It is the math behind tuning throughput on high-latency links. The MSS is derived from the MTU by subtracting 40 bytes for the IPv4 and TCP headers.

The calculation runs in your browser; no network parameters are transmitted to Haivix.

How it works

Most tools here are offline calculators: the address math, RAID capacity, permission bits, TCP window sizing and transfer-time estimate all run in your browser with no request. The RDAP lookup is the exception, because registration data lives in public registries and the domain must be sent to a lookup service. Unit conventions matter throughout: a megabyte may mean 1,000,000 or 1,048,576 bytes, and a megabit is always one eighth of a megabyte. These are reference models, not live checks, so a port can be listed without a service running, a permission mode can be valid without being appropriate, and a RAID estimate can differ from formatted capacity.

How to use TCP Window Calculator

Enter the bandwidth in Mbps and the round-trip time in milliseconds.
Set the MTU, usually 1500 for Ethernet.
Read the BDP, packet count and scale shift.
Compare the suggested buffer with the host's current setting.

When to use it

Work out the window size needed for high-latency transfers.
Understand why a fast link still performs poorly on long paths.
Check whether TCP window scaling is required.
Estimate the buffer for a long-fat-network link.
Explain the effect of RTT on single-stream throughput.
Sanity-check a tuning recommendation before applying it.

When not to use it

Do not treat the BDP as a universal OS tuning value; modern stacks auto-tune.
Do not ignore loss, congestion control or application limits.
Do not use it for UDP or for a non-TCP protocol.

Example

A 1000 Mbps link with 50 ms RTT has a BDP of 6,250,000 bytes. With a 1500-byte MTU, MSS is 1460 bytes, so about 4,281 packets would be in flight. A scale shift of 7 gives a ceiling of 8,388,480 bytes, enough for that path.

What you can do with TCP Window Calculator

Calculate the bandwidth-delay product in bytes.
Derive MSS from the MTU.
Estimate packets in flight.
Find the minimum window-scale shift.
Show the scaled window ceiling.
Flag a BDP beyond the standard maximum window.

Example: check the model against the live system

Note the unit convention before comparing figures. Mbps and MB/s differ by a factor of eight, and MB and MiB differ by about 5 percent.

Treat a lookup as a starting point. A port reference, a permission mode and a RAID calculation all describe expectations, not the state of a running host.

Confirm on the system itself where it matters: ss or netstat for listeners, ls -l for permissions, the controller utility for array status.

Keep RAID in perspective. It protects against a disk failure, not against deletion, corruption, ransomware or a lost site.

Record the inputs alongside the result so a later comparison uses the same assumptions.

Check the time unit on latency. A round-trip time is usually milliseconds, and entering seconds changes the bandwidth-delay product by a factor of a thousand.

Treat registry data as a snapshot. RDAP records can be redacted, cached or updated after a transfer, so confirm ownership and expiry with the registrar.

What to know before you start

It models the theoretical window and ignores packet loss, which limits real throughput.
It does not account for application or operating-system buffer limits.
The MSS assumes IPv4 and TCP headers with no options.
It is a sizing baseline, not a tuning instruction.

Common mistakes

Using one-way latency instead of round-trip time.
Forgetting that the MTU includes headers.
Assuming the BDP is the only limit on throughput.
Applying a buffer size without checking the host's autotuning.

Troubleshooting

If the tool rejects the MTU, it must be a whole number between 576 and 9000.

If the scale shows more than 14, the BDP exceeds the standard maximum.

If the packet count seems huge, check whether the RTT is in milliseconds.

If real throughput is lower, loss or a bottleneck elsewhere is likely.

TCP Window Calculator