Kernaali Tools
Home / Network tools / TCP MSS Calculator
Link & transport

TCP MSS Calculator

Estimate TCP MSS and the payload budget after selected header options.

Local calculation · inputs are not sent to a server.

Result

Calculating the example…

How it works

The baseline MSS option is IP MTU minus the fixed IP and TCP headers. IPv4 normally uses 20 + 20 bytes; IPv6 uses 40 + 20. TCP option and IP extension bytes reduce actual segment data; they are shown separately from the baseline MSS option.

Worked example

At IPv4 MTU 1500, the baseline MSS is 1460. With 12 bytes of TCP options and no IP options, a full-size packet has room for 1448 bytes of TCP data.

Inputs, edge cases & common mistakes

This is a header budget, not automatic MSS clamping advice. Use the effective inner MTU after tunnels. Path changes, encapsulation and option negotiation can change actual segment sizes. IPv4 options must fit the real IP header and IPv6 extension chains must be measured.

Technical references

Reviewed 9 September 2026. Found an issue? Send a correction with a reproducible example.