pjhtech Tools
Link & transport

TCP MSS Calculator

Find the TCP MSS option value for an IP MTU, and the room left for TCP data when options are present.

Input

For a tunnel, use the inner IP MTU after reserving encapsulation space.

Reduces TCP data in this packet, not the MSS option value.

Extra bytes beyond the base IP header. IPv4 options: 0–40 in multiples of four.

Reset restores the example inputs.

Local calculation · inputs are not sent to a server.

Result

Calculating the example…

Use the IP MTU available to the TCP packet, after any tunnel overhead. TCP MSS option is the value based on the fixed IP and TCP headers. TCP data with selected options is the smaller amount that fits when the packet includes those extra headers.

Example: At IPv4 MTU 1500, the MSS option is 1460 bytes. With 12 bytes of TCP options and no IP options, 1448 bytes of TCP data fit. Keep the MSS option at 1460; do not subtract the TCP options from it.

Enter only the option or extension bytes actually present. IPv4 options occupy at most 40 bytes, in multiples of four. This calculation does not discover the path MTU or choose a device’s MSS-clamping policy.

References