MTU / MSS & Fragmentation Calculator

Work out the real MSS once tunnels and headers have taken their cut.

What you provide

Timestamps take 12 bytes and are on by default on Linux and macOS. Set 0 to see the textbook figure.

Result

MSS 1448 bytes on a 1500 byte MTU

An MSS of 1448 leaves 97% of each frame carrying real data. If you are troubleshooting rather than planning, test the real path MTU directly by pinging with the do-not-fragment flag set and shrinking the payload until it gets through. The largest size that succeeds, plus 28 bytes for the probe and IP headers, is the actual path MTU.

Diagnostic telemetry
Interface MTU
1500 bytes
IPv4 header
−20 bytes
TCP header20 base plus 12 of options
−32 bytes
Maximum segment size
1448 bytes
Payload efficiency
96.5%
Fragmentation risk
low

What this cannot tell you

  • Header arithmetic on the values you choose. It does not probe a path or measure the MTU your traffic actually gets.
  • Encapsulation overheads vary with cipher and mode, particularly for IPsec. The figures here are typical rather than exact for every configuration.

Take this with you

MTU / MSS & Fragmentation Calculator on RunTheTests — free browser-based netmath tools, no sign-up.