MTU & MSS Calculator
Calculate the TCP maximum segment size (MSS) from the MTU, IP version and tunnel overhead, free online.
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MSS = MTU − IP header (20 bytes IPv4, 40 IPv6) − TCP header (20 bytes, plus options such as timestamps 12). Tunnels (PPPoE 8, GRE 24, VXLAN 50, IPsec ~73) reduce the usable MTU, so set MSS clamping to avoid fragmentation. Jumbo frames use an MTU of 9000.