MTU Calculator by Tunnel Encapsulation

MTU overhead: how much payload survives the headers

The MTU (Maximum Transmission Unit) is the largest L2 frame a link can carry, and standard Ethernet sets it at 1500 bytes. Every packet stacks up Ethernet (14 B) + IPv4 (20 B) + TCP (20 B) = 54 B of headers. That leaves 1446 bytes for payload, roughly 96.4% efficiency. Go to jumbo frames at 9000 B and efficiency climbs past 99%, which is why you find them in data centers and on NAS boxes running iSCSI/NFS. Tunnels chip away at the MTU. PPPoE takes 8 B (you're left with 1492), GRE takes 24 B, VXLAN 50 B, and IPSec anywhere from 50 to 100 B depending on the mode. If a packet is bigger than the path MTU and the Don't Fragment bit is set, the router answers with an ICMP "Fragmentation Needed" message. That's the whole basis of Path MTU Discovery (PMTUD).

Applications: VPNs, data centers and PMTUD

It helps you size VPN MTU (WireGuard, OpenVPN, IPSec) so packets stop fragmenting, turn on jumbo frames in storage networks (Ceph, vSAN), track down black-holed connections when ICMP gets filtered and PMTUD breaks, and tune data-center fabrics where east-west traffic usually runs at 9216 B.

FAQ

Why 1500 bytes? It comes straight from the 1980 Ethernet spec, where it balanced latency against error rate and efficiency on coaxial CSMA/CD.

Can I just enable jumbo frames everywhere? Only when every device on the L2 segment agrees on the same MTU. A single switch port that doesn't match will drop packets silently.

VPN slow on some sites? That smells like a PMTU black hole. Drop the VPN MTU (1400 is a safe default) or turn on MSS clamping at the gateway.