MTU/MSS Calculator
Standard Ethernet frame payload
Valid range: 68-65536 bytes.
Layer 3
Layer 4
Link/access overhead
VLAN, QinQ, and MPLS labels usually require Layer 2 headroom, not lower TCP MSS when IP MTU stays at 1500.
Tunneling
GRE and IP-in-IP entries include only tunnel header. Add Outer IPv4 or Outer IPv6 manually when modeling full outer stack.
Security
IP payload
1,480 bytes
TCP/UDP payload
1,460 bytes
Total overhead
40 bytes
Efficiency
97.3%
Packet visualization
MTU: 1,500 bytesHow it works
MTU (Maximum Transmission Unit) is maximum packet payload link can carry before fragmentation. MSS (Maximum Segment Size) is TCP data payload after subtracting transport and network headers. With standard IPv4 + TCP, MSS is usually MTU minus 40 bytes.
This calculator subtracts selected headers layer by layer from interface MTU. Added tunnel and security wrappers consume part of packet budget, so effective payload shrinks. That is why overlays like VXLAN, WireGuard, and IPsec need lower inner MTU or MSS clamping.
Use visual bar above to see exact byte split between overhead and payload. For deeper walk-through of fragmentation, PMTUD, black-hole symptoms, and field troubleshooting, readUnderstanding MTU, MSS, and Packet Fragmentation.
FAQ
What is the difference between MTU and MSS?
MTU is the largest packet size a link can carry. MSS is TCP payload after removing IP and TCP headers. On Ethernet MTU 1500 with IPv4/TCP, MSS is 1460 bytes.
Why does tunnel traffic reduce effective payload?
Tunnels add extra headers around original packet. Those bytes consume MTU budget, so less room remains for application payload.
Why can ping work while apps fail?
Small probes may pass, larger packets can exceed path MTU and get dropped when PMTUD fails. This creates classic black-hole behavior.
When should I clamp MSS?
Clamp MSS on PPPoE, VPN, and overlay edges when endpoints cannot discover path MTU reliably or ICMP is filtered.