Calculation Tools
MTU / Packet Size Calculator
Model L2, L3, tunnel, and transport overhead to estimate how much payload remains for a packet at a given interface MTU.
Useful for MSS tuning, overlay validation, and transport overhead planning.
It is intended for the kind of troubleshooting where a service “mostly works” until encapsulation stacks up and packets start fragmenting, black-holing, or forcing a smaller TCP MSS than anyone documented.
What This Page Helps You Answer
The calculator is useful when you need to estimate payload space after adding VLAN tags, MPLS labels, GRE, VXLAN, Geneve, IPsec, WireGuard, or transport headers. It gives you a quicker starting point than manually summing header sizes from memory, especially when you are comparing several overlay designs or trying to explain why a chosen interface MTU leaves less room than application teams expect.
How To Read The Result
Think of the base MTU as the budget and the selected encapsulations as consumers of that budget. The remaining payload number is the operational signal: it tells you roughly how much room is left for the packet you care about. If TCP is involved, that usually leads naturally to an MSS conversation. If tunnels are involved, it tells you whether the overlay design is likely to require a larger underlay MTU or more aggressive endpoint tuning.
What This Does Not Guarantee
Header accounting is a strong planning aid, but vendors and deployments can still differ in how they count bytes at different layers, and some security or tunnel modes include overhead that changes with configuration. For production changes, use this page to narrow the design space, then confirm with platform documentation and packet captures if the margin is tight.