MTU / MSS Calculator
Work out the TCP Maximum Segment Size (MSS) that fits inside a given path MTU. The MSS is the MTU minus the IP header, the TCP header and any extra encapsulation overhead such as VLAN tags, PPPoE or a VPN tunnel.
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How to use this calculator
- Enter the path MTU in bytes (1500 for standard Ethernet).
- Choose the IP version (4 = 20-byte header, 6 = 40-byte header) and the TCP header size (20 bytes unless you use TCP options).
- Add any extra overhead for encapsulation — 8 for PPPoE, roughly 20–60 for an IPsec/GRE/VPN tunnel — then read off the MSS.
How it works
MSS = MTU − IP header − TCP header − extra overhead. For a plain IPv4 Ethernet path that is 1500 − 20 − 20 − 0 = 1460 bytes; IPv6 drops it to 1440 because its header is 40 bytes. Each layer of encapsulation reduces the room left for payload, so the tool subtracts your 'extra overhead' as well. If the combined overhead is greater than or equal to the MTU there is no room for data and the tool reports an error.
Worked example
Worked example. A home connection over PPPoE has MTU 1500, IPv4, a 20-byte TCP header and 8 bytes of PPPoE overhead. MSS = 1500 − 20 − 20 − 8 = 1452 bytes — which is why PPPoE links are often clamped to an MSS of 1452.
Common mistakes
- Forgetting that IPv6 uses a 40-byte header, so its MSS is 20 bytes smaller than IPv4 for the same MTU.
- Ignoring tunnel or PPPoE overhead, which leaves the MSS too large and causes big transfers to stall or fragment.
- Confusing MTU (the whole frame's payload including headers) with MSS (just the TCP data portion).
Frequently asked questions
Why is the standard MSS 1460 and not 1500?
1500 is the MTU — the total IP payload. TCP over IPv4 must fit its own 20-byte header and the 20-byte IP header inside that, leaving 1500 − 40 = 1460 bytes of actual segment data.
What is MSS clamping?
A router rewrites the MSS value in the TCP handshake down to a safe size so segments never exceed the smallest MTU on the path. It is a common fix for VPN or PPPoE links where full-size 1500-byte packets would otherwise need fragmenting.
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