MTU And MSS Calculator

Work out mtu and mss instantly with clear inputs, formula shown and shareable results.

About MTU And MSS Calculator

MSS is the MTU minus the IP and TCP headers, so plain IPv4 Ethernet gives 1500 - 20 - 20 = 1460 bytes. Every layer of encapsulation reduces it further: IPsec tunnel mode costs around 73 bytes and VXLAN 50. Getting MSS wrong causes packets that cannot be fragmented to be dropped silently, which is the classic cause of connections that establish but then hang.

Formula

MSS derivation

effective MTU = path MTU - encapsulation bytes; MSS = effective MTU - IP header - TCP header (20 for IPv4, 40 for IPv6)

Worked example

With the default values below, MTU And MSS Calculator returns TCP maximum segment size of 1,460. Change any input above and every figure here is recalculated in your browser.

Inputs used

Path MTUaccepts 576–9,216 bytes: 1,500 bytes
Encapsulation in the pathone of None - plain Ethernet, IPsec tunnel mode (~73 bytes), GRE (24 bytes), VXLAN (50 bytes), PPPoE (8 bytes): None - plain Ethernet
IP versionone of IPv4 (20-byte header), IPv6 (40-byte header): IPv4 (20-byte header)

Results

TCP maximum segment size: 1,460 bytes
MTU after encapsulation: 1,500 bytes
Total header overhead per segment: 40 bytes
Payload efficiency: 97.33%

Step by step

1Values used
2MSS derivation
3TCP maximum segment size= 1,460 bytes
4MTU after encapsulation= 1,500 bytes
5Total header overhead per segment= 40 bytes
6Payload efficiency= 97.33

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What is MSS clamping?

Routers rewrite the MSS value in TCP SYN packets to fit the tunnel MTU, which avoids relying on path MTU discovery. It is standard practice on VPN and PPPoE edges.

Why does path MTU discovery fail?

It depends on ICMP fragmentation-needed messages, which many firewalls block. The sender then keeps retransmitting full-size packets that are silently dropped.