MTU / MSS Calculator
Header deduction:—
Effective MTU (after tunnel):—
MSS:—
MSS = MTU − overhead − IP header − TCP header. With tunnels (VPN, PPPoE) the usable MTU drops by the overhead.
How the MTU / MSS calculator works
The MTU (Maximum Transmission Unit) is the largest packet size a link transmits without fragmentation – usually 1500 bytes on Ethernet. The MSS (Maximum Segment Size) is the payload part of a TCP segment, derived from the MTU minus the headers.
- Formula: MSS = MTU − IP header − TCP header. IPv4 headers are 20 bytes, IPv6 40 bytes, the TCP header 20 bytes. So with a 1500 MTU and IPv4, that is 1460 bytes MSS.
- Tunnel overhead: VPNs and PPPoE shrink the usable MTU. WireGuard costs around 60–80 bytes, PPPoE 8 bytes.
- In practice: If the MSS is wrong, large transfers stall – “MSS clamping” on the router often helps.