MTU Calculator
Calculate maximum payload size and overhead for any MTU.
Standard Ethernet MTU is 1500 bytes. Jumbo frames use 9000.
What Is MTU?
Maximum Transmission Unit (MTU) is the largest packet size, in bytes, that a network interface can transmit in a single frame. The standard MTU for Ethernet networks is 1500 bytes. This value includes protocol headers but not the Ethernet frame header itself. When a packet exceeds the MTU of any link along its path, it must either be fragmented (split into smaller pieces) or dropped — depending on the protocol and flags set. Understanding MTU is critical for avoiding fragmentation, optimizing throughput, and troubleshooting network performance issues.
Why MTU Matters
IP fragmentation adds overhead and can cause performance degradation. Each fragment needs its own headers, and if any single fragment is lost, the entire original packet must be retransmitted. Tunneling protocols like GRE, IPsec, and VXLAN add encapsulation headers that reduce the available payload space. If the inner packet is too large for the tunnel's effective MTU, fragmentation occurs inside the tunnel — often causing hard-to-diagnose performance problems. Setting the correct MTU avoids these issues entirely.
Common MTU Values
Standard Ethernet uses 1500 bytes. PPPoE connections (common with DSL) reduce this to 1492 due to the 8-byte PPPoE header. Jumbo frames on data center networks support 9000 bytes for higher throughput with less per-packet overhead. IPv6 requires a minimum MTU of 1280 bytes on any link. VPN tunnels typically need an MTU reduction of 50–80 bytes depending on the encapsulation protocol used. This calculator helps you determine the exact payload available after accounting for all protocol layers.
Path MTU Discovery
Modern systems use Path MTU Discovery (PMTUD) to find the smallest MTU along a network path and size packets accordingly. This relies on ICMP "Fragmentation Needed" messages being delivered back to the sender. When firewalls block ICMP, PMTUD fails and connections stall — a common issue known as "PMTUD black hole." Manually setting the correct MTU and MSS (Maximum Segment Size) values can work around this problem.