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MTU Calculator

Calculate the effective MTU and TCP MSS after adding tunnel and encapsulation overhead to your base MTU.

Standard Ethernet = 1500  |  Jumbo frames = 9000
Presets
Encapsulation overhead
Base MTU1500 B
Total overhead0 B
Effective MTU1500 B
TCP MSS1460 B
MTU composition
Effective MTU (1500 B)

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Ping / Latency

Send real ICMP echo requests from a neutral host to any server. Runs 5 packets and reports min, avg, max, and packet loss.

About this tool

The MTU Calculator helps network engineers determine the maximum transmission unit available for actual payload after accounting for tunnel and encapsulation overhead. Each encapsulation layer adds a fixed header to every packet, reducing the usable MTU for the inner traffic. Misconfigured MTU causes silent packet drops, slow TCP connections due to fragmentation, and application timeouts that are difficult to diagnose.

TCP MSS (Maximum Segment Size) is the effective MTU minus 40 bytes for the IP and TCP headers. This is the value you configure on firewalls and routers with MSS clamping to prevent fragmentation on tunneled links. Every IPv4 host must accept packets of 576 bytes (RFC 791), while the smallest MTU a link may have is 68 bytes; a path below 576 bytes is rare and often means fragmentation.

How to test your MTU

A web page cannot send packets of a chosen size, so no online test measures the MTU of your own path. Ping can: send a packet that must not be fragmented and lower its size until it gets through. The largest payload that passes, plus 28 bytes (20 for the IPv4 header and 8 for ICMP), is the path MTU.

Windows: ping -f -l 1472 example.com. Linux: ping -M do -s 1472 example.com. macOS: ping -D -s 1472 example.com. A reply means 1,500 bytes pass; an answer that the packet needs to be fragmented, or that the message is too long, means the path MTU is lower, so try a smaller size.

MTU or MSS?

The MTU is the largest IP packet a link carries; the MSS is the largest TCP payload inside it: the MTU minus 40 bytes for IPv4 (20 for IP, 20 for TCP) or minus 60 for IPv6. When a tunnel adds overhead, lower the MSS clamp on the router by the same amount, or large TCP packets are dropped on paths where ICMP is filtered.