MTU Payload Calculator
Calculate maximum transport payload and TCP MSS from an MTU and header choices.
Find payload space inside an MTU
A network MTU limits the complete IP packet, so transport payload must fit after IP and TCP or UDP headers. This calculator uses the standard 20-byte IPv4 base header or 40-byte IPv6 base header, plus 20 bytes for base TCP or 8 bytes for UDP, then adds any option or extension bytes supplied manually before subtracting from the MTU.
With an MTU of 1500, IPv4, TCP, and no options, the maximum payload and TCP MSS are both 1460 bytes. Choosing IPv6 with base TCP would leave 1440 bytes. Header values that exceed the MTU return an error, and optional fields make the assumptions visible instead of pretending every packet has only base headers.
Base headers, options, and encapsulation
Link-layer framing, tunnels, VPNs, VLAN tags, and other encapsulation can reduce the effective MTU before this calculation. TCP options also change header length, and path MTU may be smaller than an interface setting. The math runs locally in your browser. Enter measured or configured overhead for the actual path and verify packet behavior rather than treating 1500 as universal.
Frequently Asked Questions
Why is TCP MSS 1460 for a 1500-byte IPv4 MTU?
Subtracting the 20-byte IPv4 base header and 20-byte TCP base header leaves 1460 bytes.
Does the MTU include Ethernet framing?
The entered network MTU normally describes the IP packet; link framing and tunnel overhead must be considered separately.
How are IPv6 extension headers handled?
Enter their combined size in the extra IP header bytes field so they reduce the payload explicitly.
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