How do I calculate TCP MSS from MTU?

Subtract the IP header and TCP header from the effective inner MTU. With a 1500 byte MTU, IPv4 without options, and TCP without options, the common MSS is 1500 - 20 - 20 = 1460 bytes.

Does a VLAN tag reduce MTU?

Usually no for a properly configured Ethernet path. An 802.1Q tag adds 4 bytes to the Ethernet frame on the wire, but common switches support that tagged frame size while preserving a 1500 byte IP MTU.

Why do VPNs and tunnels reduce payload size?

Tunnels add outer IP, UDP, GRE, ESP, VXLAN, or other headers around the original packet. Those added bytes consume the path MTU unless the underlay MTU is increased.

What MTU is needed to carry a 1500 byte packet through VXLAN?

VXLAN over IPv4 commonly adds 50 bytes before the inner IP packet when the inner Ethernet header is included, so an underlay MTU of about 1550 bytes is needed to carry a full 1500 byte inner IP packet without fragmentation.

Is this MTU calculator private?

Yes. Inputs are processed locally in your browser and are not submitted to a server.

MTU and Packet Overhead Calculator

Estimate effective MTU, TCP MSS, UDP payload size, tunnel overhead, Ethernet wire efficiency, and jumbo frame requirements for Ethernet, VLAN, PPPoE, GRE, VXLAN, WireGuard, IPsec, and custom encapsulation designs.

All inputs stay in your browser. Use the result as a planning estimate, then validate production paths with device MTU, tunnel, and packet-capture tests.

Inputs

Underlay link MTU (bytes) The IP MTU available before tunnel overhead is applied.

Tunnel or encapsulation preset

Encapsulation overhead before inner IP (bytes) Outer headers plus any inner Ethernet header consumed before the inner IP packet.

Inner IP header

Inner IP header bytes

Transport header

Transport header bytes

Outer Ethernet VLAN tags VLAN tags add wire bytes; they do not usually reduce the configured IP MTU.

Target per-packet application payload (bytes) Used to check whether a payload or TCP MSS fits without fragmentation.

Bulk payload to carry (bytes) Used for packet count and total wire-byte estimate.

Ethernet wire accounting

Include Ethernet FCS (4 bytes)

Include preamble, SFD, and inter-frame gap (20 bytes)

Common scenarios

MTU presets

Results

Fit status-

Effective inner MTU-

Max application payload-

Wire efficiency-

Packet sizing

TCP MSS with selected IP header:-

UDP payload with selected IP header:-

Selected IP + transport overhead:-

Total overhead before app payload:-

Target payload surplus / shortfall:-

Ethernet wire estimate at max payload

Outer Ethernet header plus VLAN:-

Ethernet FCS:-

Preamble, SFD, IFG:-

Wire bytes per full packet:-

Overhead per full packet:-

Bulk payload estimate

Packets required:-

Estimated total wire bytes:-

Estimated overhead bytes:-

Required MTU for target payload

Required underlay MTU:-

Required Ethernet frame size:-

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How to use this MTU calculator

Start with the underlay MTU: use the IP MTU supported by the real path, such as 1500, 1492, 9000, or 9216 bytes.
Add tunnel overhead: choose PPPoE, GRE, VXLAN, WireGuard, IPsec, or enter a custom byte value from your platform documentation.
Pick inner protocol headers: choose IPv4 or IPv6, then TCP, UDP, ICMP, or a custom transport header.
Set wire accounting: count VLAN tags, Ethernet FCS, and preamble/IFG when estimating line-rate efficiency.
Review the fit: compare target payload or TCP MSS with the calculated maximum and check the required MTU for a no-fragmentation design.

Formula and assumptions

Effective inner MTU: underlay link MTU - encapsulation overhead

Application payload per packet: effective inner MTU - inner IP header - transport header

TCP MSS: effective inner MTU - inner IP header - 20 for TCP without options.

Wire bytes per full packet: Ethernet header + VLAN tags + packet bytes + optional FCS + optional preamble/SFD/IFG

Wire efficiency: application payload / wire bytes per full packet

Required underlay MTU: target payload + inner IP header + transport header + encapsulation overhead

Header presets are planning values. IPsec ESP, TCP options, QUIC, MPLS, provider tags, MACsec, and device-specific tunnel implementations can change the exact byte count.

Common MTU and overhead examples

Scenario	Typical overhead before inner IP	Effective inner MTU on 1500 underlay	IPv4 TCP MSS estimate
Plain Ethernet	0 B	1500 B	1460 B
PPPoE on Ethernet	8 B	1492 B	1452 B
GRE over IPv4	24 B	1476 B	1436 B
VXLAN over IPv4	50 B	1450 B	1410 B
WireGuard over IPv4 UDP estimate	60 B	1440 B	1400 B
9000 byte jumbo, no tunnel	0 B	9000 B	8960 B

MTU, MSS, Ethernet frames, and wire rate

MTU is the largest Layer 3 packet a link can carry without fragmentation. TCP MSS is smaller because it is only the TCP payload portion of that IP packet. For a common 1500 byte IPv4 path with no tunnel, TCP MSS is 1460 bytes after subtracting a 20 byte IPv4 header and a 20 byte TCP header.

Ethernet wire usage is larger than the IP MTU. A normal Ethernet frame has a 14 byte MAC header and commonly a 4 byte FCS. Each VLAN tag adds 4 bytes. Physical line-rate accounting also includes 8 bytes of preamble/SFD and 12 bytes of inter-frame gap, which is why useful payload efficiency is lower than the MTU alone suggests.

Jumbo frames can help large transfers by reducing per-packet overhead, but every device in the path must support the configured MTU. Mixed MTU paths, hidden provider tags, tunnels, and firewalls that drop fragmented traffic are common causes of black-hole behavior.

Reference notes

Ethernet IP datagram examples use the RFC 894 1500 byte Ethernet data-field assumption.
IPv4 header sizing uses RFC 791, where the minimum correct Internet Header Length is 5 32-bit words.
IPv6 examples use the 40 byte base header and the 1280 byte minimum link MTU requirement from RFC 8200.
UDP uses an 8 byte minimum header from RFC 768. VXLAN uses the 8 byte VXLAN header and outer UDP/IP structure from RFC 7348.
GRE examples use a 4 byte base GRE header plus an outer IP header as described by RFC 2784. TCP examples use RFC 9293 header sizing.

Methodology

The calculator subtracts selected tunnel overhead from the entered underlay MTU, then subtracts the selected inner IP and transport headers to find the maximum application payload. It separately estimates Ethernet wire bytes by adding the outer Ethernet header, selected VLAN tags, optional FCS, optional preamble/SFD/IFG, and the transmitted packet bytes. Bulk estimates use full-size packets plus one final partial packet when needed.

Last reviewed: June 2026. Calculations are deterministic byte arithmetic and run locally in your browser.

FAQs

Does VLAN tagging change the TCP MSS?

Not usually. A VLAN tag adds 4 bytes to the Ethernet frame on the wire, but the IP MTU is normally still 1500 bytes when the Ethernet path supports tagged frames.

Why does VXLAN often require a larger underlay MTU?

VXLAN wraps an inner Ethernet frame in VXLAN, UDP, and outer IP headers. To carry a full 1500 byte inner IP packet, the underlay often needs about 1550 bytes for IPv4 VXLAN.

Should I set MTU or clamp TCP MSS?

MTU fixes the packet size at the interface or tunnel. TCP MSS clamping only affects TCP payload negotiation. Non-TCP traffic such as UDP may still need correct MTU and path-MTU behavior.

Can this predict exact IPsec overhead?

No. IPsec ESP overhead varies with tunnel mode, NAT-T, IV size, padding, integrity check value, and cipher suite. Use the IPsec presets as estimates unless you have exact vendor byte counts.

Is this calculator private?

Yes. Inputs are processed locally and are not submitted to a backend.

Disclaimer

MTU and overhead estimates are infrastructure planning aids. Validate production designs with device documentation, packet captures, path MTU discovery tests, tunnel configuration, and change-management review.