TCP Maximum Segment Size Utility

TCP MSS Calculator

Calculate TCP Maximum Segment Size from MTU for IPv4 or IPv6, compare the standard advertised MSS with actual payload capacity for custom TCP or IP header sizes, and measure packet header overhead.

✓ IPv4 MSS ✓ IPv6 MSS ✓ Ethernet MTU ✓ PPPoE MTU ✓ TCP Options ✓ Payload Capacity

MSS

TCP MSS Calculation

● Ready

Path / Interface MTU Enter the maximum IP packet size allowed by the path or interface.

IP Version Base IPv4 header is 20 bytes. Base IPv6 header is 40 bytes.

Actual IP Header Length Used to calculate actual TCP payload capacity for this packet.

Actual TCP Header Length Standard TCP header is 20 bytes. TCP options can increase it to 60 bytes.

Advertised Peer MSS Optional peer-advertised MSS for comparing the effective send MSS.

Result Mode

Important distinction: the traditional TCP MSS advertised during connection setup is based on the MTU minus the fixed IP and TCP headers. Actual payload capacity of an individual packet can be smaller when longer IP headers, extension headers or TCP options are present.

TCP MSS Calculation Result Calculated

Recommended / Calculated TCP MSS —

MTU —

IP Version —

Base IP Header —

Base TCP Header —

Advertised MSS —

Actual IP Header —

Actual TCP Header —

Actual Payload Capacity —

Peer MSS —

Effective Send MSS —

Header Overhead —

Payload Efficiency —

Calculation Breakdown —

Packet Layout —

Validation & Notes —

What Is TCP MSS?

TCP MSS means Maximum Segment Size. It represents the maximum amount of TCP application data that a host is willing to receive in a single TCP segment. The MSS value does not include the IP header or TCP header.

TCP endpoints normally exchange an MSS option during the SYN phase of connection establishment.

TCP MSS Formula

For the common case with no unusual headers, TCP MSS can be estimated directly from the IP MTU.

IPv4:

MSS =
MTU
− 20-byte IPv4 header
− 20-byte TCP header


IPv6:

MSS =
MTU
− 40-byte IPv6 header
− 20-byte TCP header

IPv4 MSS for MTU 1500

MTU:
1500 bytes

IPv4 Header:
20 bytes

TCP Header:
20 bytes

TCP MSS:
1500 − 20 − 20

= 1460 bytes

Therefore, 1460 bytes is the familiar TCP MSS associated with a standard 1500-byte IPv4 MTU.

IPv6 MSS for MTU 1500

MTU:
1500 bytes

IPv6 Header:
40 bytes

TCP Header:
20 bytes

TCP MSS:
1500 − 40 − 20

= 1440 bytes

Common TCP MSS Values

Network	MTU	IP	Typical MSS
Ethernet	1500	IPv4	1460
Ethernet	1500	IPv6	1440
PPPoE	1492	IPv4	1452
PPPoE	1492	IPv6	1432
Jumbo MTU	9000	IPv4	8960
Jumbo MTU	9000	IPv6	8940

TCP MSS vs MTU

MTU specifies the maximum complete IP packet size that can traverse a link without requiring a smaller packet. MSS instead represents only the TCP data portion.

IP Packet:

+-------------------+
| IP Header         |
+-------------------+
| TCP Header        |
+-------------------+
| TCP Data / MSS    |
+-------------------+

MTU covers all three.

MSS covers only TCP data.

TCP Header Options and Payload Capacity

The normal TCP header is 20 bytes, but its Data Offset field allows a TCP header as large as 60 bytes. Options such as timestamps can therefore reduce the amount of application data that fits into a specific MTU-sized packet.

MTU:
1500

IPv4 Header:
20

TCP Header with options:
32

Actual payload fitting this packet:

1500 − 20 − 32

= 1448 bytes

This calculator displays that packet payload capacity separately from the traditional MSS calculation so the two concepts are not confused.

IPv4 Options and MSS

An IPv4 header can be longer than the usual 20 bytes when options are present. That also reduces the amount of TCP application data that can fit inside a packet of a fixed total size.

MTU:
1500

Actual IPv4 Header:
24

Actual TCP Header:
20

Packet payload capacity:
1500 − 24 − 20

= 1456 bytes

IPv6 Extension Headers

IPv6 starts with a fixed 40-byte base header, but extension headers can appear between the IPv6 header and TCP. Those bytes consume packet space and can therefore reduce the TCP payload that fits in a specific packet.

You can represent such additional IPv6 header overhead by increasing the Actual IP Header Length field in this calculator.

TCP MSS Clamping

MSS clamping changes the advertised TCP MSS so endpoints avoid creating TCP segments that would be too large for a known path MTU. It is often encountered on networks involving tunnels, VPNs or PPPoE where the available IP MTU is smaller than ordinary Ethernet.

Example:

Path MTU:
1400

IPv4:

MSS =
1400 − 20 − 20

= 1360

Peer Advertised MSS

The MSS advertised by the receiving TCP endpoint limits how much TCP data the sender should place in one segment toward that peer.

Local path-based MSS:
1460

Peer advertised MSS:
1360

Effective sending limit:
min(1460, 1360)

= 1360

The calculator includes an optional Peer MSS field to make this comparison.

TCP MSS Calculator FAQs

What is the TCP MSS for MTU 1500 on IPv4?

With a 20-byte IPv4 header and 20-byte TCP header, the typical MSS is 1500 − 20 − 20 = 1460 bytes.

What is the TCP MSS for MTU 1500 on IPv6?

With the 40-byte IPv6 base header and 20-byte TCP header, the result is 1500 − 40 − 20 = 1440 bytes.

What is the TCP MSS for PPPoE MTU 1492?

For IPv4, the common result is 1492 − 20 − 20 = 1452 bytes. For IPv6 using its base header, the corresponding value is 1432 bytes.

Does MSS include the TCP header?

No. MSS describes TCP payload data and excludes both the TCP and IP headers.

What is the difference between MTU and MSS?

MTU limits the complete IP packet size, whereas TCP MSS specifies the TCP data portion that can be carried in a TCP segment.

Do TCP options reduce payload space?

Yes. A larger TCP header leaves fewer bytes available for application data inside a packet constrained to the same MTU.

What is MSS clamping?

MSS clamping reduces the MSS advertised in TCP setup packets so the endpoints send smaller TCP segments suitable for a lower path MTU.

Can MSS be larger than MTU?

A useful path-based TCP MSS must be smaller than the IP MTU because the packet also requires IP and TCP headers.