Last updated: 2026-06-24

MTU to MSS

The maximum segment size (MSS) is the largest block of application data that TCP will place in a single packet. It is derived from the path's maximum transmission unit (MTU) — the biggest IP packet the link can carry — by reserving space for the IP and TCP headers. Getting the MSS right keeps packets whole: a value that is too large forces fragmentation or, worse, silent packet loss, while a value that is too small wastes capacity on header overhead. This calculator computes the MSS and the resulting payload efficiency from three byte counts.

The formula

where $MTU$ is the maximum transmission unit in bytes, $H_{IP}$ is the IP header size, $H_{TCP}$ is the TCP header size, and $E$ is the payload efficiency — the share of each full-size packet that carries real data rather than headers.

The header sizes are the only moving parts. A plain IPv4 header is 20 bytes and a plain TCP header is 20 bytes, so on a standard Ethernet path the two together cost 40 bytes per packet. IPv6 uses a larger fixed header of 40 bytes, and TCP options such as timestamps and selective acknowledgement (SACK) can add a few bytes more.

Worked example. A standard Ethernet path with IPv4:

So 1460 bytes of every 1500-byte packet carry payload, and about 2.7% is consumed by the IP and TCP headers. Switching to IPv6 with the same 1500-byte MTU costs 20 extra header bytes, dropping the MSS to 1440 bytes and the efficiency to 96.0%.

Why the MSS matters

TCP advertises its MSS to the other endpoint during the SYN handshake at the start of every connection. Each side promises never to send a segment larger than the value it received, so neither endpoint produces a packet that the path cannot carry. When the MSS is sized correctly for the smallest MTU along the route, every segment travels intact and TCP runs at full efficiency.

Problems appear when a packet is larger than some link can carry. The router either fragments it — splitting one packet into several, which adds overhead and reassembly work — or, if the "don't fragment" flag is set, drops it and signals back with an ICMP message. If that ICMP message is filtered out by a firewall, the sender keeps retransmitting an oversized packet that never arrives, and the connection stalls. This failure is known as a path MTU black hole.

Headers and common MTU values

The MSS depends entirely on the MTU and on which headers apply. A few values come up repeatedly:

Tunnels such as VPNs, GRE, and IPsec wrap each packet in their own outer headers, which shrinks the MTU available to the inner packet. That is why VPN connections often advertise an MSS well below 1460, and why routers frequently "clamp" the MSS — rewriting the advertised value downward — to keep traffic flowing across an encapsulated link.

Payload efficiency and larger MTUs

Because the header cost is fixed per packet, a larger MTU spreads that cost over more payload and raises efficiency. A 1500-byte MTU yields 97.3% efficiency with IPv4 headers; a 9000-byte jumbo frame pushes that above 99.5%. Larger packets also mean fewer packets per second for the same throughput, which lightens the per-packet processing load on routers, firewalls, and network cards. Jumbo frames are therefore common inside data centres and on storage networks, where every device on the path can be configured for the larger MTU.

Related calculators

To turn a packet size into a packet rate for a given link speed, use the Packets Per Second Calculator calculator. To estimate how long it takes to move a file once the payload rate is known, see the Data Transfer Time Calculator calculator. For subnet and address planning on the same network, the Subnet (CIDR) Calculator calculator is a useful companion.